Why a greenhouse earns its keep

Tomatoes are a long-season crop that would really rather be growing in southern Italy. Give them a British summer and they will sulk: cool nights stall growth, wet leaves invite blight, and a single grey August can turn a promising crop into a heap of rotting stems. A greenhouse changes that calculation entirely. It holds warmth overnight, keeps rain off the foliage, and gives you a fortnight or more of extra ripening time at each end of the season. Even a small 6x4 model will comfortably crop four or five cordon plants, which is more fruit than most households can eat fresh.

The secret is not the greenhouse itself but the routine you keep inside it. Get the timing, the training and the feeding right, and you will be picking ripe tomatoes from July well into October.

Sowing seeds under cover

Sow from late February to mid-March if you can provide warmth, or wait until early April in an unheated greenhouse. Tomatoes germinate best at a steady 18–21°C, so a heated propagator, a sunny windowsill or a warm airing cupboard for the first few days works well.

• Sow individually into modules or small pots, about 1cm deep, using a peat-free multipurpose compost.
• Keep the compost moist but not sodden. Seedlings should appear in 7–14 days.
• Move them into bright light the moment they break the surface, or they will stretch into pale, leggy stems.
• Prick out into 9cm pots once two proper leaves have formed, planting slightly deeper than they were growing.
• Pot on into 12–15cm pots when roots show at the drainage holes, and keep them growing steadily until planting time.

Choose a variety that suits your space. 'Gardener's Delight' and 'Sungold' are superb cherries, 'Alicante' and 'Moneymaker' are reliable classics, and bush types such as 'Tumbling Tom' need no pinching at all.

Planting into the greenhouse

Aim to plant out in late May, once night temperatures stay above 10°C. Cold nights in early May will check growth badly, so be patient.

In a border, fork in plenty of garden compost and a handful of general-purpose fertiliser per square metre, plus a little horticultural grit if your soil is heavy. Grow bags are perfectly good too, but restrict yourself to two cordon plants per bag rather than the three often suggested — crowded roots mean smaller fruit and more blossom end rot.

• Space cordon plants 45–60cm apart, with a cane or a length of string dropping from the greenhouse roof frame.
• Water the planting hole well, then leave the plants alone for a few days to settle.
• Twist the string gently around the stem as it grows, or tie it loosely to the cane with soft twine.

Pinching out and training

Most greenhouse tomatoes are cordons, grown as a single tall stem. They will try to branch at every leaf joint, and those side shoots must go. Snap them out with your fingers while they are still small — roughly 5cm — taking care not to damage the main stem. Check weekly; in warm weather they appear almost overnight.

Bush varieties are different: leave them to branch naturally and simply support the trusses if they become heavy. With cordons, stop the growing tip in late July or early August, two leaves above the fourth or fifth truss. This directs energy into ripening fruit rather than more foliage, which matters enormously in a short British season. Once the lowest trusses begin to colour, strip away the leaves beneath them to improve airflow and let light reach the fruit.

Watering and feeding

Little and often is the golden rule. Tomatoes hate extremes: dry spells followed by a soaking cause split skins and blossom end rot. Water in the morning, directly onto the compost, and try to keep the foliage dry. In hot weather you may need to water twice a day.

Start feeding when the first truss has set its fruit — not before, or you will get leaves at the expense of tomatoes. Use a high-potash tomato feed at the dilution on the bottle, once a week at first and then at every other watering from mid-summer. If leaves yellow between the veins, a foliar feed of Epsom salts will correct the magnesium shortage that greenhouse tomatoes commonly suffer.

Ventilation, pests and the final ripening

Open vents and doors once the temperature climbs past 25°C, and damp down the path on hot days to raise humidity and deter red spider mite. Watch for whitefly on the underside of leaves and remove any foliage showing grey mould promptly, especially in a damp autumn.

As September cools, remove lower leaves, ease off the watering and stop feeding. Any fruit still green at the end of October will ripen on a sunny windowsill, and the rest makes excellent chutney. With a little attention, a British greenhouse really will give you a crop worth boasting about.