Growing Tomatoes in a Small Greenhouse or Porch

Why tomatoes love a small space

You do not need a full-size greenhouse to grow a decent crop of tomatoes. A tiny lean-to, a glazed porch, a sunny bay window or even a plastic walk-in frame can all produce fruit that tastes far better than anything from a supermarket. The trick is to work with the space rather than against it: choose varieties that stay manageable, keep the air moving, and be disciplined about watering and feeding. Get those three things right and a single plant in a six-litre pot can easily give you four or five kilos of tomatoes over a season.

Small structures heat up fast in July and cool down quickly at night, so plants in them need more attention than those in a big glasshouse. Treat that as an advantage. You can inspect every plant daily in under five minutes, which is exactly the sort of close observation tomatoes reward.

Choosing compact varieties for limited room

In a confined space, height and spread matter more than yield per plant. A traditional cordon such as 'Gardener's Delight' will happily reach two metres and needs staking the whole way, which is awkward under a low porch roof. Look instead for compact cordons, often sold as "indoor" or "greenhouse" types, which crop heavily on shorter stems.

• Compact cordons: 'Tumbling Tom' is technically a bush but works beautifully in hanging pots; 'Mountain Magic' and 'Crimson Crush' stay manageable and have good blight resistance for later in the season.
• Dwarf and bush types: 'Tiny Tim' and 'Minibel' rarely exceed 40cm and are ideal for windowsills or a shelf under the glass.
• Cherry varieties: generally more forgiving in small pots and quicker to ripen than beefsteaks, which need a long, warm season.

If space is really tight, grow one plant per large pot rather than cramming three into a trough. Tomatoes sulk when their roots compete, and cramped plants are far more likely to develop botrytis in still, humid air.

Potting on, staking and pinching out

Start with a pot of at least 10 litres for a compact cordon, and use fresh peat-free multipurpose compost with a handful of slow-release fertiliser mixed through the bottom third. Plant deeply, burying the stem up to the first pair of leaves, and firm gently. Water in well and keep the plant somewhere bright but out of direct midday sun for a few days while it settles.

Once the first truss of flowers appears, pinch out the side shoots that grow in the angle between the main stem and each leaf. Do this weekly with your thumbnail or a clean snip when the shoots are small — leaving them to grow into thick stems wastes energy and creates a tangled, airless plant. Stop the main stem two or three leaves above the fourth or fifth truss so the plant puts its effort into ripening fruit rather than growing taller. If your porch roof is low, stop sooner.

Tie the stem to a cane or a length of garden twine fixed to the roof with soft string, loosening the tie as the stem thickens. Never use wire, which cuts into the stem.

Watering little and often — but consistently

Tomatoes in containers are entirely dependent on you. The single biggest cause of split fruit and blossom end rot is irregular watering: a dry spell followed by a soaking. Aim to keep the compost evenly moist, not sodden.

• In warm weather, check morning and evening by pushing a finger 2cm into the compost.
• Water at the base, never over the leaves, and do it in the morning so any splashes dry before night.
• Stand pots in a saucer or tray with a few pebbles so excess water drains but roots can reach back into it on hot days.
• Group pots together to shade each other's root zones, and mulch the surface with a layer of compost or grass clippings.

If you are away, a simple capillary mat or a length of absorbent wick running from a water-filled bottle into the pot will keep things ticking over for a few days.

Feeding once the first trusses set

Until the first tiny fruits have formed, the slow-release fertiliser in the compost is usually enough. Once you can see small green tomatoes, start a high-potassium tomato feed at half strength twice a week, or full strength once a week, diluted exactly as the bottle says. Overfeeding causes lush leaves and few fruits, so resist the temptation to splash it about.

Yellowing leaves below the first truss are normal as the plant ages; remove them to improve airflow and reduce the chance of disease. Keep the area around the base clear of fallen leaves and rotting fruit.

Ventilation, pests and ripening

Small enclosed spaces trap humidity, and humidity plus still air equals mould. Open vents, doors or windows on any mild day from late spring onwards, but close them before evening in cool weather to keep night temperatures up. A cheap maximum-minimum thermometer will tell you more than guesswork.

Watch for aphids on shoot tips and whitefly under leaves; a squirt of water or a careful wipe with a damp cloth usually deals with small numbers. Encourage wildlife nearby — a few flowering herbs in pots by the door bring in hoverflies and lacewings that prey on aphids without any spraying.

Ripening can be slow in a shaded porch. Reduce watering slightly, remove leaves shading the trusses, and place a ripe banana skin or a ripe tomato near green fruit to speed things along. Anything stubbornly green at the end of September can be brought indoors on the vine and will colour up on a sunny windowsill. Sow a couple of seeds every four weeks from late February to May and you will have fresh tomatoes from July right through to the first frosts.