Growing Tomatoes Outdoors in a Cool Climate

Why Outdoor Tomatoes Are Worth the Fuss

There is nothing quite like the smell of a tomato plant on a warm afternoon, and nothing quite like the taste of one picked and eaten within minutes. In a cool climate you have to work a little harder for that pleasure, but plenty of British allotment holders and back-garden growers bring in a good crop every year without a greenhouse. The trick is to accept the limitations of the weather and choose your approach accordingly.

Outdoor tomatoes rarely match the sheer volume you would get under glass. What they do offer is flavour — sun-ripened fruit with a depth that forced tomatoes struggle to match — and the freedom to use space that a greenhouse cannot occupy. Get the basics right and a healthy plant will give you four to six trusses of fruit between late July and the first frosts.

Choose Short-Season Varieties

This is the single most important decision you will make. A beefsteak that ripens beautifully in the Mediterranean will still be green and hard when your first frost arrives. Look for varieties described as early, short-season or suitable for cool summers, and check the days-to-maturity figure if it is given.

• Sub-Arctic Plenty — extremely early, compact, dependable even in a poor summer.
• Latah — a bush type that starts cropping in as little as 55 days from planting out.
• Glacier — early, sweet and reliable, good for cooler northern plots.
• Maskotka — a bush cherry tomato that crops heavily and needs no staking.
• Gardeners' Delight — a cordon cherry that tolerates outdoor conditions well.
• Roma — a compact plum for sauces, paste and roasting.

Bush or determinate varieties stop growing on their own and need no pinching out, which makes them ideal for exposed sites. Cordon or indeterminate varieties keep growing and need training up a cane, but they crop over a longer period. A mix of both hedges your bets against a bad summer.

Start Early and Plant Out Late

Sow indoors in late February or early March in a warm, bright spot. Aim to have sturdy plants roughly 20 to 25 centimetres tall by the time the risk of frost has passed, which in most of the UK means late May or early June. Do not rush them outside; cold nights below about 10°C stall growth and can cause blossom drop later on.

Harden plants off properly over a week or ten days, bringing them in at night at first and gradually leaving them out for longer. When you plant, bury the stem deeper than it was in the pot — right up to the first true leaves. Tomatoes will root along the buried stem, giving you a stronger plant. Space cordons about 45 centimetres apart and water them in well.

Shelter From Wind and Find the Warmest Spot

Wind is the quiet killer of outdoor tomatoes. It shreds leaves, snaps stems and dries the soil faster than you can water it. Choose the sunniest, most sheltered position you have, ideally against a south-facing wall or fence that will hold warmth into the evening.

• Erect a windbreak of netting, woven willow or even a row of tall peas about a metre away, on the prevailing wind side.
• Use cloches or fleece for the first fortnight after planting out.
• Mulch with compost or grass clippings to hold moisture and keep roots cooler.
• Water at the base, never over the leaves, to reduce blight risk.

If you garden on an exposed allotment site, grow bush varieties in large pots against a wall. A pot warms faster than open ground and can be moved under cover if blight threatens.

Pinch Out Sideshoots and Train Your Plants

For cordon varieties, pinch out the sideshoots that appear in the angle between the main stem and each leaf. Do this weekly, when the shoot is small enough to snap off with your thumb and finger. Left alone, these shoots turn a single plant into a dense, unproductive bush with small fruit and poor airflow.

Tie the main stem to a sturdy cane as it grows, using soft string in a loose figure-of-eight so the stem is not pinched. Once four to six trusses have set, stop the plant by pinching out the growing tip in late August or early September. This directs the plant's energy into ripening the fruit you already have rather than producing flowers that will never mature.

Feed and Water Once Fruit Sets

Until the first flowers set fruit, tomatoes on reasonably fertile ground need little more than water. Once small green tomatoes appear, start feeding weekly with a high-potash tomato feed. Nitrogen-heavy feeds give you lush leaves and few tomatoes; potash gives you fruit.

Water consistently — a steady soak twice a week beats a light sprinkle daily. Irregular watering is the main cause of blossom end rot, where a dark sunken patch appears at the base of the fruit. As the lower trusses ripen, remove the leaves beneath them to improve airflow and let light reach the fruit.

In a cool, damp summer, watch for blight: brown patches on leaves and stems that spread fast. Remove affected foliage immediately, and if the whole plant succumbs, harvest the green tomatoes — they will ripen on a sunny windowsill or in a paper bag with a ripe banana, and any that refuse make excellent chutney.