Growing tomatoes in a greenhouse is one of the most rewarding projects for a British kitchen gardener. It gives you a longer season, protection from blight, and the sweetest fruit you will ever taste. The process is straightforward: start seeds indoors, pot on carefully, water consistently, and keep the air moving. This guide will walk you through each stage, from choosing the right variety to harvesting your first ripe tomato.

Choosing the Right Tomatoes for a Greenhouse

Before you sow a single seed, decide what kind of tomatoes you want to grow. For greenhouse growing, cordon (indeterminate) varieties are the most popular. They grow tall and produce fruit over a long season. Reliable choices include 'Alicante', 'Moneymaker' and 'Gardener's Delight' for cherry tomatoes. Bush (determinate) varieties stay compact and need less support, but they crop over a shorter window. If you are new to this, start with a cordon variety and give it plenty of room.

Sowing Seeds Indoors

Tomatoes need warmth to germinate, so start them off indoors in late February or early March. Fill small pots or module trays with seed compost, firm it gently, and sow each seed about 1cm deep. Water well and place in a warm, bright spot at 18–21°C. Germination usually takes 7–14 days. As soon as the first true leaves appear, prick the seedlings out into 9cm pots of multi-purpose compost. Keep them on a sunny windowsill or in a heated propagator, turning the pots daily so the plants grow straight.

Potting On and Planting into the Greenhouse

When roots start to fill their pots, move the plants on into 12–15cm pots. About a week before planting them into the greenhouse, begin hardening them off. Put them outside during the day and bring them in at night, increasing their time outdoors gradually. Plant into a greenhouse border, grow bags, or large pots of at least 10 litres. Add a handful of slow-release fertiliser and plenty of compost. Plant deeply, burying the stem up to the first set of leaves — this encourages new roots. Space plants 45–60cm apart and water them in well.

Watering and Feeding for Healthy Growth

Tomatoes are thirsty plants, but they hate extremes. Irregular watering is the main cause of blossom end rot, a dark sunken patch on the fruit. Aim to keep the compost evenly moist. In hot weather, you may need to water twice a day. A drip irrigation system or soaker hose makes this much easier. Mulching the surface with compost or straw helps hold moisture. Start feeding when the first truss of fruit has set. Use a high-potash tomato feed once a week, following the bottle instructions. In midsummer, increase to twice weekly. Avoid feeding too much nitrogen, which encourages leaves at the expense of fruit.

Ventilation and Temperature Control

A greenhouse can turn into an oven on a sunny day. Good ventilation is essential. Open vents early in the morning and close them in late afternoon. Aim for a daytime temperature of 21–27°C and a nighttime temperature of 15–18°C. In midsummer, you may need to shade the glass with netting or a wash-on shade paint to prevent scorching. Stagnant, humid air encourages botrytis and damping off, so keep the air moving. If you can, fit automatic vent openers. Avoid cold draughts on chilly nights, which can check growth and cause fruit to drop.

Supporting, Side-Shooting and Harvesting

Cordon tomatoes need support. Tie the main stem to a cane or string as it grows, using soft twine. Each week, pinch out the side-shoots that appear in the leaf axils — the V-shaped joint between the stem and a leaf. Remove the lower leaves as the fruit begins to ripen, which improves airflow. Once four or five trusses have set fruit, pinch out the growing tip to redirect energy into ripening. Bush varieties need no side-shooting, though they may still benefit from a little support. Harvest tomatoes when they are fully coloured and slightly soft; they taste best straight from the vine. Pick regularly to encourage more fruit. At the end of the season, green tomatoes can be ripened indoors on a windowsill or turned into chutney.