When to sow tomatoes

For tomatoes that will grow outdoors, sow indoors from mid-March to early April. That gives you sturdy plants of about six to eight weeks old by the time the frosts are over. If you have a greenhouse and a heated propagator, you can start in late February or early March.

Resist the urge to sow in January or February on a windowsill. Light is still poor, and early seedlings stretch into long, thin, floppy plants that sit in pots for weeks before it is warm enough to plant them out. They rarely crop any sooner than a mid-March sowing.

Tomatoes can't take any frost. In Germany, Austria and Switzerland, plant them outdoors after the Eisheiligen in mid-May; in most of the UK, late May to early June. Nights should stay above about 50°F, since colder nights slow growth and fruit set. In a greenhouse or polytunnel you can plant from mid to late April.

Sow the seeds thinly in a tray or small pots of seed compost, cover them with 0.2–0.4 in of compost and water gently. Keep them at 64–75°F, with a lid or clear bag over them; a warm windowsill or propagator works. Seedlings appear after roughly six to eleven days.

As soon as they come up, take off the cover and move them somewhere a little cooler but as bright as possible. Warmth with too little light is what makes seedlings leggy.

When each seedling has its first two true leaves, usually two to three weeks after sowing, prick them out into their own pots. Hold them by a leaf, never by the stem, and set them deeper than before: tomatoes grow roots along any buried stem.

Harden the plants off for a week or two before planting out, putting them outside by day and bringing them in at night. Plant into a sunny, sheltered bed enriched with compost, burying the stem up to the first leaves. Space cordon tomatoes 20–24 in apart with 30–31 in between rows, and bush types wider. Put the stake or string in at planting time so you don't spear the roots later.

A 120 × 80 cm raised bed takes four cordon tomatoes in two rows of two, or two sprawling bush plants. Resist squeezing in more: tomatoes need air around their leaves to stay healthy.

Even moisture is the single most important thing. Water deeply and regularly at the base of the plant, not over the leaves; in hot weather plants in pots may need water every day. Swinging between dry and soaked is what causes split fruit and blossom end rot. A mulch of grass clippings or straw keeps the soil moist for longer.

Tomatoes are hungry plants, usually counted as heavy feeders. Work compost into the bed before planting. Once the first fruits start to swell, give a high-potash tomato feed every 10–14 days, or use comfrey liquid.

Cordon tomatoes, the tall single-stemmed kind, grow a side shoot in every angle between leaf and main stem. Pinch these out while they're small, about once a week; it keeps the plant open, airy and focused on its fruit. Bush tomatoes don't need this.

Outdoors, stop a cordon plant once it has set four trusses of fruit by pinching out the tip two leaves above the top truss; the season is too short for more to ripen. In a greenhouse you can let it reach about seven. Also strip the lower leaves up to the first truss, so rain and watering don't splash soil onto them.

Outdoor tomatoes usually start to ripen towards the end of July and keep coming until the first frosts, often into October. Greenhouse plants start earlier. Pick fruit when it is fully coloured, or a little before if heavy rain is forecast, since ripe fruit splits easily.

Long hot spells well above 86°F can make flowers drop without setting fruit, because the pollen stops working. Open greenhouse doors and vents on hot days. From September, once nights are regularly below 50°F, ripening slows to a crawl.

Before the first frost, pick the remaining green fruit and let it ripen indoors somewhere warm; a ripe banana next to them speeds things up. Keep ripe tomatoes at room temperature rather than in the fridge. Cold damages their texture and flavour.

Cordon (indeterminate) tomatoes keep growing upwards all season. They need a stake, spiral or string, weekly side-shooting and stopping at the top. In return they use space well and suit raised beds and greenhouses. Most beefsteak and many cherry varieties are cordons.

Bush (determinate) tomatoes stop growing at a set height and spread sideways. You don't side-shoot them, but they take more ground, and fruit on the soil is prone to rot and slugs, so a straw mulch underneath helps. Compact bush types also suit pots and hanging baskets.

If blight is common where you live, look for varieties sold as blight-resistant. In RHS trials at Wisley, 'Crimson Crush' and 'Mountain Magic' did very well. Resistant doesn't mean immune, so keep the leaves dry as well.

Late blight (Phytophthora infestans) is the big one for outdoor tomatoes. In warm, wet weather, leaves develop brown patches and collapse within days, stems get dark lesions, and green fruit turns brown and rots. In the UK it can appear from June, and in Central Europe in any wet summer. It needs leaves that stay wet for several hours, so a rain roof, wide spacing, side-shooting and removing the lower leaves all help. Growing under cover is the most reliable protection.

Blight overwinters in potato tubers left in the ground or thrown on the compost, and spreads from potato foliage to tomatoes. Don't grow tomatoes right next to potatoes. Pick off the first infected leaves at once. Don't compost blighted plants; put them in the green-waste bin. Clean canes with hot water before reusing them.

Blossom end rot shows as a dark, sunken patch at the base of the fruit. The cause is too little calcium reaching the fruit, usually because of uneven watering rather than a lack of calcium in the soil. Split fruit has the same root cause: a dry spell followed by heavy rain or a big watering.

Tomatoes belong to the nightshade family, along with potatoes, peppers and aubergines, and they share pests and diseases. Leave three to four years before any of them grows in the same bed again. In a greenhouse where blight struck, replace the soil or use fresh compost the next year.

Basil is the classic partner for tomatoes, and planting tables add carrots, parsley, marigolds and nasturtiums. Of these, only French marigolds (Tagetes) have research behind them: they deterred whitefly in a greenhouse study. But the same study found that mixing tomatoes with other plants had a similar effect, so the benefit seems to come from variety rather than marigolds in particular. Basil next to tomatoes is tradition, pleasant and harmless.

The one rule with a real reason: keep tomatoes away from potatoes. They share late blight, and it spreads easily from one to the other.