When to Plant Sweet Peas (and Why Autumn Sowing Wins)

Ask any exhibition grower when to sow sweet peas and you'll get the same slightly smug answer: last autumn. Autumn sowing is the closest thing to a legal cheat in the flower garden — the seedlings spend winter building roots instead of racing upwards, and reward you with stockier plants, stronger stems, and flowers weeks ahead of spring-sown neighbours. But spring sowing works well too, and choosing between the two is mostly a question of what shelter you can offer over winter. Here is the full calendar.

The two sowing windows

Sweet peas are hardy annuals: established seedlings shrug off light frost, and what kills autumn sowings is almost never cold but winter wet and coddling warmth that draws them into soft, leggy growth.

How to sow, step by step

• Use deep containers — root trainers, tall 9 cm pots, or cardboard tubes — because sweet peas throw down a long taproot and hate having it disturbed.
• Fill with free-draining multipurpose compost and sow one seed per cell or two per pot, 2–3 cm deep.
• Water once, then keep barely moist. At 15–18°C seedlings emerge in 10–14 days.
• There's no need to soak seeds first; if a batch has very hard, dark coats, chipping the seed coat opposite the eye with a nail file speeds things up.
• As soon as they're up, move them somewhere bright and cool — around 5–10°C is perfect for autumn sowings.

Pinch out the tips

When each seedling has three to four pairs of leaves, pinch out the growing tip between finger and thumb. It feels brutal and it is the single best thing you will do for the display: the plant responds by throwing out strong side shoots from the base, and those side shoots carry the flowers. Unpinched sweet peas grow as a single thin, wavering stem that flowers late and sparsely.

Overwintering autumn sowings

The goal from November to March is slow, tough growth. Keep the pots in a cold frame or unheated greenhouse — bright, ventilated, and frost-free or close to it. Water only when the compost is nearly dry; cold, sodden compost rots seedlings faster than any frost. Throw fleece over the frame on bitter nights and open it up on mild days, because stale, still air breeds grey mould.

Warning: mice adore sweet pea seeds and can clear a whole tray of autumn sowings overnight. Sow into pots they can't reach — on a high shelf, under a propagator lid, or behind fine mesh — and set traps in the greenhouse before you sow rather than after the damage.

Planting out and supports

Plant out from mid-spring, once the soil is workable and hard frosts are done, hardening the plants off for a week or so first. Sweet peas are greedy: dig in plenty of compost or well-rotted manure ahead of time, and space plants about 20 cm apart at the base of their support. They climb by tendrils to two metres or more, so give them something to grip from day one — a wigwam of hazel poles or canes, trellis, or netting — and tie in the first 30 cm of growth until the tendrils take over.

Keep them flowering all summer

A sweet pea's mission is to set seed, and the moment it succeeds it winds down flowering. Your job is to keep failing it: cut bunches for the vase every few days — the more you cut, the more it flowers — deadhead anything you miss, and never let pods develop. Keep the roots evenly moist and mulched; drought is the usual trigger for powdery mildew and for flowers dropping their buds. With regular cutting, an autumn-sown row will flower from late spring into high summer.

One safety note: unlike garden peas, sweet pea seeds and pods are mildly toxic if eaten, so keep the pods away from children and pets and don't let them anywhere near the kitchen.

If your seedlings sulk, yellow, or something starts nibbling, photograph the damage in PlantMe — the app's diagnosis tool tells you in seconds whether you're looking at mildew, mice, or simple overwatering, and what to do next.