Greenhouse temperatures can rise very quickly during sunny days in May, even when outdoor conditions feel relatively mild. Roof vents and side vents should be used together to release trapped heat and maintain airflow around developing plants. Automatic roof vents are particularly useful when temperatures change while you are away, but manual vents will still need to be adjusted as conditions develop.

Shading can provide additional protection by softening intense sunlight before plants become scorched or stressed. Adjustable external blinds provide greater flexibility than permanent shading paint because they can be raised again during dull or cooler weather. Lowering the shades and damping down the greenhouse floor can help reduce temperatures quickly on particularly warm days.

Thorough watering followed by a short drying period generally produces stronger plants than applying small amounts of water frequently. A proper soak encourages roots to travel further through the compost, whereas constantly damp conditions can restrict root development and increase the likelihood of algae and disease.

Check plants individually rather than watering the whole greenhouse to a fixed schedule. Lifting pots to judge their weight, feeling the compost and watching for signs of wilting will give you a more reliable indication of when water is needed. Morning or late-morning watering is ideal because excess moisture can evaporate before temperatures fall overnight. If evening is your only option, direct the water towards the compost and avoid unnecessarily wetting the foliage.

Hardening off should begin before greenhouse-grown plants are permanently moved into the garden. Young plants raised under cover are accustomed to relatively stable temperatures, so suddenly exposing them to wind, stronger sunlight and colder nights can check their growth or damage tender foliage.

Move plants into a cold frame or sheltered outdoor position during the day before bringing them back under cover at night. Gradually increase the amount of time they spend outside as they adjust. Hardy plants will still benefit from this process, while tender crops such as tomatoes and chillies should remain protected until the local risk of frost has passed.

Slowing growth, roots filling the container and yellowing cotyledons—the first leaves produced by a seedling—can all indicate that a young plant has outgrown its pot. Potting on gives the developing roots more room while replenishing the compost and nutrients available to the plant.

Move seedlings into a slightly larger container rather than an excessively large pot, as too much wet compost surrounding a small root system can increase the risk of overwatering. Once plants are actively growing, a gentle liquid feed, such as a seaweed-based fertiliser, can support continued development without forcing overly soft or rapid growth.

May remains a productive month for sowing fast-growing annuals, tender vegetables and climbing plants. The warmer conditions allow many seeds to establish quickly, while the protection of a greenhouse gives tender varieties a more reliable start than they may receive outdoors.

Rather than sowing an entire packet at once, use succession sowing to create several smaller batches. This spreads flowering and harvesting across the summer and prevents large numbers of plants becoming ready simultaneously. May is also a good opportunity to experiment with heritage or unusual varieties that may not be readily available as established plants.

Removing the central growing tip encourages a young plant to produce additional side shoots instead of concentrating its energy into one dominant stem. This creates bushier, sturdier plants that can carry more flowers later in the season.

Pinching out is particularly useful for plants such as cosmos, antirrhinums and sweet peas once they are established and growing strongly. Use clean fingers or snips and remove the growing tip immediately above a set of leaves. Avoid pinching very small or recently stressed seedlings, as they should be allowed to establish first.

Greenhouse protection gives tomatoes a longer, warmer and more controlled growing season, which can improve fruit ripening and crop reliability during cool or wet British summers. Keeping rain away from the foliage also gives gardeners greater control over watering and helps reduce some moisture-related problems.

Tomatoes can still be grown outside successfully, but variety selection becomes particularly important. Choose reliable varieties suited to British conditions rather than types that require consistently high Mediterranean temperatures. Tender tomato plants should not be moved outside permanently until the local risk of late frost has passed.