My greenhouse in May feels like it is buzzing with potential, the warm, humid air, the trays of seedlings stretching towards the light. This May has felt a little different. There are still trays of seedlings but it has been so cold of late that there isn’t the feeling that everything is bursting to get out of doors. Temperatures are stubbornly refusing to behave like spring, or slipping towards summer and then back again into wintry, and it has made me cautious. I’m not in the rush I might have been, though I’m aware that as soon as the weather warms I will have to leap into action.
If your greenhouse is anything like mine right now, it’s probably getting crowded and that’s a good thing. May is when the space really earns its keep.
By mid-May, your greenhouse should be home to a pleasing mix of seedlings at different stages, some almost ready to face the world outside, others settling in for the summer.
Tomatoes are likely your tallest residents. If you sowed in late February or March, plants should now be well established, stocky, and possibly already showing their first flower trusses. Keep them in the greenhouse for now — tomatoes are staying put all season for most of us, thriving in the warmth and shelter. This is a good time to pot them on into their final containers (at least 10–12 litres) if you haven’t already, burying the stem deep to encourage strong roots. Start feeding with a high-potash liquid feed once the first flowers open.
Courgettes sown in April will be growing on alarmingly fast, they seem to double in size overnight. These will eventually go outside, but I am waiting for the moment. They need warm soil and settled conditions and planting out into cold soil risks stunting them or worse. Keep them in the greenhouse a little longer and let them get strong.
Cucumbers and aubergines, if you’re growing them, are happy to stay in the greenhouse all summer. Cucumbers especially will reward you with a long, productive season if kept warm and well watered. Pinch out the growing tip once they reach the roof, and train side shoots along horizontal wires.
Tender bedding plants — petunias, pelargoniums, and the like — may also be taking up bench space. These will go out eventually, but patience is a virtue this month.
Jobs to Do Now
Ventilate daily. May sunshine, even when the air outside feels cold, can heat a greenhouse rapidly. Open vents and doors on bright mornings to keep temperatures stable and reduce the risk of fungal disease.
Water carefully. Seedlings need consistent moisture but hate waterlogging. Water in the morning so foliage dries before nightfall.
Pot on anything that’s rootbound. Check pots regularly — if roots are circling the bottom or poking through drainage holes, it’s time to move up a size.
Begin hardening off — but only when the weather warms, and carefully then. Plants destined for outside (courgettes, bedding, brassicas started under cover) will need acclimatising before they go out for good. Start by placing them outside in a sheltered spot during the warmest part of the day, then bring them back in at night. A cold frame is ideal for this transitional period. Given how cool this May has been, though, there’s absolutely no need to rush this process. A plant hardened off slowly and planted out into warm soil in late May or early June will almost always outperform one rushed out into cold ground too soon.
Sow a batch of basil. It loves greenhouse conditions and you’ll be glad of it come July.