Future Growers

Module 10: Protected cropping

Briefing paper

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With thanks to Gareth Davies and Margi Lennartsson: Organic Vegetable Production – A Complete Guide.

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OVERVIEW

1. Introduction and principles 2. Standards 3. Types of protected cropping

crop covers

cloches / low tunnels

polytunnels

Spanish tunnels

Greenhouse cropping 4. Cropping

crop choice

soil fertility and management

irrigation

climate control

pests and diseases

weed control 5. Other considerations

planning permission

tunnel covers

recycling of plastics

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Introduction and principles Protected cropping is an important part of UK organic production - in 2003-04 it accounted for about 26% of the wholesale value of all UK-produced organic vegetables.

The facility to extend the season at either end is vital for most organic growers. It can improve cash flow, maintain continuity, command higher prices for crops, increase the range of crops that can be grown and help minimise the quantities of bought in or imported produce. These factors are especially important for growers selling directly to the public through box schemes, farmers markets and farm shops.

In addition the workload can be spread over a longer period and work can be found inside when inclement weather does not permit activity in the field.

Greenhouses and polytunnels can also be useful for plant raising, drying crops e.g. onions and garlic, chitting potatoes and on mixed holdings for lambing sheep and raising ducks and turkeys.

There are some critics of polytunnels and their visual impact on the countryside, though this is normally more of a problem for larger growers with acres of tunnels.

1. Standards

Following a process of consultation and development with growers, the Soil Association has published a new set of specific standards for protected cropping, which will be applied from 1 April 2012.

New standards were needed as the original organic standards applied in the UK and in Europe were designed primarily for field-scale outdoor growing, but producing crops under a protective structure differs in a number of ways. For example, the temperature is higher and can be further increased through heating; pests can be controlled more effectively with predators; and, as it is expensive to construct a glasshouse, growers tend to focus on producing a smaller range of high value salad crops.

The standards address the key issues around fertility, crop rotation, in-soil production, energy and water and aim to encourage best practice among growers. See the new standards for details.

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2. Types of protected cropping Although most people think of tunnels or glass there are a range of options for protecting crops from the elements in order to increase range, season and health of crops.

Crop covers Crop covers such as fleece have become popular with growers as a cost effective way of extending the growing season by protecting sensitive crops from the frost and raising the temperature of the growing environment around the crop. They also warm the soil thereby encouraging biological activity and the mineralization of nitrogen.

Fleece

Fleece is a lightweight non-woven porous material that is used as a “floating” mulch.

It is supported directly by the crop and has the advantage that it can also provide a barrier against insect pests e.g. aphids as well as larger pests like birds.

The disadvantage of fleece is that it is not transparent and so monitoring the crop growing underneath is not easy. In addition to which there is a tendency to forget the crop under the fleece.

Weed growth is also encouraged under fleece and removing and replacing covers to allow physical weed control is labour intensive.

Fleece is better at extending the growing season in spring rather than autumn as it encourages soft growth which can reduce winter hardiness of covered crops.

There is a risk of crops overheating under fleece, and reduced airflow is sometimes a problem with resulting disease issues.

It can be damaged relatively easily by deer, birds and other animals.

Realistically it can only be practically used for one or two seasons especially if pest exclusion is important.

If stored for re-use it should be kept out of the way of mice.

Agri meshes

Marketed under a range of names such as “Eviromesh” or “Agrimesh”. This is a tougher mesh which has the benefits of fleece without many of its downsides. It is much more expensive but will last for around 10 years so per year cost is probably less.

In particular air movement, and water reaching the plant, are better than under fleece.

Mesh netting covers are useful for pest control but have little effect on crop maturity or frost production.

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Cloches / Low Tunnels

The use of fleece and Spanish tunnels has meant that cloches and low tunnels are mostly redundant, though some growers on a very small scale still have a place for them within their system.

Use of hoops over beds can be useful where crops have a tender, exposed growing point such as peppers and would be damaged by wind abrasion from fleece.

They offer similar benefits to fleece but are more labour intensive.

Polytunnels

These are metal hoped structures covered in plastic, and can be single-span (one hoop) or multi-span (a large area open in the middle covered by a series of hoops). It is much easier to use machinery in the multi spans.

Generally speaking wider tunnels are better although excessive lengths should be avoided as hotspots can form in the middle of the tunnel if the sides are not vented.

Straight sides are an advantage as they enable a greater proportion of the growing area to be used and do not restrict the growth of low growing crops to the outside beds.

You should consider the best location for your polytunnel, usually a sheltered site but not excessively so as this can restrict air movement.

Alignment from north to south usually works best to avoid shading effects of taller crops.

A gentle slope (sited up and down rather than along the slope) will really help air movement.

In a mixed holding it is often useful to locate the tunnels near to the farmhouse for ease of management, access to water and electricity and for security.

Other things to consider when placing the polytunnel are: - avoidance of frost pockets - prior fertility building - good soil structure - absence of perennial weeds

It is best to get professional advice before erecting a polytunnel but you should make sure that the foundations and tunnel structures are secure and that the plastic is firmly stretched over the tunnel hoops. The tighter the fit the longer the cover will last.

Single-span tunnels can be used where you want to isolate crop-groups from each other to manage them separately.

On larger tunnels it is essential to install side venting, without it humidity and therefore disease will rise to unacceptable levels.

Putting up your own tunnel can be done and will save hundreds (or even thousands) of pounds. However it is a difficult and skilled job and getting it done properly could add years to the life of the polythene. If you are planning to do it yourself, try to help a skilled person put one up first so that you have a good idea of what is involved.

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Polythene versus glass

Polythene Glass

Cheaper – though plastic will need replacing every 5-10 years

Expensive to set up and maintain – though there are some good second hand prices at the moment

Heats up and cools down more quickly

Better heat retention than polytunnels, meaning they are also more useful and efficient for plant raising

Overall temperatures reached are not as high as glass and on clear frosty nights can be as low as those outside

Better light penetration through clean glass means better growth,

Risk of higher humidity, though modern polythenes have improved the conditions

Can extend the season further than is possible with polytunnels, meaning longer growing period for crops such as tomatoes, peppers and cucumbers

Potential for broken glass can create health and safety issues.

Vandalism risk – if the side is slashed you may need to replace the whole plastic cover

Vandalism risk – you will probably need to replace a few panes

Best not to use flame weeders near plastic

Can use flame weeders even right up to the edge

Spanish Tunnels

They are low cost, moveable and have multiple bays and have been use primarily for extending the strawberry growing season.

They are also suited to high value vegetable crops.

They are similar to polytunnels but the structure is usually much lighter weight and is not permanently fixed with foundations. They can also be opened at the sides which improves airflow.

When used for summer crops you can pull the roof aside to allow natural watering, ventilation (and even growing a green manure).

The improved airflow reduces humidity and potential disease problems and helps to create a more stable climate.

As they are temporary structures they do not generally need planning permission to erect.

They can also be rotated around the farm creating less need for rotation within the tunnel itself.

Location requirements are similar to polytunnels but they should be positioned lengthways to the prevailing wind.

Good drainage is also important so avoid wet areas as rainwater can be concentrated into the leg rows. In severe weather this can weaken the structure and causing it to collapse.

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Siting of Spanish tunnels is crucial, particularly on sloping ground. Run off can cause serious soil erosion and flooding.

Greenhouse cropping

High investment costs and the length of the conversion period are the main obstacle for conversion to organic glasshouse production as it is difficult to justify the loss of income during the conversion period.

Another option would be to erect a new glasshouse onto land that has already been converted and has had a period of fertility building.

There are a few examples of moveable greenhouses (on runners) that add flexibility and value to use of structure.

Solar greenhouse are designed to collect and retain solar energy and are particularly good for propagation They differ from conventional greenhouses in that the glazing is orientated for maximum solar heat gain during winter They have a steeply pitched north roof with the north facing wall is well insulated to prevent heat loss and covered or painted with reflective material.

Greenhouses covered with bubble plastic are a less expensive option than traditional greenhouses.

3. Cropping Rotation requirements of the organic standards are less rigid for protected cropping systems than for arable and outdoor horticultural crop rotations (due to the smaller number of families to which protected crops belong) but the principles are the same. Crop Choice

There are three main periods (with some overlap) for protected cash crops - spring - summer - autumn/winter

The spring to early summer period can be profitable as produce from the field can be in short supply over the hungry gap.

Early carrots, beetroot, radishes and turnips for bunching can also be good crops.

Other options for this period include sugar snap or mangetout peas, lettuce and other salads.

Climbing French beans, early celery and courgettes traverse the spring period into early summer and can be cropped until the outdoor harvest is in full swing.

During the late spring / early summer the tender, frost sensitive crops can be planted.

It is also possible to source a wide range of leafy salad vegetables which can be drilled or transplanted in modules in the autumn after the summer crops are removed. They can then be harvested from November onwards

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The main problem is that many of the salad crops are brassicas and thus care needs to be taken in planning an effective rotation to avoid disease.

When designing a rotation for a polytunnel it will be necessary to plant to spread the workload over the cropping calendar and crops should be chosen with this in mind.

Soil Fertility and Management

Manures and composts are likely to be the principal sources of soil fertility.

Manures should preferably be sourced from within the farm but if sourced conventionally will need to be composted for three months or stacked for six.

Composted manures mean that the requirement for interval between application and harvest is reduced from three months to two.

If space allows it is useful to have an area for composting reasonably close to the tunnels as they can produce plenty of organic material for composting.

In contrast with field production there is no upper limit for nitrogen application to the land area and manures and composts can be applied at any time of year.

Supplementary feeding may be needed for nutrient demanding crops such as tomatoes – there are fairly strict organic regulations surrounding liquid feeding, check the standards.

Regular soil analysis is recommended to monitor soil fertility levels and to ensure that there is no build-up of mineral salt levels

Fertility needs are very high in protected cropping situations and leaving fertility building periods is more difficult to justify financially – though the long term sustainability of your system is vital.

Irrigation

High temperatures and rapid crop growth under protection means that demand for water can be high.

Watering regularly is important so that crops do not become stressed.

Over-watering or uneven watering also creates a danger that can lead to problems e.g. splitting in tomatoes.

The two main choices are between overhead irrigation using sprinklers or drip irrigation. Sometimes a combination of the two can be used.

Sprinklers are useful for drilled crops but overhead irrigation can raise humidity levels which may lead to fungal problems.

Water marks on foliage or fruit can also reduce crop quality.

Drip (or trickle) irrigation is more efficient in the use of water as the moisture is targeted to the root zone. This is particularly useful when used under plastic mulch.

Drip irrigation needs monitoring for blockages to the holes and damage to the hoses for instance from machinery or rodents.

Water can be collected easily from glasshouses; it can also be harvested from tunnels though this is harder on single span structures.

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Automatic irrigation systems can be useful (on very large sites they are vital) but must still be checked regularly to ensure you are giving the right level of irrigation. Plants and soil should still be monitored on a daily basis.

Climate control

Temperature and humidity control is primarily by ventilation through opening end doors or panels if present.

Some crops have higher humidity requirements than others.

During the spring and autumn it is important to shut the tunnel at night to reduce heat loss.

Temperatures can rise rapidly on a spring day inside the tunnels and crops can easily be overheated if tunnels are left closed by mistake.

In glasshouses, more sophisticated computer-controlled ventilation systems are often used

Pests and diseases

High intensity cropping in warm and humid environments can often lead to increased pest and disease pressure.

In greenhouses, shorter rotations and a tendency towards mono-cropping means that regular crop monitoring is essential and training in pest and predator identification may be needed.

Pest control in protected cropping is possible using biological controls as higher temperatures and a more closed environment allow the pest predators and parasites to function well.

Regular monitoring of crops is essential as it is important that predators are introduced early before pest populations are too high.

Natural predators can also be encouraged using companion planting around the doors of the tunnels or amongst the crops.

Ducks and chickens can be used in polytunnels or glasshouse between crops to clean up slugs and snails.

Good ventilation and minimisation of overhead irrigation is necessary to avoid high humidity which encourages fungal and bacterial diseases.

Choice of polytunnel film can help by reducing drip onto plants and changing the light spectrum may reduce botrytis.

Glasshouse and polytunnel hygiene is important. Power-washing of glass and the use of biodegradable soaps to clean fixed structures is acceptable.

Weeds can act as over wintering sites for pests in both polytunnels and glasshouses.

Weed control

Weeds can grow very quickly under protected conditions and careful monitoring and management is essential.

The higher temperature in protected cropping means weeds are more likely go to seed quickly.

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Mulching with various materials is a useful method of weed control and the easiest method to use in polytunnels is black plastic or mypex mulch in combination with drip irrigation.

Biodegradable plastic mulches of various types are now available.

Mechanical weeding using inter-row equipment may be possible in glasshouses and multi-span tunnels – if mulching has not been used, otherwise hand hoeing and hand weeding are the only option.

Heavy mulching of the area between adjacent tunnels can help prevent invasion of weed roots from this area.

Stale seedbeds are possible for drilled crops if time permits.

Avoid the use of flame weeding in plastic polytunnels! 4. Other considerations Planning permission

It is important to check with your local planning authority as to whether planning permission is needed for protected cropping as requirements can vary between areas.

Normally planning permission is required when the structure is permanent on non-agricultural land.

In certain circumstances permanent polytunnels on agricultural land may require the prior notification of the local planning authority.

It is essential to take professional advice on these matters before investing any money.

Tunnel covers

There are a wide range of tunnel covers available to growers.

The most basic ones last for about four years and many much longer than that.

Many offer special properties such as improved heat retention at night, anti drip films etc.

Standard polythene is the cheapest option and is suitable mainly for growing frost-hardy crops and/or growing summer vegetable crops where condensation is not a problem.

Thermal / anti fog films give improved heat retention due to an infrared additive in the polythene and are suitable where tunnels need to be heated at night e.g. for plant raising and for over winter vegetable production.

Anti-fog films have additives to control the condensation of water on the film.

High light transmission films are designed to make more light available to plants. They can raise tunnel temperatures and also cool the tunnel environment later in the season.

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Anti-grey mould films block the entire ultra violet spectrum allowing enhanced blue light transmission. It is claimed that these properties inhibit the growth of botrytis by suppressing spore production.

Recycling of plastics

Disposal of horticultural and agricultural plastics is a big issue and burning of plastic waste is prohibited within organic standards.

There are a number of recycling schemes around the country that operate collection services for a subscription cost and/or collection fee.

Further resources

Tunnel construction http://www.murcrusto.eclipse.co.uk/polytunnel2/page_01.html