May is a dangerous month for greenhouse users. With all the talk about global warming and the evidence of it everywhere, you can easily forget that the risk of frost continues into May. The last likely frost might be in early May near the coast, but inland you are not in the clear until end of the month. Global warming is a long-term trend but there are still short-term highs and lows on the temperature graph.
Plants grow fast in May and quickly need more space. It’s tempting to move some plants out of tunnels or glasshouses to make room for the rest, but harden them off properly first and keep a sharp eye on the weather forecast. Be prepared to carry them back in on evenings when frost is predicted. And if you were tempted by the early displays of tender bedding plants in a garden centre, remember to bring them in too. Cold frames can protect plants from several degrees of frost, and plastic cloche tunnels give some protection from lighter frosts. Single small plants can be covered with upturned buckets or light materials such as straw or bracken fronds. Just make sure the wind doesn’t uncover them overnight and that you uncover them yourself next morning.