Priya Mehta
Why a greenhouse gives you the upper hand
A British summer is a gamble. One week of grey drizzle followed by a fortnight of blistering heat is entirely normal, and outdoor tomatoes often sulk, split or succumb to blight before they ever ripen. A greenhouse tilts the odds firmly in your favour. It holds warmth on cool nights, shelters flowers from wind and rain, and keeps the foliage dry enough to dodge the worst of the blight spores that drift in on wet weather.
But a closed greenhouse has its own climate, and it is not automatically a kind one. Still, humid air encourages mould, and a plant crammed against the glass will produce leaves rather than fruit. The trick is to use the structure as a tool: managing warmth, water and airflow deliberately rather than simply shutting the door and hoping. Get those three things right and even a modest 6x4 greenhouse will keep a household in tomatoes from July to October.
Choose varieties that suit a British summer
Tomatoes fall broadly into two camps. Cordon (indeterminate) varieties grow as a single tall stem and are ideal for greenhouses, where you can train them up a cane and use the height. Bush (determinate) varieties stay compact and need little pruning, making them handy for grow bags or a low shelf.
- Reliable cordons: 'Alicante', 'Moneymaker', 'Shirley' and 'Gardener's Delight' all crop well in a British season and tolerate variable weather.
- Sweet cherries: 'Sungold' is hard to beat for flavour and ripens early, giving you fruit while larger types are still green.
- Blight-resistant types: look for varieties bred for outdoor growing, such as 'Crimson Crush', if your greenhouse has a leaky roof or you garden in a damp, western spot.
- Compact options: 'Tumbling Tom' in a hanging basket or large pot suits a tiny space, though yields are smaller.
Grow a mixture. Two or three cherry plants will give you early fruit for salads, while beefsteak types need a longer season and repay patience with big slicing tomatoes in late summer.
Start them off and pot on without a check
Sow from late February to mid-March, about 1cm deep in moist seed compost, at roughly 18–21°C. Seedlings usually appear within a week or two. As soon as the first true leaves show, prick them out into 9cm pots, burying the stem slightly deeper than it was growing — tomatoes root readily along the stem and this builds a sturdier plant.
Keep young plants at around 16–18°C in the brightest spot you have, turning them regularly so they do not lean. Leggy, pale seedlings never quite catch up. Move them to their final position in late April or May, once night temperatures stay reliably above 10°C. Use one plant per grow bag, or a 10–12 litre pot of good peat-free compost. Push a strong cane in at planting time rather than later, when you risk spearing the roots.
Water little and often — consistency beats quantity
Nothing causes more disappointment than erratic watering. Let a pot dry out and then flood it, and the fruit develops blossom end rot — a sunken black patch at the base. This is not a shortage of calcium in the compost; it is the plant's inability to move calcium around when moisture swings wildly.
Water at the same time each day, ideally in the morning, aiming at the compost rather than the leaves. In hot spells you may need a second watering in the early evening. Aim for evenly moist, never waterlogged. A layer of mulch — compost, grass clippings or even torn-up newspaper — slows evaporation and keeps the roots cooler. Begin feeding with a high-potassium tomato feed when the first truss has set, once a week at first and twice weekly by midsummer. Go easy on nitrogen-rich feeds, which give you lush leaves and very few tomatoes.
Pinch out side shoots for stronger plants
Cordons are grown as a single stem, and that means weekly pinching. Side shoots appear in the leaf axils — the join where a leaf meets the main stem. Remove them while they are small, 2–5cm long, nipping them out with your thumb and forefinger or a sharp knife. Leave them and you will get a sprawling tangle of foliage, poor airflow and a scattered, late crop.
Learn to tell a side shoot from a truss. Trusses carry tiny flower buds and usually appear opposite a leaf; side shoots are pure leaf and stem. Once five to seven trusses have set fruit, pinch out the growing tip of the main stem in late summer. This stops the plant reaching for the roof and diverts its energy into swelling and ripening what is already there. Bush varieties need no pinching at all — just let them get on with it.
Airflow, ventilation and the fight against blight
Blight thrives in warm, humid, still air, which is precisely what an unventilated greenhouse provides. Open vents and doors on mild days from spring onwards, ideally early in the morning, to let the night's damp air escape before the sun seals it in. On still, muggy afternoons a small fan makes a surprising difference.
- Water the soil, never the foliage, and avoid splashing compost onto leaves.
- Remove the lower leaves below the first truss once fruit has set. This improves airflow and reduces soil splash.
- Space plants generously — crowded foliage is grey mould's favourite habitat.
- During warm, wet spells outdoors, keep the greenhouse closed to stop airborne blight spores drifting in.
- Bin diseased leaves and stems rather than composting them.
By September, strip lower leaves and remove any late flowers that will never have time to ripen. Any remaining green tomatoes will ripen on a sunny windowsill, or in a paper bag with a ripe banana. Then give the greenhouse a good clean-out, and start planning next year's crop.
