When to sow sweet peppers

Peppers need a long, warm season, so they are started indoors well before the last frost. Sow from mid-February to the end of March. Plants for a greenhouse or polytunnel go in first, in February; plants for outdoors can wait until March, which in the UK stretches into early April.

There is little to gain from sowing much earlier than February on a windowsill: in the dim light of midwinter the seedlings stretch and grow weak.

Young plants go out only when frost is over and nights stay above about 50–54°F. In Germany, Austria and Switzerland that means after the Eisheiligen in mid-May, in the UK late May or early June. In a heated greenhouse you can plant from late April, in an unheated one from mid-May.

Sow the seeds 0.2–0.4 in deep in seed compost and keep them warm and moist. Peppers are fussier than tomatoes about heat: they germinate best at about 77°F, and below about 68°F they come up slowly, unevenly or not at all, while the damp compost grows mould. A heated propagator or a spot above a radiator makes the difference.

Expect seedlings in one to three weeks. Once they are up, move them to the brightest place you have and let them grow a little cooler, around 64–72°F.

Prick them out into small pots of their own at the first true leaves, roughly three weeks after sowing, and pot on as the roots fill each pot. Harden the plants off for a week or two before they go outside, and plant them a little deeper than they sat in the pot.

Sweet peppers do best under cover in the UK and much of Central Europe: a greenhouse, polytunnel or a cold frame. Outdoors, give them the warmest, sunniest and most sheltered spot you have, such as a south-facing wall. Peppers dislike wind, and cold nights stall them for weeks.

Plant into fertile, well-drained soil with plenty of compost worked in. Space plants 16–20 in apart with 20–24 in between rows; compact varieties can go a little closer. A 120 × 80 cm raised bed takes two rows of three, six plants, or four if you grow large, bushy varieties.

In pots, give each plant at least 10 litres of compost, better 20 or more. Bigger pots dry out less quickly, which matters for blossom end rot (see below).

Keep the soil evenly moist. Peppers have small root systems and don't cope with swings between bone-dry and soaked; in hot weather, plants in pots may need water every day. A mulch of straw or grass clippings helps in beds.

Peppers are hungry. Once the first flowers open, feed with a potash-rich liquid feed, such as tomato feed: weekly in pots, every two to four weeks in well-composted beds.

German gardeners often pinch out the first flower, the one in the fork of the main stem (the Königsblüte), so the plant branches and carries more fruits. Tie the stems to a cane or two: laden branches are brittle and snap easily.

Under glass, open doors and vents on warm days. Peppers drop their flowers when it is very hot, above about 86°F, and also when nights are cold or the soil dries out. Fruit sets again once conditions settle. Where there is no wind or insects, tap the flowering stems now and then to help pollination.

Under glass the first peppers are ready from July, outdoors usually from August, and plants keep cropping until the first cold nights in October.

A green pepper is simply unripe. You can pick it once it has reached full size and feels firm and glossy, and doing so encourages the plant to set more. Left on the plant for roughly another two to three weeks, most varieties turn red, yellow or orange and become sweeter and more aromatic.

Cut the fruits off with a knife or secateurs, leaving a short stalk, rather than tugging them: the branches break easily. Before the first frost, pick everything that is left.

Sweet peppers come as blocky bell peppers, long pointed peppers and small, flattened tomato-shaped peppers. Pointed and tomato-shaped types are generally tougher and ripen more reliably in a cool summer than big blocky ones, which are best kept under glass.

For growing outdoors, choose early varieties that are sold for cooler climates. Compact varieties such as the RHS award winner 'Redskin' suit pots. Hot chillies are the same species but a different crop; they have their own guide.

Blossom end rot shows as a sunken, dark patch at the tip of the fruit. It is a lack of calcium in the fruit, but the cause is nearly always uneven watering rather than poor soil, especially in pots and grow bags. Affected fruits can't be saved; keep the compost steadily moist and later fruits come good.

Aphids cluster on shoot tips and young fruits. Under glass, glasshouse red spider mite, whitefly and thrips thrive in hot, dry air; good ventilation helps. Grey mould (Botrytis) appears in damp, stagnant air, so ventilate and clear away dead leaves and fruit.

Peppers belong to the nightshade family with tomatoes, potatoes and aubergines. They suffer from fewer soil-borne problems than tomatoes and potatoes, but it is still wise to give the nightshade group a new bed each year and leave three to four years before it returns.

Planting tables often put peppers next to basil, parsley, marigolds or nasturtiums. These pairings are tradition, with little evidence that they help peppers, though they do no harm.

What matters more is space and light: don't let taller tomatoes or climbing beans shade peppers, and keep them out of the bed where other nightshades grew last year.