Understanding the tunnel environment in winter means separating three important factors which affect plant growth; sunlight, temperature and wind chill. Wind chill is the most damaging effect outside the tunnel, stripping heat and moisture out of plants so that they suffer severe damage. Happily, plants inside the tunnel enjoy a sheltered climate and so will shrug off temperatures that would kill them outside.
Temperature determines the extent to which plants can use the limited light that they receive in winter. Because tunnels are kept warm by the greenhouse effect, daytime temperature is directly related to the amount of sunlight that the tunnel receives.
Sunlight is essential for plant growth. Without enough of it, plants stop growing and concentrate on keeping their metabolism ticking over. This means that there is a ‘no growth’ period in winter as daylight drops off towards the winter solstice. How long that period is depends on your latitude (and of course the type of plant concerned); the closer to the pole, the less light you get.
For professional growers it is feasible to provide artificial heat and light to continue to grow through the winter. However, this comes at a fierce environmental cost; tomatoes grown in heated British polytunnels actually have a higher carbon footprint than tomatoes grown in unheated tunnels in Spain and Portugal, which are then shipped to the UK by sea and road. Although there are circumstances when it may be justified (for example a commercial grower using a large and highly insulated structure to bring on early seedlings in bulk) artificial heat and light should not be viewed as a neat ‘get around’ in winter.
Instead, view your polytunnel as a walk-in store cupboard during the no-growth period. If you have planned your winter plantings successfully there should be produce ready to pick, and provided you protect the less hardy plants (below) the cold temperatures will keep the food just as it is until conditions improve. Remember, food on the plant is still absolutely fresh – there is no wilting or loss of nutrients unless the plant itself perishes. Much better than the salad compartment of a fridge!
A cold frame or fleece (floating row cover) cloche in the tunnel will keep plants much warmer than it would outside and is often all you need to keep the interior frost-free. If you use a fleece cloche, make sure that the supports are tall enough to keep the cover off the plants inside – the fabric tends to get damp and may then freeze on cold nights, damaging any foliage it touches. When you harvest, make sure that the protection is replaced immediately.
A lot less watering is needed in winter and there is a risk of freezing damage to pipes, connectors and timers in cold conditions. Disconnect and store all susceptible equipment until the spring, and water with a can only until the risk of frost reduces.
Once temperatures drop there is less risk of mould growth in the tunnel, and warmth/wind protection becomes more important. You should still make sure that at least one end of the tunnel is opened at first light (allowing trapped cold air to flow away), and that the tunnel is closed up properly an hour before sunset to keep night-time temperatures from falling too quickly. If the temperature outside is forecast to remain below freezing all day, open both ends of the tunnel briefly at first light and then close them again to make the most of the greenhouse effect. If possible, briefly ventilate the tunnel again around noon.