Tomatoes are sun-loving plants that relish the warmth and shelter of a greenhouse. In the British climate, growing under glass gives you a dependable crop, protecting plants from cool winds, heavy rain and blight. The secret is to mimic their ideal conditions: bright light, steady moisture and good airflow. Follow these practical steps and you'll be picking ripe fruit from midsummer well into autumn.

Choosing the right tomato varieties for your greenhouse

For a British greenhouse, cordon (indeterminate) tomatoes are the best choice. They grow tall, produce fruit over a long season, and are easy to train up a string or cane. Reliable varieties include 'Gardener's Delight' for sweet cherry fruits, 'Moneymaker' for a heavy crop, 'Ailsa Craig' for classic flavour, and 'Shirley' F1 for early ripening. If you prefer bush (determinate) types, try 'Totem' or 'Tumbling Tom' in large pots or hanging baskets. Look for disease-resistant F1 hybrids if you've had problems with mildew or botrytis before. Always buy fresh seed from a reputable supplier, as tomato seed loses viability quickly.

Sowing seeds indoors and potting on carefully

Start your seeds indoors from late February to early April. A heated propagator or a warm windowsill at 18–21°C is ideal. Sow thinly, 1cm deep, in moist seed compost. Germination usually takes 7–14 days. As soon as the first true leaves appear, prick out seedlings into 9cm pots, holding them by a leaf rather than the stem to avoid damage. Pot on into larger containers as roots fill the pot, finishing in 23–30cm pots or growbags. Bury the stem deeper each time to encourage extra roots. Don't move plants to an unheated greenhouse until night temperatures stay above 10°C, usually late May in southern England and early June further north. Harden them off over a week if they've been indoors.

Watering and feeding consistently for steady growth

Consistency is everything with tomatoes. Irregular watering leads to blossom end rot and split fruit. Aim to keep the compost evenly moist, watering little and often—twice a day in hot weather. Water at the base, avoiding the leaves, to reduce the risk of fungal disease. A drip irrigation system or capillary matting can help if you're busy. Begin feeding with a high-potash tomato fertiliser when the first truss has set, every 7–10 days. Don't overdo nitrogen, or you'll get lush leaves at the expense of fruit. In growbags, plants need more frequent feeding and watering because there's less compost. Check moisture by pushing your finger into the soil; if it's dry 2cm down, water.

Managing temperature, ventilation and humidity

British greenhouses can overheat surprisingly quickly, even on a cloudy day. Keep the temperature between 18–27°C. Above 30°C, pollen becomes sterile and fruit won't set. Ventilate by opening doors and roof vents on warm days, but close them before evening to trap warmth. Use shade netting or greenhouse blinds in high summer. Humidity should be around 60–70%. On hot days, damp down the path to raise humidity, which helps pollination. Avoid high humidity at night, as this encourages botrytis. A max-min thermometer is a worthwhile investment. In early spring, protect young plants with fleece or bubble wrap. In autumn, a small heater or bringing green tomatoes indoors can extend your harvest.

Pollination, support and pruning for a healthy crop

Tomatoes are self-pollinating, but in a still greenhouse they need a little help. Tap the support or trusses gently at midday, or use an electric toothbrush to vibrate the flowers. Cordons need support: twist the main stem around a string or tie it to a cane. Remove side shoots that appear between the main stem and leaves every week—these sap energy. Once five or six trusses have set fruit, pinch out the growing tip to concentrate energy on ripening. Remove lower leaves below the first truss to improve airflow. Bush varieties need no pruning. Watch for whitefly, red spider mite and aphids; biological controls or a soap spray can help.

Harvesting and troubleshooting common issues

Harvest tomatoes when they are fully coloured and slightly soft. Cut them off with scissors or twist gently. Regular picking encourages more fruit to ripen. If problems arise: blossom end rot indicates irregular watering—add a little calcium-rich feed if needed. Split fruit also comes from erratic watering. Greenback is caused by too much sun or heat; shade the plants. Leaf curl is usually a stress response to temperature or moisture changes. To ripen green fruit at the end of the season, remove leaves shading the trusses or place a ripe banana nearby. You can also pick green tomatoes and ripen them indoors in a drawer with a banana. With a little care, your greenhouse will reward you with a delicious crop from July to October.