Greenhouse tomatoes go in late April and need stopping by August

Greenhouse tomatoes go in from late April. Sow them in late February or March, plant them into an unheated greenhouse once the nights are frost-free under glass, and train them up strings or canes. By early August, stop them growing upwards so they ripen the fruit they have.

A greenhouse adds weeks to the season in the UK, keeps rain off the leaves, which keeps blight away, and lets you grow varieties that rarely ripen outside.

Sowing

Sow in late February for a heated greenhouse, or in March for an unheated one. Sow seeds 0.5 cm deep, keep them at about 20 °C, and pot them on into 9 cm pots when they have their first true leaves, burying the stem up to the seed leaves.

Choosing varieties

Cordon (indeterminate) varieties suit a greenhouse best. They grow as a single tall stem and use the height. Good choices:

• Sungold: very sweet orange cherry.
• Gardener’s Delight: a reliable red cherry.
• Shirley and Alicante: heavy-cropping standard tomatoes.
• Ailsa Craig and Moneymaker: old favourites.
• Beefsteak types, such as Big Beef, ripen far better under glass than outside.

Planting

Plant into an unheated greenhouse from late April, once the night temperature inside stays above about 10 °C. In a heated greenhouse, you can plant earlier.

• Border soil: dig in compost. Change or refresh the soil every few years, or diseases build up.
• Growbags: two or three plants per bag. Use planting rings or bottomless pots to add depth.
• Large pots: at least 30 cm across, one plant each.

Space plants about 45 cm apart. Plant deeply, and tie the stem loosely to a cane or wind it around a string tied to the roof. French marigolds and basil make good greenhouse neighbours.

Training

Every week, pinch out the side shoots that grow in the angle between a leaf and the main stem. Do it when they’re small, with your fingers. Twist the stem gently around its string, or tie it to its cane, as it grows.

By early August, or when the plant reaches the roof, pinch out the top. That’s called stopping. It tells the plant to ripen the fruit it has rather than making more that won’t ripen before autumn.

Watering and feeding

Water little and often, ideally at the same time each morning. Uneven watering causes split fruit and blossom end rot. Plants in growbags may need water twice a day in hot weather.

Once the first fruits have set, feed weekly with a high-potash tomato food.

Ventilation

Open vents and doors on warm days, and shut them in the evening. Above about 30 °C, flowers drop without setting fruit, and the plants suffer. Shade paint or netting on the glass helps in a hot summer.

On hot days, damp down the floor by wetting it in the morning. It raises the humidity, helps the flowers set, and keeps red spider mite at bay.

Harvest

The first fruit ripens from July, and plants crop until October. Pick when fully coloured. In autumn, pick any remaining green fruit and ripen it indoors.

Common problems

Blossom end rot. Black patches on the base of the fruit. Water evenly.

Greenback. Hard, green shoulders on the fruit. Too much sun or too little potash. Shade the glass and feed regularly.

Whitefly. Tiny white flies under the leaves. Hang yellow sticky traps.

Grey mould. Fuzzy grey patches in cool, damp conditions. Ventilate and remove affected leaves.

For sowing dates, see when to sow tomato seeds, and set your own frost dates on the tomato page.