Growing Tomatoes in a Small Greenhouse or Conservatory

Why a Small Greenhouse or Conservatory Suits Tomatoes So Well

Tomatoes are warmth-lovers from South America, and in most of the UK they need some protection to crop reliably. A greenhouse or conservatory gives you that protection, plus a longer season and shelter from the wind and rain that can ruin outdoor plants. The catch is that small spaces heat up fast, dry out quickly and can become humid overnight, so success comes down to choosing the right varieties and staying on top of watering and feeding. Get those right and a modest 6ft by 8ft greenhouse can easily keep a household in salad tomatoes from July to October.

Choose Compact Varieties for Tight Spaces

In a small space, every plant has to earn its place. Bush varieties and dwarf cordons are the obvious starting point because they stay manageable without endless tying-in and pinching.

• Tumbling Tom and Tumbler — trailing bush types, ideal in a hanging basket or a pot on a conservatory windowsill.
• Minibel and Garden Pearl — genuinely tiny plants producing a surprising number of cherry-sized fruit.
• Red Alert — a compact bush with good flavour and early cropping.
• Losetto — a bush variety with decent blight resistance if you also grow a few outdoors.

For classic full-sized fruit in a limited footprint, train a cordon such as Shirley or Alicante straight up a single cane and keep it to one stem. Two or three cordons in a small greenhouse will out-crop a dozen sprawling bushes. Cherry types such as Sungold are worth a place too, as they ripen early and keep going late.

Sowing, Pricking Out and Growing On

Sow from late February to mid-March if you have gentle heat, or early April in a cool conservatory. Fill small pots or modules with moist seed compost, sow the seed about 1cm deep and keep at 18–21°C. Seedlings usually appear within a week to ten days.

As soon as the first true leaves appear, prick the seedlings into individual 9cm pots, burying the stem slightly deeper than it was growing. Grow them on at around 15–18°C in good light — a windowsill that gets morning sun is ideal. Pale, leggy plants are a sign of too much warmth and too little light, so move them somewhere cooler and brighter if that happens.

Move plants into their final positions in late April or May, once night temperatures stay above about 10°C. Use 10-litre pots, growbags or a border soil enriched with compost. Plant deeply, right up to the first pair of leaves, and water in well.

Watering Consistently — the Single Biggest Factor

Tomatoes hate boom and bust. Alternating between drought and a soaking causes split skins and, more seriously, blossom end rot, where a leathery brown patch appears at the base of the fruit. That is a calcium uptake problem triggered by irregular watering, not a shortage of calcium in the compost.

• Water little and often; in hot weather a plant in a pot may need watering twice a day.
• Aim for consistently moist, never waterlogged, compost. Check by pushing a finger in up to the second knuckle.
• Water in the morning, directly onto the compost, keeping foliage dry to reduce disease.
• Consider a drip line or self-watering container if you are out at work all day — it makes a real difference.
• In a conservatory, group pots together and stand them on a gravel tray to raise humidity slightly without wetting the leaves.

Feeding: Hold Back, Then Switch to High Potash

Young plants in fresh compost need nothing beyond water. In fact, too much nitrogen early on gives you a magnificent leafy plant with almost no fruit. Start feeding only once the first trusses have set fruit — when you can see small green tomatoes rather than just yellow flowers.

At that point, switch to a high-potash tomato fertiliser and feed weekly, following the dilution on the bottle. Once plants are cropping heavily, increase to twice a week. Potash drives flower and fruit development, and a well-fed plant will produce bigger, better-flavoured tomatoes with fewer problems. If leaves start to look pale and yellow between the veins, a general-purpose feed once a fortnight alongside the tomato feed usually corrects it.

Ventilation, Pollination and Summer Care

Ventilation is not optional. Tomatoes set fruit poorly above about 30°C, and stagnant humid air encourages botrytis and blight. Open vents and doors in the morning and close them in late afternoon, aiming to keep daytime temperatures around 21–27°C. Shade the glass with a wash-on product or a roll of fleece during the hottest weeks.

Tomatoes are self-pollinating but need a little movement. Tap the supporting cane or gently flick each flower truss at midday, when the pollen is dry, to help fruit set. On cordons, pinch out the side shoots that grow between the stem and the leaves, and stop the growing tip two trusses above the last set fruit in late summer so the plant puts its energy into ripening what is already there.

Watch for whitefly and aphids on the undersides of leaves, and for red spider mite in dry, hot conservatories — raising humidity and misting the floor helps. Remove lower leaves as fruit trusses ripen to improve airflow, and pick tomatoes as they colour, or leave them on the plant for the fullest flavour. At the end of the season, green fruit will ripen indoors in a paper-lined drawer with a ripe banana, and any that refuse make excellent chutney.