A few days of sunshine and the greenhouse becomes a real summer hot house, just perfect for all your sun loving plants. And that’s of course why we have a greenhouse, to harness the energy of the sun and use it to boost plant growth and improve conditions for our hothouse flowers.
While ventilation is essential to keep good airflow around the plants, so is reducing overcrowding by spacing out the plants and tending to your individual plant’s needs. Not everything thrives in extreme heat and now the challenge is to temper the temperature and keep things on an even keel. Those with brick base glasshouses or even stone have a head start. The energy from the warm summer sun is stored, radiator style in the walls and generates the heat back gently to the plants through the night, tackling that coldest point, just before dawn. Tempering the heat of the midday sun is the trickier part; external shades provide the most efficient means to prevent excessive heat build up. Circulatory air fans help move hot air around, but for the best airflow you need top and bottom vents that mimic the natural movement of air. The low vents draw cooler air in at the bottom while the hot air is expelled at the apex.
With a greenhouse full of leafy salads, herbs and young plants, not all should remain under glass. Mixed salads by mid summer are quick to bolt (set seed) as are spinach and spinach beets as well as many herbs such as coriander, rocket and mustards which will all quickly run to seed in hot weather. You can delay this process by cutting them back hard to about 2 – inches from the soil surface and they will reshoot with smaller fresh leaves that can be harvested. By sowing little and often and now planting out into the garden at the first opportunity your plants will flower later and you can keep cropping the leaves indefinitely. Older, larger leaves tend to be hotter, stronger and more fibrous, so keeping plants well picked and rejuvenated will provide a rich diversity of flavours and tenderer shoots.
With the last frost a distant memory in most places, anything ready to plant out in the garden can free up some vital greenhouse space. Harden them off in a cold frame for a few days but take great care to ensure they don’t cook in the heat of the sun. If it’s too hot then just place them out in the daytime and into the cold frame at night. Beans, courgettes and anything tender can be readied for planting out as well as container plants, hanging baskets and plants for bedding displays.
Things that will simply thrive in the greenhouse warmth are the ones that hail from hotter climes. Tomatoes are an obvious candidate, but the Mediterranean herbs such as rosemary, lavender, basil and thyme also love a little extra heat. It’s so much easier to germinate seeds from hotter climates when the temperatures mimic their origins. Try growing these delightful plants from seed now and you’ll have a forest of flavour in no time.
With an eye on water use, the greenhouse gardener needs to ensure that plants under glass are properly cared for. A blip in watering stresses the plants and can set them back severely. But overwatering causes even more issues with sour compost, compost flies and waterlogged roots affecting young plants. The ideal is to tend to your plants daily when it’s hot but to avoid overwatering when it’s not. Basil in particular hates having wet roots at night. Far better to water it in the morning, just a little and let it dry out than to administer too much water. It’s a balancing act, but one that is easy to achieve by regular observation and time spent in the greenhouse and with your plants.