With plenty of light in summer, heat is the main problem in greenhouses and conservatories, unlike other seasons when lack of light is limiting. Ventilation is the best defence, allowing excessively hot dry air to exit and cooler, moister, air to enter.
Commercial greenhouses have roof vents of at least 20 per cent of the floor area. They seldom use sidevents, which are less effective. Unfortunately, home greenhouses have less ventilation, relying instead on shading to reduce temperatures. A maximum of 25°C is ideal, with over 30°C suppressing plant growth while favouring red spider mite and powdery mildew disease.
Shading, however, cuts out some light. Plant growth is directly linked to light levels. In practice, enough shading is used to limit temperatures on bright, hot days, where ventilation is insufficient to allow heat to escape, while minimising shade. Unfortunately, excessive shade on dull days is unavoidable. Temporarily removing some panes of glass from a greenhouse’s sides or gable can be effective.
Shade paint is the cheapest heat control, typically applied lightly in spring then more thickly by high summer. Exterior blinds provide shade without allowing heat into the greenhouse and are ideal if expensive. Interior screens, though less expensive, allow more heat in.
Shade-avoiding plants such as tomatoes are happy with as much light as they can get. In practice, excluding 40 per cent of light is satisfactory. Cucumbers may be scorched by strong sun, so 50 per cent shading might be appropriate. Shade-tolerant plants including begonia, coleus and impatiens are typically grown with more than 50 per cent shade to avoid stress.
Shade netting is used on plastic greenhouses and tunnels, as shade paint often cannot be washed off for winter.
Humidity is the amount of water vapour in the air – hot air can hold much more water than cold air. Relative humidity is a percentage of the amount of water the greenhouse air can hold: it is often excessive from autumn until spring, leading to rot and moulds. In summer, insufficient humidity stresses plants, reducing photosynthesis while encouraging red spider mites and powdery mildew.
Plentiful ventilation allows in moister external air. Wetting down (spraying with water) floors, benches and plants is much less effective as the water vapour quickly disperses. Plants also humidify air very effectively, so keeping greenhouses filled with well watered leafy plants will help prevent damaging dryness.
Summer watering must keep roots constantly moist – unlike in winter they should not dry out between waterings.
The usual indicator of drying out is a change in surface colour, but with peat-free potting compost the surface may dry out and yet the root zone remains wet. Lifting some pots to assess weight is the easiest way to monitor pot water content. Alternatively, squeeze some of the growing media to see how much water runs out. Evening watering in summer minimises evaporation losses, but in overcast weather morning watering reduces disease risks as plants remain dry overnight.
Ideally, to save water and avoid water stress, little and often watering is ideal for greenhouse plants. Realistically, even the most diligent waterer may have some plants dry out. Deeper but less frequent watering when plants are about to get too dry – until a little water runs out of the pot – is easier to manage. Deeper watering particularly suits large, thirsty plants which resist occasional saturation.
If pots do dry out, stand smaller ones in a bucket of water for an hour. For larger pots, adding some special detergent called wetting agent to the water speeds up rehydration.