Why tomatoes thrive under glass

Tomatoes are warmth-loving plants from South America, and in most of the UK they spend the summer waiting for a decent spell of weather. A greenhouse takes that gamble out of the equation. It shelters plants from wind and rain, keeps foliage dry enough to dodge blight for longer, traps heat well into autumn, and gives you a ripening season that can stretch from July to October. Even a small unheated 6x6 house is enough for four or five cordon plants, which is more fruit than most families can eat fresh.

The key is to treat the greenhouse as a managed environment rather than just a shelter. Tomatoes reward consistency: steady moisture, steady warmth, steady feeding. Get those roughly right and the crop looks after itself.

Choosing varieties and sowing the seed

For greenhouse growing, cordon (indeterminate) varieties give the best returns. They grow tall, crop over a long period and are easy to train up a cane. Reliable choices include 'Gardener's Delight' for sweet cherry fruit, 'Alicante' and 'Shirley' for classic medium tomatoes, and 'Sungold' if you like something really sugary. Bush or determinate types such as 'Tumbling Tom' suit pots and grow bags where headroom is limited, but they crop in a shorter, heavier flush.

Sow from late February to mid-March. Fill small pots or modules with moist seed compost, sow seeds about 1cm deep, and keep them at 18-21C. Germination usually takes seven to fourteen days. As soon as the seedlings appear, move them into the brightest spot you have — a windowsill or a heated propagator lid removed — because weak, drawn-up seedlings never fully recover.

Potting on and planting into the greenhouse

• Prick out seedlings into 9cm pots when the first true leaves appear, planting slightly deeper than they were growing.
• Move on into 2-litre pots once roots fill the compost, or plant straight into grow bags or the border.
• Plant into the greenhouse from late April in mild areas, or mid to late May elsewhere, once night temperatures stay above 10C.
• Bury the stem up to the first pair of leaves. Tomatoes root along the buried stem, which makes for a stronger plant.
• Allow two cordon plants per standard grow bag, and put a cane in at planting time so you never disturb the roots later.

If you grow in border soil, add plenty of compost and a handful of general-purpose fertiliser before planting. Rotate the position each year if you can, as tomatoes leave behind soil pests and diseases.

Watering and feeding: the heart of the routine

Irregular watering is the number one cause of disappointment. It leads to split fruit and blossom end rot, where the base of the tomato turns black and leathery. Aim for compost that is consistently moist but never sodden. In cool spring weather, once every couple of days is plenty. By July, plants in grow bags may need water twice a day, ideally in the morning, with a second check in early evening on hot days.

Start feeding once the first truss of flowers has set fruit. Use a high-potash tomato feed at the manufacturer's strength, once a week at first and twice weekly once plants are carrying several trusses. Don't feed before fruit set, or you will grow a magnificent leafy plant with very few tomatoes. A little Epsom salts diluted in water once a fortnight helps prevent yellowing between the veins on hungry plants.

Ventilation, temperature and pollination

Open vents and doors early on sunny mornings and aim to keep the house between 21C and 27C. Above about 27C, tomato pollen becomes sticky and fails to set, so fruit simply doesn't form. Close up again in mid-afternoon to trap warmth for the evening. Damping down the floor with a watering can raises humidity and helps flowers set in hot spells.

Tomatoes are self-pollinating but need a nudge indoors where there is no breeze. Tap each support gently at mid-morning, or mist the flowers lightly, and you will see far better fruit set. Good air movement also keeps botrytis (grey mould) at bay, especially in a crowded house.

Training, pests and harvesting

Cordon plants need a single stem. Tie it to its cane as it grows and pinch out the side shoots that appear in the leaf joints every week or so — they are easy to snap out with your fingers while small. Once four or five trusses have set, stop the plant by pinching the growing tip two leaves above the top truss. Remove leaves below the first truss as they yellow, but don't strip the plant bare.

Watch for whitefly on the underside of leaves, red spider mite in hot dry conditions, and the first signs of blight in wet August weather. Healthy, well-ventilated, well-fed plants resist all of these far better than stressed ones.

Pick fruit when it is fully coloured and slightly soft, and ripen it on a windowsill rather than in the fridge, which kills the flavour. Harvest little and often to keep plants cropping. At the end of the season, any green tomatoes will ripen indoors in a drawer with a ripe banana.