Growing tomatoes in a British greenhouse

Why grow tomatoes in a greenhouse?

Nothing from a shop comes close to a tomato picked warm from the vine, still smelling of the stem. Supermarket fruit is picked under-ripe and chilled for transport, which is why it so often tastes of very little. A greenhouse changes that entirely — and in a British summer, it is usually the difference between a decent crop and a disappointing one.

Glass or polycarbonate keeps rain off the foliage, which is your single biggest defence against blight, the disease that turns outdoor tomato plants to brown mush by August. It also traps warmth, so fruit ripens earlier and keeps coming well into autumn, and it shelters plants from wind that snaps stems and chills the roots. Even a modest 6x4 greenhouse will hold two growbags, which is easily enough for a family. Get the basics right and you will be picking tomatoes from late June through to October.

Pick the right varieties for your space

Tomatoes fall into two main habits, and knowing which you have governs everything that follows.

• Cordon (indeterminate) varieties grow tall and keep producing until stopped. They need a cane or string support, and their side shoots must be removed. Good choices include 'Alicante', 'Shirley', 'Gardener's Delight' and the orange cherry 'Sungold'.
• Bush (determinate) varieties stay compact and bushy, need almost no pruning, and are ideal for pots, growbags and hanging baskets. 'Tumbling Tom' is excellent trailing over a container edge, while 'Roma' is a reliable plum for sauces.
• Beefsteak types such as 'Costoluto Fiorentino' produce huge, meaty fruit for slicing, but they demand very steady watering or the skins split.

Cherry tomatoes are the most forgiving and the quickest to ripen, so they are a sensible choice for a first season or a cool, shaded greenhouse. As a rough guide, allow one cordon plant per growbag planting ring, or space plants 45–60cm apart in a border.

Planting out: timing and technique

Do not rush the move. Wait until late May, ideally early June, when night temperatures stay above about 10°C. Cold nights check growth badly and can trigger the purplish leaf tint that signals a struggling plant. Harden young plants off over a week by leaving them in a sheltered spot by day, bringing them in at night.

Plant deeper than they were in their pots, burying the stem right up to the lowest leaves — tomatoes root along the buried stem, which gives you a stronger plant and better drought tolerance. Water the hole well before planting, firm the compost gently around the roots, and water again. If you use growbags, stand them on a shallow tray or use bottomless pots sunk into the bag for ring culture, because the compost volume in a bag is small and dries out fast.

Watering and feeding: the two habits that matter most

Inconsistent watering is the cause of most greenhouse tomato troubles. Let plants dry out and then flood them, and you invite blossom end rot, the sunken black patch on the base of the fruit. It is not a disease but a calcium uptake problem triggered by irregular moisture.

Water little and often, aiming for compost that feels like a wrung-out sponge, and always at the base rather than over the leaves. Morning is best, as it gives any splashes time to dry. In hot weather you may need to water twice a day.

Begin feeding once the first truss has set fruit — small tomatoes the size of peas. Use a high-potash tomato feed once a week, diluted according to the bottle, and increase to twice weekly as the plant carries several trusses. General-purpose feed is the wrong tool here: too much nitrogen gives you a forest of leaves and precious little fruit.

Side shoots, supports and pinching out

On cordon varieties, a side shoot appears in the angle between a leaf and the main stem. Left alone, each one becomes another sprawling stem, sapping energy and blocking light. Snap them out with your fingers while they are still small — a clean pinch, no scissors needed — and check every plant weekly, as they reappear fast in warm weather.

Tie the main stem to its cane or wind it around a length of string as it grows, using soft twine in a loose figure of eight so the stem is not pinched. Once five or six trusses have set, pinch out the growing tip. This tells the plant to stop climbing and put its energy into ripening the fruit you already have. Bush varieties need none of this — leave them to get on with it.

Keeping the greenhouse healthy — and getting the crop home

Ventilation matters as much as watering. Open vents and doors on warm mornings and close them in late afternoon to trap the evening heat. On very hot days, damp down the path to raise humidity, which helps flowers set fruit. Watch for whitefly on the underside of leaves, and act early with sticky traps or a soap spray rather than waiting for an infestation.

Check plants daily during humid spells for brown or grey patches on leaves and stems. If blight appears, remove affected foliage immediately and bin it — never compost it. Yellowing between the veins usually means magnesium deficiency; a dilute foliar spray of Epsom salts will perk the plant up within a fortnight.

Pick fruit when it is fully coloured and gives slightly to a gentle squeeze, and keep harvesting — the plant will keep producing. As autumn closes in, bring any green tomatoes indoors and ripen them on a sunny windowsill alongside a ripe banana, or turn them into chutney. That way, nothing goes to waste and you will already be planning next year's varieties.