HorticultureUnit 5

GROWTH STIMULANTS, RETARDANTS AND ROOTING

HORMONES

Growth Regulating Substances

Called hormones: organic chemicals which act and interact to affect growth rate

Auxins: accelerate growth by stimulating cell enlargement

Gibberellins: stimulate growth in stem and leaf by cell elongation

Cytokinins: stimulate cell division (only works with auxins present)

Inhibitors: inhibit seed germination, stem elongation and hasten ripening of fruit.

Growth Hormones

Organic chemicals principally produced by actively growing plant tissue such as short tips and young leaves.

They move throughout the plant and can be found in many parts.

They react with one another.

Apical Dominance

The terminal bud secrets chemicals which inhibit or prevent growth of lateral buds.

This causes the plant to grow tall and not send out side branches.

Seems to be a genetic program directing the plant to grow above competing plants.

Once the plant reaches flowering age, the terminal bud becomes a flower bud and the chemicals are no longer secreted.

Pinching the terminal bud has the same effect and side branching occurs sooner.

Major Discoveries

Chemical and natural stimulants that cause plants to grow taller or faster

Chemical retardants that cause plants to grow slower

Hormones that cause plants to root faster

Dwarfing rootstock for fruit trees

Chemical Stimulants

Enables plants to grow taller. Most common is gibberellic acid Gibberellic acid causes the stems

of plants to stretch out The nodes (the joints at which

bulbs, leaves and branches) are further apart

Natural Stimulants

Chemical known as alcohol Triacontanol

Found in alfalfa and has been known to nursery men for 30 years.

Stimulation is brought about by mulching with alfalfa hay or by watering plants with an alfalfa tea

Chemical Retardants

Used to retard the growth of plants making them shorter and more compact and therefore more attractive.

One of the newest applications for growth retardants is on lawns, a chemical called “Limit” is used new and it restricts grass growth for 6 to 8 weeks.

A new plant regulator called “Sumagic” reduces the height of plants by inhibiting production of the hormone Gibberellic acid. (which cause stems to elongate)

Rooting Hormones

When propagating plants from cuttings, it is very important that they root as quickly as possible

Indoleacetic acid (IAA) is a natural plant hormone that causes roots to form

Chemical hormones, indolebutyric acid (IBA) or naphaleneacetic acid (NAA) are the most common used today.

Rooting Hormones

Rooting hormones are either mixed with talc and used as powders or mixed with liquid and used as a wet dip

All rooting hormones should contain a fungicide.

Dwarfing Rootstock

As the cost of labor has increased, growers are looking for smaller trees.

Discovered trees from certain types of roots, do not grow as large and bear fruit earlier.

A complete series of rootstock known as malling rootstock was developed.

Commercially Used Plant Growth Regulators

Trade Name Effect Used onA-Rest Shortens stem length Mums, l ily, poinsettia

B-Nine Shortens stem length Mum, hydrangea, petunia, azalea ,

poinsettia

Cycocel Shortens stem on poinsettia,

induces heavier flower bud set on

azalea

Poinsettia , aza lea, geranium

Phosfon Shortens stem length L ily, mum

Florel Induces basal breaks on roses ,

retards growth on poinsettia ,

inc reases number of branches on

carnation, speeds ripening of

tomatoes, assists in faster rooting

Rose, poinsett ia, carnation ,

tomato, cuttings

Bonzi Shortens stem length Poinsettia , geranium, petunia,

mum, impatiens , coleus , pansy,

snapdragon, celosia

Sumagic Shortens stem length Poinsettia , mum, ce losia , coleus ,marigold, periw inkle, petunia,

ageratum, dahlia

Student Activities

Complete self-evaluation at the end of this unit.

Using nursery catalogs and books, make a list of available dwarf fruit trees and details.