Start Them Off Indoors, Not in the Greenhouse

Tomatoes are tender plants and a UK greenhouse in February is still a cold, damp place. Instead of sowing direct, start seed indoors on a bright windowsill or in a heated propagator, then move plants out once the weather has properly turned. Sow from late February to mid-March for the best balance: early enough for a long season, late enough that plants are not sitting leggy and sulking while light levels are still low.

Fill small pots or module trays with a good peat-free seed compost, sow two seeds per pot about 5mm deep, and keep them at roughly 18–21°C. Germination usually takes seven to fourteen days. As soon as the seedlings are up, move them somewhere cooler and brighter, around 15°C, to stop them stretching. Thin to the strongest seedling per pot by snipping the weaker one off at soil level rather than pulling it, which disturbs the roots of the one you are keeping.

Varieties worth trying in a small greenhouse include reliable cordon types such as Gardener's Delight, Shirley and Moneymaker, plus Sungold for sweet cherry fruit. If space is tight, bush or tumbling types will crop happily in a large pot on a staging shelf.

Pot On Steadily Rather Than Leaping to the Final Position

Tomatoes resent a sudden move from a tiny pot to a grow bag, so pot them on in stages. Go from module to a 9cm pot, then to a 12–15cm pot, and only then into the greenhouse border, a large pot or a grow bag. Each move should happen when roots show at the drainage holes, but before the plant becomes pot-bound.

Bury the stem slightly deeper at each potting on, right up to the seed leaves. Tomatoes will produce roots along a buried stem, which builds a stronger plant. Water the pot before moving and again after, and keep them in good light to prevent weak, drawn growth.

Harden plants off for a week before they go into an unheated greenhouse. Bring them in during the day and back indoors at night at first if the forecast is cold, and do not plant out until night temperatures stay above about 10°C. In most parts of the UK that means late April in the south and mid to late May further north.

Planting, Spacing and Support

• Use fresh growing medium each year; old compost carries disease and has lost its nutrients.
• Space cordon plants about 45cm apart. Cramming them in reduces airflow and encourages botrytis.
• If using grow bags, limit yourself to two plants per bag and cut drainage slits in the base.
• Consider ring culture: a bottomless ring of compost standing on a gravel-filled base. You water the gravel for roots and feed the ring for the plant's feeding roots.
• Put supports in at planting time. A cane, string line or greenhouse wire is far easier to install before the roots fill the space.

Tie cordon varieties in loosely with soft string as they grow, and pinch out the growing tip once four or five trusses have set, so the plant's energy goes into fruit rather than more leaves.

Water Little and Often Once Flowers Appear

This is where most greenhouse tomatoes go wrong. Up to flowering, water steadily and moderately. As soon as the first flowers open and fruit begins to set, switch to little and often — small amounts regularly rather than a drench every few days. A plant that dries out and is then flooded will take up water faster than the fruit can swell, and the skins split.

In a warm July, that can mean watering twice a day: a modest amount in the morning and another in the early afternoon. Aim for consistently moist compost, never sodden and never bone dry. Damping down the greenhouse floor and paths on hot days raises humidity, which helps flowers set and reduces stress, but avoid wetting the foliage in the evening.

Consistency matters most in the final weeks of ripening. Irregular watering at that stage also encourages blossom end rot, a dark sunken patch at the base of the fruit caused by poor calcium uptake rather than a shortage in the compost.

Feeding, Side-Shooting and Fruit Set

Start a high-potash tomato feed once the first truss has set, roughly weekly, following the dose on the bottle. Feed into moist compost, never onto dry roots. If leaves yellow between the veins, a magnesium-rich foliar feed helps where the compost has run short.

Remove sideshoots from cordon varieties weekly. These are the small shoots growing in the join between a leaf and the main stem; pinching them out when small keeps the plant tidy and directs energy into fruit. Bush varieties do not need this.

Tomatoes are largely self-pollinating, but they need a nudge in a still greenhouse. Tap each support or truss gently at midday, when pollen is dry and the air is warm, or brush trusses with a soft paintbrush. Keep the greenhouse ventilated on sunny days — open vents early and close them in late afternoon to hold the warmth overnight.

Late Season and Ripening

By late August, stop feeding and reduce watering slightly to concentrate flavour. Remove leaves below the lowest truss to let light and air in, and take the growing tip out if it has not already been pinched. Fruit that will not ripen before the nights turn cold can be picked green and ripened indoors on a sunny windowsill, or used green in chutney.

Watch for blight in damp, muggy spells, particularly after rain and warm nights; good ventilation and prompt removal of affected leaves are your best defences. Clear the greenhouse of spent plants, compost them properly, and wash down the glass and staging before winter. A clean greenhouse is the quiet secret to next year's crop.