Page 38 - Simply Vegetables Summer 2022

Polytunnels
It is only in the last ten years that I
have become fully acquainted with polytunnels, partly through using one
at work and now building my own. I
have been particularly surprised at the amount of growth some plants put on in a polytunnel and in some cases better than a greenhouse. They can be used alongside a greenhouse or on their own and if you have the space and especially if you cannot afford a greenhouse (although these have come down in price a lot over the last few years) a polytunnel is well worth considering. Polytunnels have a more humid environment than greenhouses which can be both an advantage and disadvantage depending on which crops you are growing.
Most polytunnels are easy to erect and also move if required and are cheaper than greenhouses especially if you want a good sized one and are available in single bay and multi-bay structures so you can have different temperature in each bay. They are very versatile and can be used for growing from propagation to cropping. Like with greenhouses choose the biggest polytunnel you can afford and have space for as once you have it you
the hoops use anti-hotspot tape on the top of the hoop to reduce sun damage to the polythene cover.
Internal partitions – if you want more than one growing environment polytunnels can have partitions fitted so that different growing regimes can be achieved.
After the hoops the most important consideration is the covering or cladding as it is called. There are a number of different types of polythene that can be used including different thicknesses. A thicker gauge polythene will last longer than thinner gauge, the standard gauge is 720 and this should last 4 to 6 years and even longer if looked after. It is the most commonly used in the U.K. and has a 95% light transmission and is very good for summer growing vegetables.
Thermal/anti fogging polythene also 720 gauge and can have UVA and UVB filters included in the polythene. This is better
for winter growing and early sowings as it retains heat better and can be up to 2°C higher on a cold night. It also warms up quicker in the morning and cools slower
in the evening. The anti-fogging helps to reduce condensation as it runs down the
any machinery or equipment through. The partitions can also have a door fitted. Ventilation – small tunnels and the
lower priced ones will just have the doors for ventilation but if possible, try to get a tunnel with side vents as these do not get
a hot during the summer. The side vents consist of a fine net skirt fitted around the base of the tunnel with a polythene cover that can be raised or lowered as required. Multispans have roof ventilators fitted along the full length of the roof and side vents on the two sides.
Location
Choose a level site or get it levelled this both makes construction easier and also temperature control as hot air rises to the higher level of the tunnel and in the winter the lower end will be colder. Like with greenhouses avoid shaded areas from trees, buildings or tall hedges but also avoid exposed areas where wind could damage the tunnel. Check the direction
of the prevailing wind and avoid having the tunnel end on as it is more likely to be damaged. If a windy site wind breaks can be planted or erected to give some protection.
Check if planning permission is required, it is not usually for small tunnels but is for the larger commercial ones. Note the soil conditions as this affects how the hoops
/ tubes are anchored, if the ground is firm they can be hammered in, if soft concreted in; I would always concrete in to be sure.
Decide which direction to site the tunnel, east / west tends to be slightly warmer as the sun warms the whole of the side. North / south may be better in the summer as it may be slightly cooler.
will want a bigger one! Polytunnels are very
widely used commercially for growing both fruit and vegetables and for early and all the year-round cropping. They extend the growing season both at the beginning and end of the season and are especially useful in the spring for starting and hardening crops to be planted outside. Some tender plants can be overwintered in polytunnels and they can be used to dry crops like onions, shallot and garlic as well as squash.
Polytunnels are very widely used commercially for growing both fruit and vegetables and for early and all the year- round cropping
inside of the polytunnel rather than form on the polythene and drip off. This helps to reduce fungal diseases like Botrytis
and still has good light transmission.
White polythene UVI this reduces light transmission to 70% and is useful for shade loving plants and reduces leaf scorch. It gives a cooler temperature inside but is not widely used in vegetable growing.
Mesh and net tunnels
are more expensive than polythene and again reduce the light levels inside so are
If choosing a polytunnel there are a wide range of sizes and configurations, as well as single and multi-bay types other factors to consider is whether you want a straight sided or rounded side. Both the single span and multispan come in different widths from 2.44 metres up to 9 metres for single spans and wider for multispan.
Are ventilators required, if so, end vents or side vents and is mesh covering wanted? Looking at other factors note:
Metal tubes – these come in various diameters and either galvanised on the outside or inside and outside. If you live in a windy area, choose a larger diameter tube as it is both more wind and snow resistant. Also space the tubes/hoops closer together on windy sites. Before covering
useful for shade loving plants. They are sometimes used as fruit cages for some soft fruits and also to keep pigeons off Brassica crops on allotments. The size of the mesh dictates what it keeps out.
Other polyethene’s, that are now available in green or light blue and are mainly experimental at present but there is strong evidence that it can give good control of some pests and does not affect plant growth too much so could be something for the future.
Looking at the other requirements for tunnels it is wise to have doors at both ends as this makes access easier and helps to ventilate the tunnel in hot weather. Single and double doors are available but make sure they are wide enough to get
38 Simply Vegetables