Growing Tomatoes in a Greenhouse

Why greenhouse tomatoes are worth the effort

A greenhouse transforms tomato growing in the UK. Outdoor tomatoes ripen in a race against the first frosts, and blight can wipe out a crop in a wet August. Under glass or polycarbonate, you control the water, the warmth and the airflow, which means healthier plants and a far longer picking season. Whether you have a small lean-to, a walk-in polytunnel or a modest 6x8 foot greenhouse on an allotment, the principles are the same. Get watering, ventilation and feeding right, keep the side shoots in check, and you will be eating your own tomatoes from July right through to October.

Choosing varieties that suit a greenhouse

Indeterminate, or cordon, varieties are the classic greenhouse choice because they keep growing and cropping until you stop them. They need staking and regular attention, but they reward you with fruit over many weeks.

• Reliable croppers: 'Gardener's Delight' for sweet cherry fruit, 'Alicante' and 'Shirley' for medium slicers, 'Moneymaker' for a steady, dependable supply.
• Beefsteak types: 'Big Boy' or 'Ferline' if you want large fruit for sandwiches. Ferline has useful blight resistance for later in the season.
• For flavour: 'Sungold' is exceptionally sweet, though the thin skins mean it splits if watering is uneven.
• Bush or determinate types: 'Tumbling Tom' works in a large pot or grow bag and needs almost no pruning, handy if space is tight.

If you are growing in grow bags or containers, choose compact varieties and be prepared to water far more often than you would for plants in border soil.

Planting out and getting the watering right

Move plants into the greenhouse once night temperatures stay above about 10°C, usually from late April in southern England and mid to late May further north. Harden them off for a week first. Plant deeply, burying the stem up to the first set of leaves, because tomatoes root along the buried stem and that builds a stronger plant.

Watering is where most greenhouse crops succeed or fail. The golden rule is little and often, and never let the compost swing from bone dry to sodden. That fluctuation causes blossom end rot, the black sunken patch on the base of the fruit, and it also splits skins. Aim to keep the soil consistently moist, watering in the morning so that any splashes on the leaves dry before evening.

Once plants are established in border soil, a good soak every two or three days in warm weather is usually enough. In grow bags or pots, you may need to water twice a day in July and August. Push your finger two inches into the compost before deciding. A simple drip irrigation kit on a timer takes the guesswork out of it and is well worth the investment if you are away at work all day.

Ventilation, shade and keeping the air moving

Tomatoes dislike stagnant, humid air as much as they dislike drought. Poor airflow encourages botrytis, the grey mould that turns stems and fruit soft and furry. Open vents on the leeward side first thing in the morning, even on cloudy days, and close them in late afternoon to hold some warmth for the evening.

• On hot days: open doors and roof vents together so hot air escapes at the top and cooler air is drawn in low down.
• In prolonged heat: damp down the floor with a watering can to raise humidity slightly and cool the space.
• In strong sun: use shade netting or paint the glass with a diluted shading wash from mid-June to prevent scorching and fruit that ripens unevenly.
• Keep the space tidy: remove fallen leaves and spent trusses so mould has nowhere to take hold.

Pinching out side shoots and supporting the plants

Cordon tomatoes produce a side shoot in the joint between each leaf and the main stem. Left alone, these turn into competing stems and the crop becomes a jungle with small, slow-ripening fruit. Snap them out by hand when they are small, ideally once a week, and you will keep the plant focused on one strong leader.

Tie the main stem to a cane, string line or overhead wire, using soft twine in a loose figure of eight so the stem is not pinched. Once the plant has set four or five trusses, usually in late July or August, pinch out the growing tip two leaves above the top truss. This tells the plant to stop stretching and put its energy into swelling and ripening the fruit already set.

Remove the lower leaves below the first truss once fruit has set, as they are of little use and only reduce airflow. Do this with a clean knife and take only a few at a time.

Feeding, setting fruit and harvesting

Start feeding once the first truss has set small fruits. Before that, the plants have enough reserves in the compost. After flowering begins, use a high-potassium tomato feed at the strength recommended on the bottle, usually once a week, increasing to twice a week in peak season. Overfeeding causes leaf curl and lush growth at the expense of fruit, so resist the urge to double up.

To help fruit set, tap the supports or gently shake the trusses at midday when flowers are open, which moves pollen around in a still greenhouse. A soft brush works too. Keep watering steady as the fruit swells, and pick when the colour is deep and even. Tomatoes ripen best at room temperature, so leave them on the windowsill rather than the fridge, and enjoy the crop that your care has produced.