Polytunnel cropping
Protected cropping in polytunnels is an increasingly important part of organic vegetable production because:
- Growth can be rapid during the summer with the potential for several crops in the season
- It extends the growing season by allowing extended season and over-winter production. This improves cash flow during the year, also enhanced by increased returns from higher value out of season crops
- It allows more exotic, higher value niche crops to be produced, leading to higher returns
- Importation of produce for box schemes and farm shops can be reduced or avoided
Production principles are just the same as for field production, and certification bodies will require you to show in particular:·
- How crops will be supplied with nutrients
- A suitable crop rotation and cropping sequence in the tunnels
Husbandry and management requirements of protected cropping
Protected cropping is a specialised operation and because of the modified climate in the house will require particular care and attention in a number of areas.
Temperature control
When the sun comes out the temperature in a polytunnel can rise very rapidly to 40ºC or more.
Very few crops can tolerate such temperatures for very long, so it is important that the temperature is brought down to between 21ºC and 25ºC for most crops.
Frost protection
During early and late season production there is always a risk of a sudden, unexpected sharp frost. For most crops the tunnel will give adequate protection, but for some crops such as cucumbers frost is lethal. So there may be a need for providing supplemental heat for frost protection purposes.
Humidity control
High internal tunnel temperatures and frequent watering can lead to high humidity in tunnels. A few crops such as cucumbers thrive in high humidity, but for many others the damp conditions lead to an increased risk of fungal and bacterial disease infection, which can develop and spread very rapidly in the warmth.
Need for water
Because of protection from natural rainfall, high temperatures and rapid crop growth, the need for water in a tunnel must be monitored regularly. Crops should be supplied with water as required.
It is highly important that crops are not allowed to suffer from drought stress due to water shortages as plants can be easily damaged and yield and quality dramatically reduced.
Similarly, over-watering can also cause serious problems with some crops, notably tomatoes.
Monitoring temperature and humidity
Regular and frequent monitoring of the climate in the tunnel must be carried out. If weather conditions change during the day, the climate should be checked to see if any action is required. Do not forget to do this seven days a week.
Ventilation
Ventilation of tunnels by opening end doors, raising end panels or by opening vents in the polythene cover is the key to controlling both temperature and humidity. This can be automated, at a cost. For most organic growers regular monitoring of the climate and manually adjusting the ventilation level suffices.
If ventilation is provided during the day, always remember to close up the tunnels in the evening, especially during spring and autumn.
Choice of a tunnel and associated equipment
Polytunnels come in a range of sizes, shapes, specifications and qualities. Which to select will depend on personal preferences, growing requirements and finance available.
Types and sizes of tunnel
For commercial purposes a tunnel of not less than 18m long by 6.5m wide makes sense.
Whilst fewer, larger tunnels are acceptable, several tunnels of this size do give flexibility in terms of climate control for specific crops, as well as for crop rotations. Tunnels narrower than this have less head room and using machinery in them is more difficult, particularly along the sloping sides.
If you require a large area of tunnel space, tunnels of approximately 40m x 9m may be more suitable.
Some modern tunnels have sides that are more upright than on those previously available. These give excellent access to the ground right across the width of the tunnel.
Some modern tunnels have polythene arranged in strips across the tunnel rather than a single sheet. These make installing and replacing the polythene relatively easy, and ventilation can be provided along the tunnel.
Tunnel costs
Tunnel costs vary considerably, depending mainly on the quality and specification. Typical costs of commercial sized tunnels (excluding erection costs) are from £6 to £18 per m2. In addition, erection costs also vary considerably depending on who supplies the labour.
The cost is mainly determined by the size (and thus strength) of the supporting tubes, the number of them along the length of the tunnel, the amount of cross-bracing and support for the polythene cover, door construction, polythene type and thickness and other aspects of the overall specification such as ventilation systems.
Consider very carefully what your needs are, and especially how exposed the intended site for the tunnel is. Cheap tunnels may not be as strong as more expensive one, a fact that may only become apparent during a storm.
Siting a tunnel
Exposure to winds and low temperature is the dominating factor that will quickly help to decide where to site a tunnel. Try to site your tunnels where there is some shelter from the prevailing winter storms. You should also consider installing wind fencing if the site is particularly exposed.
Avoid shade due to trees, hedges, and buildings. For maximum benefit, tunnels should receive as much sunlight as possible. In addition, aligning tunnels East-West maximises light availability.
Steep slopes should be avoided and if a sloping site is inevitable, then the tunnel should be aligned up rather than across the slope.
Water supply and watering system
Another factor in determining where to site a tunnel is the availability of a convenient water supply. Considerable quantities of water will be required, and water piped to the tunnel is essential. This may require installation of a new, or extension of an existing pipe.
Choice of watering system is important - you will not be able to water everything by hand.
Overhead spray lines are perhaps the simplest watering system, but can lead to high humidity, increased disease levels and water marks on foliage and fruit which reduce quality. This is particularly important when you cannot use fungicides to control diseases.
Trickle irrigation using, for example, 'T tape' along the ground will avoid wetting the foliage of your crops and help prevent disease. More precise irrigation lines to individual plants are feasible, but more expensive.
Machinery requirements for use in polytunnels
Hand tools are, of course a prime requisite in tunnel growing, but for incorporating manures and green manures, and for general cultivation purposes, a wheeled rotavator is more or less essential. Only in the largest tunnels will a tractor be suitable, though a mini tractor is an option.
Tunnel maintenance
The polythene on a tunnel will typically last three to four years before it needs replacement. As it ages, polythene tends to become opaque, reducing light for plant growth. It also tends to wear on the frames and eventually split.
One of the most important parts of tunnel maintenance is ensuring that any tears, holes or loss of tension in the polythene are corrected as soon as they are found so that the wind cannot get in and cause more serious damage.
Supplying nutrients and crop rotations
Polytunnel growing is the most intensive form of organic production and as a result the fertility of the soil must be adequately maintained. Your certification body will monitor this very carefully.
Manures and composts are likely to be the main source of nutrients in polytunnels, supplemented by green manure crops. Unless you have your own organic manure, you may need to import some. Ideally this should be from another organic farm, but can come from a conventional farm, providing it is not from an intensive system, and it is properly composted.
There are currently plentiful supplies of organic poultry manure available locally.
All manures should be properly composted before use as per organic standards.
The source and quantity requires clear prior approval from your certification body. Standards now place particular emphasis on ensuring that brought-in manure is not contaminated with GMO or their derivatives. Of specific importance here will be ensuring, and proving, that the animals that produced the manure have not been fed GMO feed.
Manures are probably best incorporated into the soil before planting, though surface mulches are possible. You must avoid any contamination of edible parts of crops with manure material or splashes from it.
Regular soil analysis is particularly important in polytunnel growing so that you can be sure that mineral salt levels are not building up unexpectedly, and that overall fertility is being maintained.
Green manures
Green manures, particularly legume green manures, have a clear place in polytunnels and if at all possible they should be incorporated into cropping sequences and crop rotations. In a very specialist role, white clover can be grown as a living mulch below tomatoes, cucumbers and other tall crops such as sweetcorn.
Green manures are incorporated into the soil exactly as they would be in a field situation.
Supplemental feeding
Supplemental feeding with liquid feed may be required in some nutrient demanding crops such as tomatoes. When applying a liquid feed, particularly a home-made feed, be very careful to avoid getting it on the leaves or fruits of plants in which these parts are eaten, to avoid contamination.
Crop rotations and cropping sequences
As in field production crop rotations and cropping sequences should be planned so that crops in the same family are not planted in the same ground until the next rotational cycle. Since tunnels can be growing crops all year round, the crop rotation concept is extended to include a yearround continuity of crops, without a nonproductive 'resting' period which is common in field rotations.
A four year crop rotation should be the minimum, ideally it should be longer, and should preferably include short or longer-term green manures. This means that cropping sequences should be planned carefully. They should also contain a certain amount of 'slippage', which may be needed if weather conditions slow down crop growth.
Crop protection
Weed control
Weeds can grow very rapidly in tunnel conditions and also provide hiding places for slugs.
Since ground conditions can be controlled easily in tunnels, stale seedbed techniques are easily incorporated in weed control regimes. However, there will inevitably be a need for removing weeds mechanically whilst crops are growing. Keep a careful watch out for weed seedlings and, because they grow so fast, remove them promptly at an early stage.
Pest and disease monitoring and control
Humidity and high temperatures can lead to rapid pest and disease infestation, so inspect crops frequently.
Good ventilation to avoid high humidity and temperatures is the first step in keeping fungal and bacterial diseases to a minimum. It will also help to reduce pest levels.
Biological control
Biological control agents play an important role in pest control in tunnels where the higher temperatures allow the various pest predators and parasites to function well. Biological control should be the preferred option over the very limited range of pesticides available to organic growers.
Biological control agents are readily available for a number of tunnel pests including white fly, red spider mite, aphids, slugs, caterpillars and vine weevil.
Whilst not biological control a such, bumble bees are also available for assisting with pollination.
Commitment needed for tunnel production
Polytunnel production is very attractive, but it demands a high level of commitment and planning if it is to be successful and generate optimum returns.
Monitoring and controlling temperature and humidity, as well as watering crops is an on-going process and requires a daily commitment to having someone carry out these vital tasks. It is not just a once-a-day job either, as tunnels may need opening up to provide ventilation in the morning, and then closing up in the evening. Watering will probably be required several times a day when the weather is hot.
Contact for further information
Adrian Saunders
Greenmount Campus, ANTRIM, BT41 4PU
Greenmount Campus, ANTRIM, BT41 4PU
April 2008