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Weed Management

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TopicsinThischapTer

By Chip Bubl, Extension Agent, Columbia

County, Oregon State University.

Adapted by Thomas R. Jahns, Extension Faculty,

Agriculture and Horticulture, Cooperative

Extension Service, University of Alaska

Fairbanks.

Why are weeds a problem?

Why do we all have weeds?

Weed identication

Weed life cycles

Shade trees

Weed competition

Invasive plants

Managing weeds

E

very garden has weeds, and every gardener wonders

what to do about them. This chapter will explore the

origin of weeds, their adaptations and impacts, andthe techniques you can use to manage weeds in your land-

scape.

Why are weeds a problem?The plants we call weeds are aggressive, pushy, in-your-

face plants. There are no shrinking violets in this group.

This is not to say that they dont have some redeeming

qualities, because many weeds have a substantial set of vir-

tues. However, these virtues often are overwhelmed by their

weedy attributes.The most useful denition of a weed is a plant that is a

hazard or a nuisance or one that causes injury to people,

animals or a desired crop. A plant may be dened as a weed

in some cases and not in others. Clover can be a valuable

addition to a pasture and is viewed with tolerance in many

lawns. But if you are a park manager faced with reducing li-

ability due to bee stings, you may feel compelled to remove

clover in some parts of the park. Tree seeds such as aspen,

cottonwood or birch that sprout in an herbaceous peren-

nial ower bed are weeds in that context but may well have

come from trees that are part of the planted landscape.Weeds can cause a range of problems in your garden or

community. They:

Compete with desirable crops

Reduce the aesthetic qualities of a landscape

Obstruct sight lines on roadways

Interfere with water drainage from roads

Create a re hazard along railways and next to power

substations


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Chapter 20Weed Management 397

plants are categorized as noxious with a

legal designation.

It should be noted that not all non-nativeplants are considered harmful or invasive.

Of the 4,000 species of exotic plants that

have become established in the United

States, only about 10 percent have been

identied as a threat. Examples of non-

native plants that are not invasive include

apple, potatoes and peonies.

While Alaska is fortunate to not have

many noxious weeds or invasive plants, we

need to be vigilant as more species become

introduced. Once weedy species are es-tablished in a region, it is virtually impos-

sible to remove them completely. Instead,

gardeners must live with them and work to

lessen their negative impacts. Fortunately,

new techniques offer a range of options to

achieve that goal.

Weed identication

If you can accurately identify a plant thatis causing problems in your landscape, you

have taken the rst step toward a good man-

agement plan. Fortunately, the list of weeds

that cause most problems is surprisingly

short. Often, a little time spent with a good

reference (see For more information) will

help you put a name to some of the chal-

lenging plants you encounter. You may nd

it helpful to give some of your worst weeds

specic nonsense names until you get a

more accurate identication. Then it is easyto mesh your new knowledge with your old

identication.

Weed books usually classify plants rst

into two groups and a miscellaneous cat-

egory:

Figure 1.Reed canarygrass

(Phalaris arundinacea), an

example of a monocot.

Figure 2.Garlic mustard

(Alliaria periolara), an example

of a dicot.

Figure 3.Field horsetail (Equisetum

arvense).


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398 Weed ManagementChapter 20

Monocots (Figure 1) include grasses,

lilies and the like.

Dicots (Figure 2) are broadleaf species.

Other weeds include mosses, horse-

tails and ferns (Figure 3).

Plants are further subdivided within each

major category byfamily for example,

the buckwheat, rose and borage families.

Within these family groupings are thegen-

era, and within the genera,species.

Scientic (botanical) names are in Latin

and list the genus name followed by the

species name. For example, common dan-

delion is known as Taraxacumofcinale.

The scientic name is used worldwide and

eliminates the confusion caused when the

same common name refers to several plants.

You dont have to be a botanical expert

to work through a weed identication book,

Common name Botanical name

Annual weeds

Grass weedsAnnual bluegrass Poa annuaCheatgrass Bromus tectorum

Broadleaf weedsCommon chickweed Stellaria mediaCommon groundsel Senecio vulgarisLambsquarters Chenopodium albumPineapple weed Matricaria matricarioidesProstrate knotweed Polygonum aviculareShepherds purse Capsella bursa-pastoris

Hempnettle GaleopsisbidaCorn spurry Spergula arvensis

Biennial weeds

Grass weeds

None

Broadleaf weedsCommon mullein Verbascum thapsusTansy ragwort Senecio jacobaeaWhite sweet clover Melilotusalba(M.ofcinalis)

Common name Botanical name

Perennial weeds

Grass weedsQuackgrass Elymus repensReed canarygrass Phalaris arundinaceaFoxtail barley Hordeum jubatum

Broadleaf weedsCanada thistle Cirsium arvenseCreeping buttercup Ranunculus repensCommon toadax Linaria vulgarisCommon plantain Plantago majorDandelion Taraxacumofcinale

Red sorrel Rumex acetosellaOrange hawkweed Hieracium aurantiacum

Other weedsHorsetail Equisetum arvense

Table 2.Weeds classied by life cycle.

Note: This list is not exhaustive. It does include many of the most common weeds that gardeners should recognize.

although it does help to learn some of the

key characteristics of the major families.

Once you know a plant, you can gather

important details about its life cycle and

how it spreads within the landscape or

garden. With practice, you can learn to dis-

tinguish weed seedlings from your planted

vegetables and owers.

Weed life cyclesMost gardens have a mix of annual and

perennial weeds, with a few biennials

thrown in to keep things interesting. When

you understand the life cycle of trouble-

some weeds, you can begin to make intel-

ligent decisions about control strategies.

Later sections of this chapter examine some

of the management strategies that can make

your gardening life easier.


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Table 2 lists some of our worst weeds by

common and botanical name. It also shows

their life cycles.

Annuals

Annuals go from seed to seed in less than

1 year, often in periods as short as 45 days.

Once they have thrown off their usually

prodigious quantities of seed, the plants

die. As you might suspect, there are lots

of annual weeds since their growth habits

parallel our agricultural cropping patterns.

Annual weeds can be found throughout

the gardening season, as germination andowering often start early and end late to

maximize reproduction.

Many annuals germinate from late fall

through early spring and go to seed in

spring and early summer. Some common

examples include little bittercress, several

mustards, downy brome, annual bluegrass,

shepherds-purse and chickweed (Figure 4).

Clearly, the cooler temperatures and lower

light intensity are not an obstacle to them. A

few annuals, especially common groundsel,

seem to germinate in ushes throughout the

year.

Most annuals get started in the spring and

summer and go to seed in the summer and

fall. Many of our most annoying weeds are

in this group, including chickweed, shep-

herds purse, pineapple weed, hempnettle

and lambsquarter. Most annuals germinate

quickly, ahead of many ower and veg-

etable seeds. Others do not germinate until

May or June and then grow aggressively

and complete their life cycle without delay.

While it often appears that weeds are con-

stantly germinating, in reality an established

seed bank is always ready to take advantage

of good growing conditions.

As discussed below, annuals have sophis-

ticated mechanisms to ensure good year-to-

year seed survival.

Biennials

Biennial species are less common. They

take more than 1 year but less than 2 years

to complete their life cycle. Most start from

seed in the spring and grow through the

summer, fall and the next spring. Bienneals

overwinter as rosettes. In the second sum-mer, they ower, set seed and die. Examples

include white sweet clover, bull thistle and

the livestock-poisoning weed tansy ragwort

(Figure 5).

Perennials

These weeds often are the most

difcult to manage. Woody species

generally go dormant in the winter

and begin growth in spring from

aboveground stems. Aboveground

parts ofherbaceous perennials may

die back, but their underground stor-

age organs survive the winter. Many

are deep rooted and survive summer

droughts as well. The life span of

perennials varies. They spread from

seed and often from roots, tubers,

bulbs, stolons and rhizomes as well.

Figure 4.Shepherds purse(Capsella bursa-pastoris), an

example of an annual weed.

Figure 5.Tansy ragwort(Senecio jacobaea), an

example of a biennial weed.

Figure 6.Field

bindweed (Convolvulus

arvensis), an example of

a perennial weed.


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Tilling perennial weeds often spreads them,

and mulches may have little impact. Weeds

in this group include blackberries, quack-grass, hawkweeds, Canada thistle, horsetail

and bindweed (Figure 6).

Weed competitionGardeners work to create landscapes that

provide food and are a source of beauty.

Weeds can disturb the appearance of a land-

scape and injure desirable plants. A veg-

etable garden is supposed to produce lots of

good vegetables. This can be difcult whenweeds gain the upper hand.

Most garden soils contain a lot more

weed seeds than the vegetable or ower

seeds you plant. Studies on commercial

vegetable farms show that viable weed seed

populations in the top 6 inches of soil aver-

age 900 per square foot, with some elds

having in excess of 7,000 viable seeds per

square foot!

What is worse, weed seeds germinate

very quickly when the ground is tilled. Twoweeks after planting, it may be very difcult

to nd your owers and vegetables amid

all the weeds. Some poorly tended gar-

dens may end up with 10 to 20 times more

weight in weeds than in desired plants.

Weeds compete directly with garden

plants for light, nutrients and water. A suc-

cessful weed grows aggressively to capture

whichever resource is in the shortest supply.Crop plants may end up stunted and unable

to produce a normal product. Paired test

plots of vegetables that were either weeded

or left untouched give some eye-opening

results (Table 3).

Research in England showed that a delay

in weeding could reduce nal yield by 3

percent per day, depending on the crop,

weed species and weed density. Some

plants (onions, for example) need at least

a certain leaf area to produce a marketablecrop, while others (e.g., potatoes and arti-

chokes) can send up new leaves and recover

to produce a near-normal yield if weed

competition is removed early enough.

Some characteristics that help weeds

compete include:

Aggressive vegetative growth from seeds

Abundant and rapid reproduction

Good means of dispersing seeds

Long-lived seeds and other plant parts

Wide adaptability to soil types andclimates

Ability to time germination to coincide

with favorable conditions

Ability to thrive in disturbed or bare soil

Rapid root and top growth

Weeds grow quickly to capture sunlight,

water, space and nutrients. They often can

alter their branching pattern, leaf size and

leaf orientation to win the resource battle.

Weed root growth can stunt crop roots in

moisture-short soil.

Sophisticated reproductive strategies

Gardeners know that weeds can produce

tremendous numbers of seeds. Weeds also

have ways to prevent all of their seeds from

germinating during years with less favor-

Table 3.Vegetable yields in weeded and

unweeded plots.*

Yield

Crop Weedy NonweedyCarrots 27.9 lb 503.3 lbBeets 45.9 lb 240.3 lbCabbage 129.1 lb 233.6 lbOnions 3.6 lb 67.7 lbPotatoes 52.7 lb 148.3 lb

*Plot sizes not specied, but weedy and nonweedy plots

were equal in size. With the exception of weed manage-

ment, both plots were treated the same.


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able weather. Seeds can be buried in undis-turbed soil for an amazing length of time

and still be able to germinate. Table 4 shows

weed seed production and survival rates.

Just so you dont give up, these survival

rates are under optimal conditions, and most

weed seeds are long gone before they reach

these ages. Nevertheless, as the saying goes,

1 year of weeds leads to 7 years of hoeing.

Chemical warfare between plants

Some weeds, especially some herbaceousperennials, produce compounds in their

leaves and/or roots that can kill or harm other

plants. This phenomenon is called allelopa-

thy. The compounds arent effective on all

the species with which a particular weed

might be competing, but they at least can

improve the weeds odds. Some weeds that

have allelopathic qualities include quack-

grass, Canada thistle, garlic mustard, purs-

lane and some hawkweeds. As if these plants

didnt have enough going for them already!

Invasive plantsInvasive plants are weeds that, in addition

to being a nuisance in the garden, degrade

the economic and ecological value of the

areas they invade. There are many terms

used in the discussion of invasive plants.

Native or indigenous plants

These plants have evolved with the eco-system within which they grow. Although

native, some species can dominate natural

areas, especially after a disturbance. In

Alaska, reweed (Epilobium angustifolium)

is a good example of a native plant that

dominates natural areas following distur-

bances, typically re.

Non-native or exotic plants

Non-native plants are plants that have been

introduced from one geographically isolatedarea to another with the help of a human

vector. Not all non-native plants are invasive,

but some have the potential to invade natural

areas and cause ecological damage.

Invasive plants

These aggressive plants eventually

dominate an area, displacing native species,

altering ecosystems and degrading the eco-

nomic value of the land. Spotted knapweed

(Centaurea stoebe) is an invasive plant thathas begun to establish in Alaska. In areas of

the Lower 48 where is has invaded, it has

been found to degrade wildlife habitat and

reduce the agricultural value of land.

Noxious weeds

Noxious weeds are plants that have been

legally designated by a federal, state or coun-

ty government as injurious to public health,

agriculture, recreation, wildlife or property.

There are 72 terrestrial, 19 aquatic/wetland

and 5 parasitic weeds listed on the current

Federal Noxious Weed List. In Alaska, there

are 12 prohibited noxious weeds (see Table

5).

Table 4.Weed seed production and seed

survival in soil.

No. of seeds Seed survivalWeed species per plant (years)Lambsquarter 72,450 40Purslane 52,300 40Dandelion 15,000 6Canada thistle (per stem) 680 21Crabgrass 25,000 3

Note: Seed survival means that some viable seed remains.

Generally, however, most seeds germinate or lose viability

within 3 to 10 years or less, depending on soil conditions. A

few, however, will hang on to aggravate future gardeners.


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Invasive Species Laws

Federal

In 1999, President Bill Clinton signed an

Executive Order (13112) directing Federal

agencies to use their authority to prevent

the introduction of invasive species, control

their spread and monitor and restore native

plant communities affected by invasive spe-

cies. The National Invasive Species Coun-cil, which includes 20 federal agencies,

developed a National Management Plant

(NMP), that outlines how federal agen-

cies will work with other groups to prevent

infestation, educate the public, manage

information and conduct research. The

NMP does not dene how invasive spe-

cies should be managed and controlled;

that responsibility is left to the states.

State

The Alaska Division of

Agriculture is authorized to

prevent the importation and

spread of pests that are injuri-

ous to the public interest and

to protect the agricultural

industry. Regulations

relating to noxious

weed control are found in Title 11 Chap-

ter 34 of the Alaska Administrative Code.

The rules for establishment of quarantines,inspections, noxious weeds lists and con-

trol measures are all provided for in current

state regulations.

Private and public landowner control of

noxious weed infestations is encouraged.

Public education rather than enforcement

is the preferred method for dealing with

noxious weed problems in Alaska.

Prevention

Due to the difculty involved with re-moving/eradicating invasive plants, pre-

venting their establishment is crucial, which

is why education and early detection are

key components of a successful invasive

plant management plan.

Learn how to identify invasive plants

of concern by visiting the Alaska Com-

mittee for Noxious and Invasive Plant

Management website (www.cnipm.org).

A plethora of resources are available on

this site, including photos of invasive

plants, a downloadable pocket guide of

invasive plants, an events calendar with

invasive plant identication workshops

and much more.

Reduce the opportunity for invasion by

not planting known invasive plants.

What can homeowners do?

Remove invasive plants on their property

and dispose of them properly. Cuttingsand/or whole plants should not be thrown

into the woods or compost, where they

may survive and spread. Instead, place

invasive plants into garbage bags, securely

tying the top to ensure that the plants do

not break through the bag.

Volunteer at weed removal projects in

your area.

Table 5.Prohibited noxious weeds in the state

of Alaska

Quackgrass (Agropyron repens)Whitetop and its varieties (Cardaria drabe,

C. pubescens, Lepidium latifolium).Russian knapweed (Centaurea repens)Canada thistle (Cirsium arvense)Field bindweed (Convolvulus arvensis)Leafy spurge (Euphorbia esula)Galensoga (Galensogaparviora)Hempnettle (Galeopsis tetrahit)Blue-owering lettuce (Lactuca puchella)Austrian eldcress (Rorippa austriaca)Horsenettle (Solanum carolinense)Perennial sowthistle (Sonchus arvensis)


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Report establishing invasive plant popu-

lations you observe at www.cnipm.org.

Invasive plants do not recognize propertyboundaries; successfully battling invasive

plants requires partnerships between public

and private landowners, government agen-

cies, industries, academia and non-govern-

ment organizations. As a Master Gardener

you can play a crucial role in limiting the

impact of invasive plants in Alaska by edu-

cating others about invasive plant issues and

controlling invasive plants on your property

(see chart below for a list of invasive orna-

mentals and alternatives).

Managing weedsWeeds are part of the dynamic and shift-

ing garden landscape. As discussed earlier,

many weeds are especially adapted to a

cultivated environment.

A gardener should plan for weeds. It is

possible to develop a fairly comprehensive

weed management strategy that takes intoaccount landscape objectives, weeds al-

ready present, available tools and personal

gardening philosophy.

In the broadest sense, weed management

strategies have three objectives:

To prevent the introduction of new weeds

To discourage weeds so they cant com-

pete with desired plants

To stop weeds from going to seed, thus

reducing, over time, the weed seed bur-

den in the soil

Prevention

While most garden soils already have

many weeds present, and other weed seeds

can be blown in, the alert gardener can take

some steps to prevent the introduction of

new weeds. First, examine any soil you plan

Invasive ornamentals Alternative

Dont Plant in Anywhere in AlaskaRampion bellower (Campanula rapunculoides) Peach-leaved bellower (Campanula persicifolia), Milky

bellower (Campanulalactiora)Creeping Charlie, Ground Ivy (Glechoma hederacea) Bugleweed (Ajuga reptans), Hermann's Pride (Lamiastrum

galeobdolon)Butter and Eggs, Toadax (Linaria vulgaris) Annual snapdragonsPurple loosestrife (Lythrum salicaria, L. virgatum) Blazing star (Liatris spicata), Salvia,Delphinium

or native lupinesOrnamental ribbongrass (Phalaris arundinaceae, Picta) Japanese silver grass (Miscanthus sinensis, Variegatus)Japanese knotweed (Polygonum cuspidatum),Bohemian knotweed (Polygonum x bohemicum)Common tansy (Tanacetum vulgare)Common mullein (Verbascum thapsis) Verbascum bombyciferum, V. chaxii

Oxeye daisy (Leucanthemum vulgare) Shasta daisy (Leucanthemum Xsupurbum), Arctic daisy(Dendranthema arcticum)Ornamental jewelweed, Washington orchid, Impatiens nolitangere

Policemans helmet (Impatiens glandulifera)

Don't Plant in Southeast AlaskaScotchbroom (Cytisus scoparius)St. John's wort (Hypericum perforatum)Foxglove (Digitalis purpurea)Creeping buttercup (Ranunculus repens)Sweet rocket (Hesperis matronalis)

Species list compiled from CES publication FGV-00146


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to bring to your lot. Pay particular attention

to the presence of horsetail, which will

cause years of agony if it nds yourlandscape.

Second, watch container plants

from nurseries, garden exchanges and

plant sales. They can be a source of several

tough plants, particularly chickweed, grass-

es, creeping charlie, buttercups

and even Canada thistle. Remove

any sign of these plants before

placing new trees, shrubs or

owers in your landscape.

Finally, cultivate a relation-ship with your neighbors that allows

for a coordinated approach to creep-

ing perennial weeds such as orange

hawkweed, creeping buttercup, eld bind-

weed, rampion bellower and oxeye daisy.

Cultural and mechanical control

The alert gardener looks for opportunities

to reduce the impact and spread of weeds.

This section will look at ways to achieve

those goals without the use of herbicides.

Gardening involves lots of small deci-

sions that can have a cumulative effect on

weed problems. Working from the premise

that weeds thrive in disturbed ground, you

can manipulate the landscape in ways that

reduce weed success. These techniques gen-

erally are called cultural control methods.

Several are discussed below.

Rotation

Rotating the vegetable garden can reduceweed infestations. Noncompetitive crops

such as carrots and onions should follow

more aggressive vegetables such as potatoes.

Fallowing(keeping part of the garden clean-

tilled or in a summer cover crop) can help

reduce weed problems for the next season.

Weeding

Cultivating soil to control weeds has been

practiced for thousands of years. Agriculturalyields improved tremendously when cultivat-

ing equipment became widely available.

Advantages: A germinating weed is very

vulnerable to root disturbance from hoe-

ing, hand pulling or rototilling. A secondary

benet of tilling is that it helps break up soil

crusts that may limit vegetable or ower

seedling growth.

Disadvantages: Mechanical weed re-

moval must be done early and often to be

effective. Also, tilling brings new weedseeds to the surface. In addition, excessive

cultivation destroys soil tilth and causes

compaction.

When tilling, take care not to damage

roots of desirable plants. For example, it

generally is unwise to hoe close to shallow-

rooted perennials, rhododendrons and

blueberries.

Bed planting

In this method of garden design,planted areas are designed with un-

tilled areas between them. Soil work-

ing is reduced and mulches are used

extensively. Flower or vegetable

crops often are planted close together

to compete against weeds. Shading

is effective in preventing some weed

species from germinating. Trans-

plants are used extensively.

Advantages: The reduction in

tillage reduces the number of weedseeds brought to the surface. Leav-

ing untilled areas (either in sod or

heavily mulched) reduces the area

that needs to be managed for weeds.

Disadvantages: Most

of the work (tillage and

weeding) must be done by

See Chapter 8,Vegetable

Gardening.


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hand. Some gardeners plant too closely and

stunt their crops much as weed competition

would have done.

Using transplants

Vegetables or owers can be started in a

cold frame or greenhouse and set out into

the freshly worked garden.

Advantages: The transplant has a head

start against germinating weed seeds and, if

managed well in the transplanting process,

should effectively compete against many

weeds. This can be one of the best weed

management strategies.Disadvantages: It takes time and plan-

ning to produce transplants, or money to

purchase them. Also, not all plants trans-

plant well.

Mulches

Mulching should be part of any land-

scape planting for the weed suppression it

provides. Mulch materials may be organic

(such as shredded bark or leaves, spruce

needles, compost or newspapers) or syn-

thetic (such as plastic or landscape fabrics).

They can be applied in the spring, summer

and fall for year-round weed control. Mulch

materials may be home generated or pur-

chased; however, purchased materials can

be somewhat expensive.

Advantages: If used effectively, mulches

reduce weed-pulling time signicantly.

When bare soil is covered, many weed

seeds either wont germinate or cant grow

through mulch. Also, when mulching is

used instead of hoeing or rototilling, fewer

weed seeds are brought to the surface. Done

correctly, mulching also can cool the soil

and help manage soil moisture.

Disadvantages: Mulches are not without

problems. If organic mulches are applied

too thickly they may produce acids that

can hurt plants and cool soil temperatures.

Around some deep-rooted trees

and shrubs, and on well-drained

and aerated soils, plastic mulchescan work well. However, around

shallow-rooted species such as

rhododendrons, the inability of

plastic mulches to breathe can

create a zone of lifeless soil that

often leads to root disease prob-

lems. In general, plastic mulch

is not the best choice for woody

landscape beds.

Synthetic mulches generally are covered

with an organic mulch to improve theirappearance. However, over time weed

seeds blow in and often germinate in the

organic material on top of the weed barrier,

thus reducing weed control. In addition,

both plastic and landscape fabrics can give

shelter to mice or voles and run the risk of

signicantly reducing soil temperatures.

Mulches for woody landscapes

Yearly applications of organic mulches

such as shredded plant materials go a longway toward reducing weed problems. As

the material decays, it improves soil tilth

and aeration around landscape plants. Fall

applications of mulch can reduce spring

weed problems.

Landscape fabrics are better than plastic

mulches in a woody landscape bed, since

water and air can pass through them. Al-

though more expensive, laminated fab-

rics such as Typar are porous, wick water

through the ber and prevent weed rootsfrom penetrating and becoming established.

Mulches for annual gardens

Plastic mulch does have a place in annual

vegetable and ower gardens. It usually is

used with drip irrigation and bed planting.

Make slits in the plastic and place trans-

plants next to the water source. Add fertil-

izer to the soil prior to transplanting or later


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through the irrigation system. Dis-

pose of the plastic after the grow-

ing season.Be sure to stretch the

plastic tightly over the soil

surface to get the benets

of soil warming as well as

weed control. Black plastic

is preferred for weed control

because it reduces light to the

soil, thus preventing weed

growth. Clear plastic mulch

warms the soil faster than black

plastic but does not control weeds.Infrared-transmitting mulch warms the

soil and helps control weeds.

Landscape fabrics can be used in the

same way, but since they allow moisture to

pass through, they dont require a drip sys-

tem. However, they are thicker than plastic

and tend to cool the soil instead of warm-

ing it. This can be a problem for crops that

require warm soils.

Organic mulchesOrganic mulches cool the soil but also

improve the soil as they break down. Ex-

amples of organic mulch include grass clip-

pings, leaves and straw that is not weedy.

Be careful with woodchip mulches and

sawdust; they contribute to nitrogen de-

ciency as they decompose. Woody mulches

work best in landscape situations, not in

vegetable gardens or ower gardens that

include annuals.

Water managementWeeds need water to germinate and grow.

When a garden is irrigated with sprinklers,

the entire area usually is watered and must

be weeded. When water is directed only to

desired plants, such as with drip irrigation,

much of the garden stays dry. In those drier

areas, weed problems are much reduced.

Drip systems often are combined with bed

planting and/or plastic mulches in vegetable

and annual ower plots.Advantages: Drip systems are excellent

tools for reducing weed growth. They are

fairly easy to set up and have come down in

cost.

Disadvantages: Time must be spent man-

aging clogged noozles and xing leaks.

Cover crops

Cover crops generally are

grown in the late summer on

annual vegetable and owerbeds. The cover crop may be a

grain, a legume or a combina-

tion of the two.

Advantages: The cover crop smothers

much late weed growth. Fast-growing crops

such as buckwheat are the most com-

petitive. Cover crops also capture excess

fertilizer and improve soil tilth and increase

organic matter when turned under. They

generally are very inexpensive. In warm

areas, some cover crops can be cut andleft on the surface as a weed-suppressing

mulch, and an early crop such as lettuce can

be transplanted through the residue. Crop

residue left on the soil surface enhances the

environment for benecial soil insects.

Disadvantages: The only major dis-

advantage is that gardeners in most areas

have to till in the cover crop before they

can plant a spring/summer garden. In a wet

spring, this can delay planting. Cover crops

are by no means 100 percent successful inweed suppression, so there may be some

weeds to contend with. Also, slugs may

prosper if the residue isnt turned under.

Chemical control (herbicides)

Herbicides are another tool for managing

weeds. The extent to which you choose to

use them depends on personal philosophy,

See Chapter 3,Soils and

Fertilizers.


13/20


garden objectives and the particular weed

problems in your garden. Herbicides may

have a place in your garden, but alwaysunderstand what you are trying to achieve,

what the alternatives are and what decisions

make the most sense given your situation.

All herbicides have detailed label instruc-

tions on mixing, application timing, weeds

controlled, plants around which they can be

used and other signicant issues concerning

their safe and effective use. It is crucial that

you read these instructions before you pur-

chase a product and follow them as you mix

and apply it. The label is the legal documentthat denes the use of a product and your

responsibilities as a consumer.

Herbicides control weeds by interfering

with critical plant functions, thus resulting

in the death of the plant. Not all herbicides

act in the same manner.

You need to understand some important

terms and concepts before purchasing and

applying herbicides.

Aselective herbicide controls certain

plants and not others. For example, mostlawn herbicides control broadleaf plants,

such as dandelions, without damaging

lawn grasses. A few products control some

(though not all) annual weed grasses with-

out harming turf. Other selective herbicides

affect germinating seeds and sprouting her-

baceous plants but not established woody

trees and shrubs. A handful of products

control grasses without damaging broadleaf

species.

If an herbicide is selective, the label willgive extensive information about which

plants it is safe to use around and which

weeds and plants it is likely to affect. In ad-

dition, there will be instructions on when to

use the herbicide to get the desired results

and avoid problems.

Nonselective herbicides potentially can

damage any type of plant. Some last a long

time (have some residual effect); others

do not. Again, the label will detail how the

herbicide acts.It is important to remember that some

products may be selective at certain rates

but lose that selectivity as the amount ap-

plied increases.

Most herbicides aresystemic, which

means they move from the point of initial

absorption to other parts of the plant. They

may be sprayed on the leaves and move to

the roots or vice versa. These products cir-

culate through the plant to have their effect.

Nonsystemic orcontactherbicides affectonly the part of the plant they touch. When

a contact product is sprayed on leaves, it

kills those leaves but does not travel to the

root system. Thus, contact herbicides may

control young annual or biennial weeds, but

established perennial weeds that lose leaves

due to a contact herbicide application will

resprout. Products containing soap, clove

oil and vinegar act as contact herbicides.

They are most effective on annual weeds

when they are young.Herbicides also are classied by the way

they move into plants. Many common prod-

ucts arefoliar-active, meaning they enter

through leaves. For example, glyphosate

(e.g., Roundup) must enter through green

tissue. If this product is applied to bare soil,

it has no effect on germinating seeds. Also,

glyphosate cannot be picked up by roots in

most soils. Soil-active products, on

the other hand, are ab-

sorbed through roots orthrough the growing

tips of germinating

seeds.

A number of

products are both

soil-active and foliar-

active, although one

See Chapter 22,Understanding

Pesticides.


14/20


absorption route usually is more important.

Again, the label will describe how to apply

the herbicide.Finally, it is important to know when

to use an herbicide relative to the growth

of the weeds and the crop. Apreemergent

herbicide is put on before weeds sprout.

The desirable plants (landscape trees and

shrubs, vegetables, owers, bulbs, etc.) may

or may not be present. Careful label reading

is important!

Apostemergentproduct is used after

weeds are up. Selectivity of postemergent

herbicides may depend on crop age, pres-ence of bark, rate applied or other factors.

Its important to understand application

timing as it relates to both weed and crop

growth. Again, read and follow the label.

Herbicide interactions

Herbicide results are inuenced by a

number of factors. A plant with a waxy leaf

(such as Japanese knotweed) or a hairy leaf

(such as hempnettle) may not absorb an her-

bicide as well as does a plant with a smoothleaf. Likewise, a plant with narrow, upright

leaves may be hard to cover adequately

with spray mixture. Sometimes, a weeds

growing point is protected under the soil

surface.

Environmental condi-

tions can affect an herbi-

cide, rendering it useless

against the target plant or

causing unintended dam-

age to nontarget plants.In general, as tempera-

ture increases, herbicides

work faster. However,

some herbicides can be-

come volatile (gaseous)

at certain temperatures.

Dichlobenil (Casoron) tends

to become volatile when temperatures

exceed 55F. While this particular chemi-

cal does little harm as it volatilizes, it also

doesnt provide much weed control.On the other hand, some formulations of

2,4-D and some of its relatives can come off

leaves shortly after application if tempera-

tures climb into the mid 80s. Since small

amounts of this herbicide can visibly distort

sensitive plants, high temperature combined

with a little wind can cause serious prob-

lems in your own garden or your neighbors

garden.

Wind drift by itself can be a problem when

using herbicides. As you pump a sprayer, thepressure increases, which in turn creates a

smaller spray droplet. The smaller the drop-

let, the more likely it is to become airborne

and move away from where you want it to

land. Again, this drift can cause unintended

consequences and neighborhood problems.

Do not spray if conditions do not permit, and

always use safety precautions when calculat-

ing wind speed to avoid spray drift.

Moisture in modest quantities is needed

to move soil-applied herbicides into thesoil. However, if a downpour occurs shortly

after the material is applied, the chemical

may wash downslope instead of entering

the soil. This runoff can damage lawns or

other plants in its path.

When you use foliar chemicals, there

must be enough time between application

and rain to allow the plant to absorb the

herbicide. Normally, 8 hours of dry weather

is enough. However, there are many stories

of gardeners misapplying herbicides andthen trying to wash them off only to nd

that the plant already was damaged. The

take-home message is that while 8 hours is

ideal, signicant plant damage can occur in

considerably less time.

Sometimes, herbicides work poorly in

dry weather. Plants that are drought stressed

conserve water by reducing transpiration


15/20


through leaves. In this condition, they are

less able to absorb herbicides. Several sys-

temic products have specic statements onthe label about their reduced effectiveness

when plants are moisture stressed.

Specic herbicides

Note:Trade-name products are men-

tioned as illustrations only. This does

not mean that the University of Alaska

Fairbanks Cooperative Extension Service

endorses these products or that they intend

to discriminate against products not men-

tioned.2,4-D

This systemic, foliar, post emergent her-

bicide affects broadleaves, especially her-

baceous annuals, biennials and perennials.

It is a common component in lawn products

since it doesnt damage established grass.

It is sold under numerous trade names and

often is combined with other closely related

chemicals (meco-prop and dichlorprop) to

broaden the spectrum of weed control.

Dicamba

Dicamba has the same effect on plants

as 2,4-D. It also is systemic, postemergent

and foliage applied. Dicamba is very ac-

tive against broadleaves, especially some

of the harder-to-control lawn weeds. It is

used in some lawn herbicide mixtures. It is

more dangerous to woody plants than 2,4-D

and can be absorbed through roots. It is not

uncommon to see landscape tree and shrub

injury when dicamba is used extensively onlawns surrounded by woody plants. Di-

camba also is sold under many trade names.

Normally, it is mixed with other herbicides

at very low rates.

Dichlobenil

A systemic, preemergent, soil-applied

herbicide, dichlobenil is available as gran-

ules. It controls annual, biennial and peren-

nial broadleaves and grasses but generally

does not damage woody plants. It is used in

landscape beds where there are no under-plantings of herbaceous owers or bulbs.

Dichlobenil is best used before weeds

emerge. It needs light rain and cold tem-

peratures to move it into the soil. If tem-

peratures are too warm when it is applied,

it becomes volatile and does not provide

much weed control. If it rains too much just

after it is applied, it may travel downslope

and damage nontarget plants. Generally, it

is best applied as a pre-emergent. Dichlo-

benil persists in soil for 6 to 9 months ormore, depending on rainfall, soil conditions

and application rate. Its use may limit later

planting options. It is sold under the trade

names Casoron and Noxall for home use.

Glyphosate

Glyphosate is a systemic, nonselective,

foliar-applied herbicide. It potentially af-

fects any plant with which it comes in con-

tact. Glyphosate is absorbed through leaves,

green bark (usually a very young tree orshrub) or freshly cut stems. The chemistry

of glyphosate is such that it becomes tied up

on soil particles. There is little chance that

it can be picked up by roots unless they are

exposed in some manner.

Glyphosate tends to follow the ow of

sugars in plants. If it is applied to an actively

growing woody plant, the chemical tends to

move to the new leaves (where the sugars are

going) and stunt the terminal growth. In that

circumstance, it may not move to the rootsystem in sufcient quantities, and the plant

survives. Thus, it is best used on perennial

plants as they begin owering or in the fall

as they start storing sugars for winter.

Glyphosate can be used on some grasses

almost any time they are green and on annu-

als when they are actively growing. It works

more quickly (7 to 10 days) in warm weath-


16/20


er. It also can be effective in colder weather,

although the results might not be evident for

a month or more. It is sold under the nameRoundup and several other brands.

Triclopyr

This product is systemic and predomi-

nately foliage absorbed. In some cases, it

can be absorbed through bark. Triclopyr is

active against broadleaf plants, especially

woody species. It has no effect on estab-

lished grass. It is mixed with 2,4-D in the

product Crossbow and is sold alone in other

brush killer and lawn formulations. Read

the label carefully to see whether the formu-

lation is an ester or an amine. Avoid apply-

ing esters in hot weather to reduce potential

volatility and drift.

Oryzalin and trifuralin

These preemergent compounds act on

germinating seeds. There is a fairly com-

plex list of weeds (some, but not all, grasses

and broadleaves) that they control and

plants around which they can be used. They

are most effective on annual weeds.It is very important to read the labels

carefully when purchasing and using these

products. They differ in their need for

incorporation into the soil, the weeds they

control and the plants around which they

can be used. Triuralin is sold as Preen,

Trean and several other names. Oryzalin is

marketed as Suran and other prod-

uct names. Plants sensitive to these

herbicides cannot be planted until at

least a year after use.

Fluazifop

A fairly new herbicide, uazifop

controls many grass species but does

not harm most broadleaf plants. It is

applied after grasses emerge and are

rapidly growing. It takes 7 to 14 days

or more to see results. It is sold under

the name of Ortho Grass-B-Gone and many

other brand names.

Sethoxydim

Sethoxydim controls many grass species

but does not harm most broadleaf plants.

It is applied after grass has emerged and is

growing rapidly. The most common trade

name is Poast.

Corn gluten meal

Corn gluten meal (CGM) is a natural by-

product of the corn milling process. It has

been used as livestock feed for many years.

CGM has preemergent herbicidal activity;it inhibits root growth of susceptible germi-

nating seeds. It affects a variety of annual

and perennial broadleaf and grass seedlings.

It is not effective on established perennial

weeds. CGM has been used on existing turf

and around established plants, transplants

and deeply seeded vegetables in vegetable

and ower gardens. It usually is surface

applied and lightly worked in. If worked

in too deeply, it might damage nontarget

plants. Read the label cautions and use rec-ommended rates and application techniques.

Weed management for specicsituations

Lawns

Choose grass species suited

for your area. A vigorous lawn

reduces weed invasion. Good

fertility, watering, mowing

and aeration programs will keep your turf

in top condition. Plan an annual program of

overseeding weak areas, especially those in

some shade.

If the grass you plant can tolerate a mow-

ing height of 2 inches or more, the shade

cast by the grass will inhibit many broadleaf

weeds. Use a fertilizer low in phosphorus

to avoid stimulating clover (assuming you

See Chapter 14,Lawns.


17/20


dont want clover in your lawn). Some

broadleaf weeds can be managed by hand

pulling, but clover and dandelions mayprove difcult to control this way.

Weedy perennial grasses such as quack-

grass can be very invasive. There are few

options short of complete renovation once

these plants get established. Some garden-

ers spot spray the patches with glyphosate

and then overseed them.

Annual bluegrass is common in lawns.

Some herbicides are available to help man-

age this species.

Moss grows where grass is weak. Usu-ally, lawns are weakest in shady areas and

where drainage is a problem. Control moss

either with products containing iron or by

dethatching. In shady areas, overseed lawns

with red fescue to maintain a viable turf.

Many homeowners use broadleaf herbi-

cides, either alone or in combination with

fertilizer (weed-and-feed). To reduce

the amount of herbicide used, it generally

is much better to spot spray weedy areas

rather than to apply herbicides over theentire lawn each time you fertilize.

If young children or pets use the lawn,

be very cautious in the use of herbicides!

Renovation

Lawn renovation often is done to reestab-

lish turf grasses where weedy grass species

have taken over. The weedy species must be

killed (especially perennials) before a new

lawn is planted.

A single rototilling generally spreadsrather than controls problem grasses. Re-

peated tillage over 3 to 4 weeks can give

acceptable control in dry weather.

Some homeowners use glyphosate to

kill an existing weedy lawn. They then

dethatch, aerate and overseed. Rototill-

ing is not used unless the lawn needs to be

reshaped.

With good temperatures and water, a

new lawn can be up and growing in 3 to 4

weeks.

Woody landscape areas

Weeds in landscape beds can be man-

aged with a mix of techniques that include

mulching, water placement, competitive

planting, hand pulling and herbicides ap-

plied as both spot and broadcast treatments.

Mulches should be your rst line of

defense against weeds. They reduce the

germination of weed seeds and protect the

soil. Organic mulches such as bark probablyare the most effective and cause the least

problems. Hoeing in mulch is very easy

and disrupts most annual weeds. Landscape

fabrics are ne for use on pathways, but

in landscape plantings are only useful for

deterring weeds for a few years. Do not use

black plastic around trees and shrubs. Pe-

rennial weeds generally are not deterred by

organic mulches and may defeat landscape

fabrics as well.

Drip irrigation places water around desir-able plants but doesnt water everything.

The potential for weed growth in the dry

areas is greatly reduced.

When bare areas are planted with robust

plants, weeds struggle to compete. Complex

landscapes that cover most of the ground

generally have fewer weed problems as the

plants get established. This can be a great

reason to buy more plants! It does help to

have a plan and to know which species work

best in this role. In some cases, competitiveplantings may limit your herbicide choices.

The herbicides most commonly used in

woody landscapes are oryzalin, triuralin

and spot application of glyphosate. Be sure

you understand how these products work to

avoid damaging desirable plants.

See Chapter 11,Woody Landscap

Plants.


18/20


Annual ower and vegetable gardens

Annual vegetable and ower gardens can

be weed nightmares. Working the garden inthe spring offers an opening for weed seeds.

Their aggressive growth can quickly domi-

nate a garden.

Weeds are best managed in ower and

vegetable gardens by a combination of hoe-

ing, hand pulling, vigorous plants (includ-

ing extensive use of transplants) that shade

the ground as they mature, drip irrigation,

mulches and relentless attention that keeps

weeds from going to seed.

Some gardeners use glyphosate beforethe rst spring cultivation to control persis-

tent perennial weeds, especially quackgrass.

Triuralin (Preen or Trean) is labeled for

use aroundsome (but not all!) owers and

vegetables. Preen with corn gluten meal as

an active ingredient is also available as a

preemergent. Oryzalin has some home-gar-

den labels for owers. Read the labels very

carefully and follow instructions if you use

these products.

For more information

UAF Cooperative Extensionpublications

Dont Plant a Problem, FGV-00146.

Lawn Establishment, HGA-00036.

Lawn Maintenance, HGA-00334.

Lawn Maintenance and Pesticides, HGA-

00236.ManagingAlaskanLawns:WeedIdentica-

tion, Prevention and Control,

HGA-00239.

Moss Control in Lawns, HGA-00133.

NoxiousWeedIdenticationandPreven-

tion, FGV-00144.

Reducing the Spread of Non-Native Inva-

sive Plants in Alaska: Voluntary Codes

of Conduct for the Gardening Public,FGV-00142.

Reducing the Spread of Non-Native Inva-

sive Plants in Alaska: Voluntary Codes

of Conduct for Nursery Professionals,

FGV-00147.

Other references

Invasive Thistles in Alaska, USDA Forest

Service Leaet R-10-TP-132.

Japanese Knotweed, USDA Forest Service

Leaet R-10-TP-133.Orange Hawkweed, USDA Forest Service

Leaet R-10-TP-129.

PacicNorthwestWeedManagement

Handbook,WEED.

Perennial Sowthistle, USDA Forest Service

Leaet R-10-PR-17.

PlantsofthePacicNorthwestCoast, Pojar

& Mackinnon; Lone Pine Press.

Plants of the Western Boreal Forest, John-

son et al. Lone Pine Press.

Rice, P.M., J.C. Tonye, D.J. Bedunah and

C.E. Carlson. 1997. Plant community

diversity and growth form responses to

herbicide control of Centaurea macu-

losa.Journal of Applied Ecology: 34:

1397-1412.

Selected Invasive Plants of Alaska, USDA

Forest Service Leaet R10-TP-130B.

Spotted Knapweed, USDA Forest Service

Leaet R-10-TP-125.

Weeds of the Northern U.S. and Canada,

Royer & Dickinson; Lone Pine Press.

Weeds of the West, Western Society of Weed

Science.

See Chapter 8,Vegetable

Gardening, and

See Chapter 9,Annual and

Perennial Flowers.


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Documents

20 Weed Management