Growing Tomatoes in a British Greenhouse

Why a Greenhouse Makes All the Difference

British summers are a gamble. One week brings Mediterranean heat, the next a cold, damp spell that turns outdoor tomato plants sulky and prone to blight. A greenhouse changes the odds entirely. It traps warmth, keeps rain off the foliage and extends your season by weeks at both ends. Even an unheated lean-to against a south-facing wall will outperform the open plot. The key is to work with the space you have, not against it. Ventilation matters as much as warmth: on sunny days, open the door and roof vents by mid-morning to stop the air becoming stale and humid, which invites botrytis and mildew. Aim for a steady 18–24°C by day and no lower than 10°C at night.

Sowing Under Cover

Start seeds indoors or in a heated propagator from late February to mid-March for greenhouse cropping. Fill small pots or modules with peat-free seed compost, sow two seeds per pot at about 1cm deep, and water gently. Keep the compost moist but not sodden. Germination takes seven to fourteen days at 18–21°C. Once the seedlings have their first true leaves, prick them out into 9cm pots, handling them by the leaf, never the stem. Grow them on in good light to prevent leggy stems. If you have a windowsill that faces south, that will do until the greenhouse is reliably frost-free, usually from late April in the south and mid-May further north. Harden plants off over a week before planting out, bringing them in at night at first.

Planting Out and Supporting Cordon Varieties

Most greenhouse tomatoes are cordon (indeterminate) types: 'Gardener's Delight', 'Alicante', 'Moneymaker'. They grow as a single tall stem and need support from the moment they go into their final position. Plant them into grow bags, large pots or a border bed with plenty of compost. Space them 45–60cm apart. Push a sturdy cane or length of string up to the greenhouse roof, then twist the growing stem around it as it climbs. For string, tie it loosely to a horizontal wire at the top and wind the plant clockwise. Remove side shoots that appear between the main stem and each leaf – pinch them out when small, ideally on a warm morning so the wound dries quickly. Once the plant has set four or five trusses (or reaches the roof), pinch out the growing tip to direct energy into fruit. Bush varieties such as 'Tumbling Tom' need no side-shooting, just a little room to sprawl.

Watering: Consistency Beats Quantity

Tomatoes hate dramatic swings between drought and flood. Irregular watering is the main cause of blossom end rot – a sunken black patch at the base of the fruit. Water little and often, aiming for a consistently moist root zone. In warm weather, greenhouse plants may need water twice a day; in cool, cloudy spells, once every two or three days is enough. Water in the morning, directly onto the soil, keeping the leaves dry. A mulch of straw or grass clippings over the surface will lock in moisture and suppress weeds. As the plants grow, increase the volume of water, but never let the compost become waterlogged. If you use grow bags, consider a drip irrigation kit on a timer – it pays for itself during a busy week.

• Water in the morning, aiming at the compost, not the leaves.
• Check moisture daily by touch – if it's dry 2cm down, water.
• Use a mulch to slow evaporation and keep roots cool.
• Never let pots stand in a saucer of water for long.

Feeding Weekly Once Flowers Set

Start feeding when the first little yellow flowers open and small green fruits begin to form – usually around six to eight weeks after planting out. Use a high-potassium tomato fertiliser, diluted according to the bottle, and apply it once a week with your watering. If the leaves start to yellow between the veins, a dose of magnesium sulphate (Epsom salts) will green them up. Stop feeding by early September to let the final trusses ripen. To help pollination, gently tap each flowering truss with your finger or a soft brush at midday. Greenhouse air can be still, and without a little vibration the pollen stays put. A small fan on low, or opening vents, also aids air movement. Avoid overhead watering on flowers, which can cause them to drop.

Harvest and Late-Season Care

Pick fruit when it is fully coloured and slightly soft, or leave it on the plant for a day or two longer for deeper flavour. Regular picking encourages more to ripen. As autumn approaches, remove lower leaves that have finished photosynthesising to let light and air around the fruit. If frost is forecast, pick any green tomatoes and ripen them indoors in a paper bag with a ripe banana, or use them in chutney. With a little attention, a British greenhouse can yield ripe tomatoes from July right through to October – a taste of summer long after the outdoor crop has given up.