Growing Tomatoes in a Greenhouse

Tomatoes are the crop that makes a greenhouse worth having. Under glass they ripen weeks earlier than outdoor plants, crop more heavily over a much longer season, and — best of all — stay largely clear of the blight that so often wrecks outdoor tomatoes in a wet British August. If you have a greenhouse, even a small unheated one, tomatoes should be near the top of your planting list.

The trade-off is attention. A greenhouse in July gets hot, dries out fast, and concentrates pests if you ignore it. None of it is hard, but tomatoes reward a daily five-minute habit. This guide walks you through the whole season under glass, and sits alongside the broader greenhouse growing guide and the main how to grow tomatoes guide if you want the wider picture.

Best varieties for under glass

Most greenhouse tomatoes are grown as a cordon — a single upright stem trained up a string or cane, with side-shoots removed. Cordon (also called "indeterminate") varieties keep growing and fruiting all season, which is exactly what you want when you've gone to the trouble of a protected space. These are the reliable UK favourites:

• Sungold — a small, intensely sweet orange cherry. An F1 hybrid, so it crops early and heavily and shrugs off a few problems. The variety most people fall in love with.
• Shirley — a dependable F1 that sets fruit even in cooler early-summer spells, which makes it forgiving in an unheated greenhouse. Good medium-sized salad fruit.
• Alicante — an old, easy, open-pollinated favourite. Smooth round fruit, heavy crops, and seed you can save yourself.
• Gardener's Delight — a vigorous cherry with old-fashioned flavour; tolerant and beginner-friendly.

If you only want one plant, pick Sungold for flavour or Shirley for reliability. For a fuller rundown across types, see our tomato varieties guide. Bush ("determinate") tomatoes also exist, but they suit pots and hanging baskets more than a greenhouse border, so stick with cordons here.

Planting

You have three sensible options for where the roots go, and each has its place.

Border soil gives the strongest, least thirsty plants because the roots range freely. The catch is disease build-up: grow tomatoes in the same border every year and you risk soil-borne problems. Either swap the top 30cm of soil annually, or rotate to growbags every few years to break the cycle.

Growbags are the classic greenhouse choice — clean, cheap and disease-free each year. Two plants per bag is plenty; three is a stretch that leaves them hungry and dry. Growbags hold little water, so they need the most frequent attention.

Large pots (at least 30cm / 10 litres each) sit between the two: more root room than a growbag, still fresh compost each year. Good peat-free multipurpose compost is fine.

Whichever you choose, wait until nights are reliably above about 7–8°C before planting — that's mid-April to mid-May for most of the UK, later in the north or a cold spring. Check your area with the frost-date checker and the planting calendar. Space plants about 45cm apart, water them in, and give each one its support before the stem gets tall.

Training cordons

This is the part that feels fussy at first and becomes second nature within a fortnight. The aim is one clean stem carrying a ladder of fruit trusses.

Support. Either push a tall cane in next to each plant, or — neater in a greenhouse — tie a length of soft string to the roof and loop the base loosely around the plant's stem, then twist the growing tip around the string as it climbs.

Side-shooting. Tomatoes throw out shoots in the 45-degree angle between the main stem and each leaf. On a cordon, pinch these out while they're small — a quick snap between finger and thumb. Leave them and you get a tangled bush, lots of leaf and far less ripe fruit. Do a sweep every few days; it takes seconds.

Tying. Add a soft tie every 20–30cm as the plant grows, keeping it loose enough not to cut into the swelling stem.

Stopping. In an unheated UK greenhouse, pinch out the growing tip two leaves above the fourth or fifth truss, usually in August. This stops the plant wasting late-summer energy on fruit that will never ripen before the light fails, and pushes everything into the trusses already set.

Watering and feeding

Consistency is the whole game here, and it's where most beginner crops come unstuck. Tomatoes want steady, even moisture — not a drought followed by a flood. The two commonest faults both trace straight back to erratic watering:

• Split fruit happens when a dry plant suddenly gets a big drink: the skin can't stretch fast enough and cracks. The fix is little and often rather than occasional soakings.
• Blossom end rot — a sunken brown patch on the base of the fruit — is a calcium problem, but the real cause is usually irregular watering stopping the plant moving calcium around. Keep the moisture even and it largely disappears.

In high summer a greenhouse plant in a growbag may need watering twice a day. Aim for evenly damp, never waterlogged. Mulching the surface or standing pots on damp capillary matting buys you breathing room on hot days.

Tomatoes are hungry once they start fruiting. From the moment the first tiny fruits set, feed weekly with a high-potash liquid tomato feed (the bottle will say "tomato food" and list a high K figure) — potash drives flowering and fruiting rather than leaf. Our watering and feeding tomatoes guide goes into the routine in detail. Stick to the label dilution; overfeeding does more harm than skipping a week.

Ventilation and pollination

A closed greenhouse on a sunny June day can hit 35°C, and tomatoes stop setting fruit when it gets that hot. Ventilation is not optional. Open roof vents and the door every morning once the weather warms, and on hot days leave them open all day — a louvre vent or two and an automatic vent opener take the guesswork out of it while you're at work. Good airflow also keeps humidity down, which discourages disease.

Outdoors, wind and insects shake pollen loose for free. Inside, you sometimes need to lend a hand. Keeping vents and doors open lets bees and hoverflies in to do the pollination for you, which is the easiest route. On warm days you can also tap or gently shake each flowering truss daily, or lightly mist the flowers in the morning — both help the pollen drop and the fruit set. If whole trusses flower and then drop without setting, heat and dry air are the usual culprits, so ventilate harder.

Problems under glass

A greenhouse swaps one set of problems for another. The warm, sheltered conditions that suit tomatoes also suit several pests, so a quick daily look pays off.

Whitefly are tiny white flies that rise in a cloud when you brush the plants, leaving sticky honeydew behind. Hang yellow sticky traps early, encourage ventilation, and consider the biological control Encarsia if they take hold.

Red spider mite is the classic hot-greenhouse pest, thriving in dry air. The first sign is fine pale mottling on the leaves and, later, faint webbing. Mist plants and damp down the floor on hot days to raise humidity — they hate it — and act early, as a bad infestation is hard to shift.

Blight is much rarer under glass but not impossible, especially if you grow outdoor tomatoes or potatoes nearby and spores blow in through open vents. Watch for fast-spreading brown blotches on leaves and stems in warm, humid spells. Keeping foliage dry (water the soil, not the plant), ventilating well, and removing lower leaves all reduce the risk. If you spot it, our tomato blight guide explains what to do next.

Stay on top of the daily basics — water evenly, feed weekly, ventilate hard, snap out side-shoots — and a greenhouse will give you the easiest, heaviest tomato crop you can grow in the UK. For more on getting the most from the structure itself, head back to the greenhouse growing guide and browse the rest of our vegetable guides.