Never mind how cold it is in your garden; the sun is getting higher every day and even short clear spells will allow it to build up a lot of heat in your glasshouse or polytunnel. You might think this is a good thing, but not always so. Lettuce seed won’t germinate if it’s too warm, and temperatures above about 35°C (95°F) will destroy the red pigment in ripening tomatoes so they never colour up properly. Overheated plants need more water to keep cool. As well as that, high temperatures put plants under stress and promote diseases such as grey mould and pests such as red spider mites. You can’t stand there 24/7, thermometer in hand, to open and close doors and vents with each change in temperature. Ordinary mortals adjust the ventilation once in the morning for the expected daytime temperatures and once in the evening for the night, and that’s near enough most of the time. The weather forecast can help you decide how much ventilation to give in the morning. A recording thermometer can show you how you did today, helping you to fine-tune your settings for tomorrow. End doors alone give enough ventilation for most polytunnels up to about 20 metres long. Side windows, roll-up vents and louvre vents are good but not cheap. Glasshouse vents can be fitted with gas-filled struts that automatically push them further open when they get warmer, without needing electricity. The best ventilation comes from a through flow of air, so open both ends of a polytunnel or more than one window in a glasshouse. Open many vents a little bit instead of opening two vents wide to reduce draughts. Be cool!

Peter Whyte (Lanscape and Garden Consultant) B Agr Sc (Hort), Nat Dip Sc (Apic), Dip Tr & Ed, MI Hort