We had some unseasonably cold weather in the UK recently, and where I live near the south coast of Devon frosts at the end of April are almost unheard of. But we had some all the same, and it caused a few problems. For me it wasn’t so much the night time lows, but the general cold, when the sun wasn’t out greenhouse temperatures during the day were too cool for plants to grow quickly, so everything is behind.
There were a few touch and go nights though, and towards the end of April I had just too many plants to bring indoors, so they had to run the gauntlet of some near freezing temperatures.
The lowest my max/min thermometer read was -1°c in the greenhouse, but don’t take this as definite. Have you ever browsed the range of thermometers available in a garden centre? Have a close look, last time I bought one the readings on all the thermometers on offer ranged from 18 to 21°c, and they were all hanging from the same shelf! I bought an average one, but there is no telling how accurate it really is. I suspect the temperature didn’t quite reach as low as -1°c, or if it did it wasn’t for very long, maybe only 15 minutes before dawn.
So did my plants survive? Yes, they all did, (not so the courgettes planted outside). And what does this tell us about the lowest temperatures young chilli plants can endure?
My experience tells me that as long as there is no cold wind or rain blowing directly against the plants, which means they need to be in a greenhouse not outdoors, and as long as the cold temperatures only go on for an hour or two, then the air temperature can go down to just 2 or 3°c and still recover happily, but who wants to gamble on a couple of degrees this way or that? And remember this is far from desirable, they aren’t actually growing at these temperatures, as they would if they were bottoming out at 19°c, so warmer is always better.
Another factor that can seriously jeapordise the well being of small tender plants is how quickly they warm up. Generally the soil in their posts will retain a little heat to insulate the roots. The plant might wilt slightly when it draws down fluids into the roots as a reaction against the cold. When the sun suddenly starts to warm the greenhouse then the wilting leaves can’t transpire quick enough to offset the rapidly increasing temperature and the drooping leaves shrivel. In the past I have seen chilli plants looking healthy before sun-up, but two hours later they have died. This is a particular problem where a greenhouse is sheltered from the rising sun and remains cold until later in the morning when suddenly the strong sun peeps over nearby buildings or trees and the temperature rises from nothing to 30°c in a matter of seconds, rather than a gradual warming as the sun rises. This is why instructions on constructing greenhouses always tell you to site it somewhere away from such situations.
Finally, of course, different varieties react differently, tepin, most Capsicum pubescens, (so rocoto), and many aji varieties will be slightly more tolerant than others.