Why tomatoes love a UK greenhouse

Tomatoes are tender South American plants, happiest in steady warmth, bright light and dry air around the leaves. The British outdoors manages that occasionally. A greenhouse manages it most days from May to September, which is why the same variety grown under glass will usually crop a fortnight earlier and keep ripening into October. Shelter from wind and rain also cuts the risk of blight, the fungal disease that can level an outdoor crop in a wet August. The trade-off is that you become the weather. Under glass there is no rain to top up the soil, no breeze to dry the foliage and fewer insects working the flowers, so watering, ventilating and a daily tap of the trusses are your jobs. Get those right and a modest 6x8 greenhouse will keep a household in salad tomatoes from July to the first frosts.

Choosing varieties: cordon or bush

Decide the habit before you sow, because it determines how much work you are signing up for. Cordon (indeterminate) types grow as one tall stem, throwing a new truss of fruit roughly every other leaf. They need a cane, weekly tying-in and the removal of side shoots, but they crop over a long season and use the full height of a greenhouse. Bush (determinate) types stop on their own at a set height and branch freely, needing almost no training — ideal for grow bags and low frames under a sloping roof. A useful mix for most greenhouses:

• Reliable cordons: 'Gardener's Delight' for sweet cherries, 'Alicante' and 'Moneymaker' for dependable medium fruit, 'Shirley' for an early slicer.
• Good bush types: 'Tumbling Tom' for hanging baskets and pots, 'Totem' and 'Red Alert' for compact plants that need no pinching.
• Flavour first: 'Costoluto Fiorentino' and 'Black Russian' are less tidy but taste superb warm from the vine.

Two or three cordons of a cherry type plus a beefsteak or two is a sensible balance for a small house.

Planting, watering and feeding

Move plants into their final positions once the first flowers open on the second truss — usually late May in a heated house, early June in a cold one. Use fresh peat-free compost, never soil from last year's bed, and plant deeply with the stem buried to just below the seed leaves; tomatoes root along it and gain stability. One plant per grow bag or a 10-litre pot is plenty. Water little and often rather than drenching once a week, aiming for consistently moist compost and never a flooded then bone-dry cycle, which invites blossom end rot. In hot spells that may mean twice daily. Start feeding only when the first truss has set — before that, food encourages leaves at the expense of fruit. Switch to a high-potash tomato feed weekly, following the bottle rather than the instinct, and add a pinch of Epsom salts if leaves yellow between the veins.

Ventilation, shading and pollination

Tomatoes set fruit poorly above about 27°C and above 80 per cent humidity, so ventilation is not optional. Open roof vents and doors on any morning that promises sunshine, ideally by mid-morning, and close them in late afternoon to trap the evening warmth. Automatic vent openers are worth every penny if you are out at work. From June, shade the glass on the sunny side with a paint-on shade or horticultural netting to prevent leaf scorch and stop the house turning into an oven. Pollination needs help indoors: gently tap each cane or flick the flowers with a finger around midday, when the pollen is dry and loose. A quick mist of the floor in the early afternoon raises humidity briefly and helps pollen stick.

Pinching out and training

For cordons, pinch out the side shoots that appear between the leaf and the main stem while they are still small enough to snap off with finger and thumb. Check weekly — a shoot left for a fortnight becomes a woody distraction. Let the plant grow one stem, tied to a cane or string every 20cm, and stop the growing tip in late July or once six or seven trusses have set, so the plant's energy goes into ripening what is already there. Remove the lower leaves below the first ripening truss to improve airflow. Bush varieties need none of this: leave them alone apart from a light trim if they crowd their neighbours.

Pests, problems and picking

Whitefly clouds rise when plants are disturbed, and their sticky honeydew encourages sooty mould. Hang yellow sticky traps early, and inspect the underside of leaves weekly. Grey mould follows cool, damp, still air, so keep up the ventilation and remove damaged foliage promptly. Blossom end rot — a sunken black patch at the base of the fruit — is a calcium transport problem caused by erratic watering, not a shortage in the compost, so steady the can. Pick fruit when fully coloured, or at first blush and ripen it on a sunny windowsill if blight threatens or frost is forecast. Ripening slows once nights drop below 10°C, which is your cue to strip the plants, clear the debris and start planning next year's sowing.