Shading and ventilating The changeable weather in April can cause violent temperature fluctuations in the greenhouse. Remove winter insulation if you have not done this already. Shade young seedlings and newly ported plants from the sun with horticultural fleece or sheets of newspaper. Keep the greenhouse well ventilated, but only open those lights facing away from the wind if there is any risk from hailstones. Watering and feeding plants Give increasing amounts of water to all plants in pots or containers, which should now be established and growing rapidly. Continue to feed established plants such as zonal and regal pelargoniums, annuals in pots, fuschias and other summer-flowering plants. Apply liquid fertiliser at ten-day intervals. Hardening off half-hardy plants Move any half-hardy plants into a cold frame for hardening off. This will also provide space for sowing melons and cucumbers at the end of the month or in early May. Plants in frames require regular watering and the lights propped open on sunny days to give them extra ventilation. Move tall plants to the back of the frame so that they do not touch the glass. Protect plants from slugs and snails. Controlling pests and diseases Introduce Phytoseiulus persimilis as a biological control if red spider mite is a problem - chemical controls are often less effective. Sowing seeds Make further sowings of dahlias, dianthus, annuals, lilies, shrubs and vegetables. Sow winter cherry at about 15°C (60°F). Prick out the seedlings when they are large enough to handle. Potting cuttings Pot up rooted cuttings, and pot on cuttings and young plants which have made good growth and produced a mass of roots. Cucumbers and melons Cucumbers are a worthwhile greenhouse crop, and a single plant will be sufficient for most households. 'Butcher's Disease Resisting' is an excellent variety. Melons of the cantaloupe type are usually grown in a cold frame or under cloches. Sow melons and cucumbers in a temperature of 15-18°C (60-65°F). In each case sow the seeds individually about 3 (1 1/2 in) deep in 8 cm (3 in) pots of John Innes No. 1 potting compost. Climbing cucumbers should be grown up canes, and the lateral shoots which carry most of the fruit should be trained along horizontal wires 30 cm (12 in) apart. Pinch out growing tip and sideshoots two leaves beyond a female flower. Where a single cucumber plant is being grown, plant it on a mound. Add John Innes base fertiliser or a slow-release fertiliser at the rate recommended by the manufacturer. Female types will not require lateral training as they only fruit on the main stem. Peppers and aubergines Prick out into individual pots when the seed leaves are fully expanded. Tomatoes In a cool greenhouse plant tomatoes in the bed prepared last month, allowing 35 cm (14 in) between plants, alternatively use grow bags. Water the tomato plant before planting. If you are growing plants by the ring culture system, use john Innes No. 3 potting compost and bottomless containers which are 20 cm (8 in) deep and 20-25 (8-10 in) in diameter. Keep the gravel base moist after planting watering the plants in the pots just sufficiently to keep them growing. Once the roots have penetrated into the gravel, no further watering via the containers will be needed, except when the plants are fed. Train tomato plants up tall canes or up strings. You can hang these from wires on glazing bars down to hooks of thick wire pushed into the soil alongside the plants. If you are growing tomatoes in a growing bag, take care not to push the canes through the bottom. From the time the first tomato flowers open, spray the plants lightly at about midday during sunny weather. During dull weather, dust the flowers with a feather duster or a paint brush. Remove all sideshoots of cordon tomatoes from the axils of the leaves when they are 2.5 cm (1 in) long. Leave the sideshoots on bush varieties. Vines Ensure vines get as much light and heat as possible once growth begins but try to maintain adequate ventilation to assist pollination.