Growing Tomatoes in a British Greenhouse

Choosing Varieties That Suit a British Greenhouse

The British summer is a short, moody thing, which is exactly why a greenhouse earns its keep. Under glass you gain several degrees of warmth, a longer season at both ends and, crucially, protection from the rain that spreads blight spores. But not every tomato enjoys the same conditions, so pick with your greenhouse in mind.

In an unheated greenhouse, cordon (indeterminate) varieties are the mainstay. They grow tall, crop over many weeks and can be trained up a cane to make the most of limited floor space. Reliable favourites include 'Gardener's Delight' for sweet cherry trusses, 'Shirley' for heavy crops of medium fruit, and 'Moneymaker' for sheer dependability. If you fancy something more unusual, try 'Sungold' for apricot-coloured cherries or a beefsteak such as 'Big Boy' for slicing.

Where blight has been a problem in nearby outdoor crops, or if your greenhouse stays humid, look for resistant types such as 'Crimson Crush' or 'Mountain Magic'. Bush (determinate) varieties like 'Tumbling Tom' suit grow bags and containers, needing no side-shooting, which makes them handy for a small glasshouse or a sunny porch.

Sowing, Potting On and Planting Out

Timing matters more than most gardeners expect. Sow indoors or in a heated propagator from late February to mid-March, aiming to plant out in the greenhouse from late April to mid-May once night temperatures stay reliably above 10°C. Sow too early and you will nurse leggy, pale plants through a gloomy March.

Sow seeds about 1cm deep in moist seed compost at around 18–21°C. Once seedlings show their first true leaves, prick them out into 9cm pots, burying the stem slightly deeper than it was growing to encourage roots. Move them to a bright, frost-free spot and keep them ticking over until the greenhouse is ready.

• Harden off gradually: even greenhouse-raised plants benefit from a few days of daytime visits before permanent planting.
• Use fresh growing media: grow bags, ring culture pots or large containers of peat-free multipurpose compost with added John Innes. Never reuse last year's compost — it carries disease.
• Space generously: two plants per standard grow bag or one per 10–12 litre pot. Crowding is the quickest route to mould and poor air movement.
• Water the hole, not the leaves, when planting and firm the compost gently around the stem.

Supporting and Training for a Long Season

Cordon varieties need a cane or string support reaching the greenhouse ridge. Tie the stem in loosely every 20–30cm as it climbs, using soft twine in a figure-of-eight so it never strangles the plant.

Remove the side-shoots that appear in the leaf axils, pinching them out while small and snapping them sideways rather than tearing downwards. Once four or five trusses have set fruit — six in a good year — pinch out the growing tip to channel energy into ripening rather than more foliage. Keep the fruit trusses clear of leaves below them so air circulates and ripening is even. Bush varieties need none of this, but a light thinning of crowded foliage helps.

Watering and Feeding Consistently

Irregular watering is the root of most tomato disappointments: split skins, blossom end rot and leathery fruit all trace back to wet-dry swings. Little and often is the golden rule, ideally once or twice a day in high summer.

• Water in the morning so any splashes on foliage dry before evening, when humidity lingers.
• Aim at the compost, never over the leaves, and let the top centimetre dry slightly between waterings.
• Mulch the surface with compost or straw to slow evaporation.
• Shade the roots — grow bags bake in direct sun, so drape a light cloth or place them lower down.

Start feeding once the first truss has set fruit, switching to a high-potash tomato fertiliser every seven to ten days. Weak, yellowing leaves with green veins usually signal magnesium deficiency; a foliar feed of Epsom salts (about a teaspoon in a litre of water) often corrects it. Yellowing lower leaves, by contrast, are simply old and can be removed.

Ventilation, Humidity and the Blight Battle

Blight thrives in warm, damp, still air, so ventilation is your best defence. Open vents and doors early in the morning, even on cool days, to flush out humid air, then close them mid-afternoon to trap warmth for the night. In a heatwave, open everything and consider shade netting or a wash of white shading paint on the sunny side.

Aim for humidity around 60–70%. Damping down the greenhouse floor and path on hot days raises moisture without wetting foliage. Water the ground in the morning, not at dusk. Remove any leaves showing brown blotches immediately, bag them and bin them — never compost them. If blight strikes badly, clear affected plants at once to protect the rest of your crop.

Watch for pests too. Whitefly and aphids can be discouraged with yellow sticky traps and regular inspections of leaf undersides. Red spider mite appears in hot, dry conditions, so a light daily damping down helps keep it at bay. A few marigolds or basil tucked among the tomatoes may not ward off every pest, but they do no harm and certainly brighten the bench.

Harvesting and Extending the Season

Pick fruit when it is fully coloured and slightly soft at the shoulder, twisting gently rather than pulling, and harvest in the morning when sugars are at their peak. Regular picking encourages more trusses to ripen.

As September cools, strip the last green tomatoes and ripen them indoors on a windowsill or in a paper-lined drawer with a ripe banana. Alternatively, leave them on the vine, lay a fleece or bubble wrap over the plants at night, and keep the greenhouse vents closed to hold in warmth. With a little attention to variety, water and air, a British greenhouse will reward you from July right through to the first frosts.