Fill your watering cans with water and allow them to stand in the greenhouse for at least 24 hours to warm before watering your plants. You may need to water two or three times while the weather is hot especially if you are behind on your potting on and large plants are in small pots. Water seedlings with tap water. If there is a shortage of rainwater, save it for plants that need soft water like citrus fruit.
I hope that your greenhouse tomatoes are growing well. As they become established, remove the side shoots of ‘cordon’ varieties, leaving a single productive stem. Just snap them off with your finger and thumb. Keep them well fed and watered. You may need to water twice a day – one large soaking often leads to water-logging. Erratic watering causes calcium deficiency known as ‘blossom end rot’ – where the end of the fruit furthest away from the stem turns black. Try to keep the compost constantly moist.
Ventilate the greenhouse early while the weather is hot but avoid chilling drafts. If the wind is cold, open the ventilation on the leeward side of the greenhouse. Dampen the floor of the greenhouse regularly on hot days to increase humidity levels. This not only benefits plant growth but also reduces the risk of pests like glasshouse red spider mite.
It is really important to check for pests and diseases as temperatures rise. Check the greenhouse daily for red spider mite, whitefly, scale and mealybug. Control them with biological controls or environmentally friendly sprays. Mist to raise humidity to control red spider mite, make sure that plants are not crowded and keep the glasshouse well ventilated.
Transplant Aubergines, peppers and chillies into pots one or two sizes larger as they grow. Wait until they have a good root system but before they become pot bound, finally transplanting into 23 cm pots or two per growing bag. Keep a bucket or bag of compost in the greenhouse so it is warm for transplanting. Pinch out the tips from the main stems of aubergines when they are 30-40cm tall. Stake or tie into supports as necessary.
Pinch out the growing tip of peppers and chillis when they are about 15cm to encourage branching and heavier cropping.
Aubergines and tomatoes can be pollinated by tapping the flowers, ideally when the pollen is dry on a bright sunny day. Allow five or six aubergines to form, on each plant.
Outdoor ridge cucumbers can be planted out this month after ‘hardening off’. They benefit from a site that has been enriched with lots of organic matter to help retain food and water and they need plenty of both.
Move seedlings from the greenhouse to cold frames or into a sheltered spot for ‘hardening off’ during the day, bringing them in at night, for ten to fourteen days. When night temperatures increase and in sheltered spots, you may want to try planting out vegetables and if necessary, covering them at night with horticultural fleece.
Happy gardening. Matt