Growing Tomatoes in a Greenhouse or Polytunnel

Why Grow Tomatoes Under Cover?

There is nothing quite like the taste of a tomato picked warm from the vine, still smelling of the greenhouse. In the UK, growing under glass or polythene turns a gamble into a reliable harvest. Our summers are often too short and too cool for outdoor varieties to ripen properly, but a greenhouse or polytunnel gives you a protected microclimate with warmer days, gentler nights, and shelter from wind and rain. That extra warmth can add weeks to the start and end of your season, and it also reduces the risk of blight, which thrives in wet, humid weather. Whether you have a small lean-to greenhouse or a roomy polytunnel, the principles are the same: warmth, steady watering, good ventilation, and sturdy support.

Choosing the Right Varieties

Not all tomatoes suit every space. In a greenhouse or polytunnel, cordon (indeterminate) varieties are the classic choice. They grow tall on a single stem and keep cropping until the first frost, which makes the most of your valuable under-cover room. Popular options include reliable beefsteaks, sweet cherry types, and traditional salad tomatoes.

• Cordon varieties for tall growing and long cropping — ideal for the back wall of a greenhouse.
• Bush (determinate) varieties for pots and hanging baskets, needing little or no pinching out.
• Heritage or beefsteak types for flavour, though they need a longer season and more warmth.
• Blight-resistant varieties as a sensible insurance policy in damper regions.

If you are growing in a polytunnel, remember that the plastic filters out less light than glass, so plants can grow faster and taller. That is a blessing, but it means you must stay on top of training and support.

Sowing, Potting On, and Planting Out

Sow seeds from late February to mid-March indoors, in a warm room or on a heated propagator. Tomato seeds germinate best at around 18–21°C. Once the seedlings have their first true leaves, prick them out into individual 9cm pots of multi-purpose compost and grow them on somewhere bright and frost-free.

Do not be tempted to plant them into the greenhouse border or grow bags too early. The soil or compost needs to be consistently above 12°C, so aim for late April in the south and mid to late May further north. Cold roots sulk, turn purple, and stall for weeks. If in doubt, wait. A plant set out in warm soil will overtake one planted early almost every time.

Before planting, work plenty of well-rotted compost into the border, or use fresh grow bags. Add a handful of blood, fish and bone or a slow-release tomato fertiliser. Space cordon plants about 45cm apart, and water them in well.

Watering and Feeding for Steady Growth

Tomatoes hate feast and famine. Irregular watering is the main cause of split fruit and blossom end rot, where a dark, sunken patch appears at the base of the tomato. The answer is consistency, not quantity. Aim to keep the compost evenly moist, watering little and often in warm weather.

• Water in the morning so foliage dries before evening, reducing the risk of disease.
• Water the soil, not the leaves, and avoid splashing compost onto the stems.
• Start feeding when the first truss of flowers sets fruit, using a high-potash tomato feed once a week.
• Reduce watering slightly as fruit ripens, which concentrates flavour and discourages splitting.

In a polytunnel, the soil can dry surprisingly fast on a sunny day, so check moisture by hand every morning. A soil moisture meter is a cheap and useful ally if you are forgetful.

Training, Support, and Pinching Out

Cordon tomatoes need a tall cane or a length of string hung from a overhead wire. Twist the stem gently around the support as it grows. Remove the side shoots that appear in the leaf joints every few days — these are the little shoots between the main stem and a leaf. Pinching them out keeps energy going into fruit rather than foliage. Stop the plant by removing the growing tip two or three trusses above the last set of flowers, usually in late summer, so the remaining fruit has time to ripen.

Once four or five trusses have set, you can also remove some lower leaves to improve airflow and let light reach ripening fruit. Do not strip the plant bare; the leaves feed the crop.

Ventilation, Pollination, and Pests

Good airflow is the single best defence against mould, botrytis, and blight. Open vents and doors on warm days, but close them before evening to trap warmth. On still days, tap the canes or gently shake the flowers at midday to help pollination — tomatoes need a little movement to release pollen. A soft paintbrush works too.

Keep an eye out for whitefly, red spider mite, and aphids. Damp down the floor on hot days to raise humidity and deter spider mite. Companion planting with basil, marigolds, or nasturtiums can help, and a healthy, well-fed plant shrugs off pests far better than a stressed one.

Harvesting and Late-Season Tips

Pick tomatoes when they are fully coloured and slightly soft at the shoulder. Regular picking encourages more fruit to set. As autumn approaches and light fades, remove any remaining flowers so the plant concentrates on ripening what is already there. Green tomatoes can be ripened indoors on a sunny windowsill, or turned into chutney and green tomato relish. By giving your plants warmth, steady water, and sensible support from the start, you will be rewarded with punnets of fruit that taste of summer sunshine — and far better than anything from the shops.