Future Growers Module 10: Protected cropping Briefing paper With thanks to Gareth Davies and Margi Lennartsson: Organic Vegetable Production – A Complete Guide. 2 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 3 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. 4 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. 5 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. 6 Polythene versus glass Polythene Glass Expensive to set up and maintain – though Cheaper – though plastic will need there are some good second hand prices at replacing every 5-10 years the moment Better heat retention than polytunnels, Heats up and cools down more meaning they are also more useful and quickly efficient for plant raising Overall temperatures reached are not Better light penetration through clean glass as high as glass and on clear frosty means better growth, nights can be as low as those outside Can extend the season further than is Risk of higher humidity, though possible with polytunnels, meaning longer modern polythenes have improved growing period for crops such as tomatoes, the conditions peppers and cucumbers Potential for broken glass can create health and safety issues. Vandalism risk – if the side is slashed Vandalism risk – you will probably need to you may need to replace the whole replace a few panes plastic cover Best not to use flame weeders near Can use flame weeders even right up to the plastic 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. 7 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.