Sow sweet peas in autumn for the earliest flowers

Sow sweet peas in October or November in a cold frame or unheated greenhouse. Autumn-sown plants build deep roots over winter and flower from early June. If you miss autumn, sow under cover from mid-January to mid-March, or straight outdoors from early March to mid-April.

These dates are for a typical lowland garden in England and Wales, with the last frost around 20 April and the first around the end of October. In Scotland, the north of England and colder spots, spring sowings outdoors and planting out are better left two or three weeks later. In the south-west and sheltered towns, you can go a little earlier.

Autumn sowing under cover

Seedlings sown in October and November grow slowly over winter, putting their effort into roots instead of top growth. They go out in spring as sturdy, bushy plants that flower earlier and usually for longer than spring-sown ones.

Sweet peas are hardy to about -5 °C, so they don’t need heat. In fact warmth makes them leggy. A cold frame, unheated greenhouse or porch is ideal. Open the lid or door on mild days for ventilation, and cover the frame with fleece or old carpet during a hard frost.

Mice love sweet pea seed. If they’re a problem where you overwinter seed trays, set traps or keep the trays somewhere they can’t reach.

Sowing in spring instead

Seed sown indoors from mid-January to mid-March catches up quickly in the longer days. The plants flower a few weeks later than autumn-sown ones, which can be useful: a second batch sown in spring keeps flowers coming into late summer after the autumn sowings tire.

You can also sow straight into the ground where they’re to flower, from early March to mid-April. This is the least effort, but mice, slugs and birds take more seedlings outdoors, so sow a couple of extra seeds at each station.

How to sow

1. Use deep pots or root trainers. Sweet peas make long roots early on, and deep cells let them grow down without circling. Cardboard toilet roll tubes packed tightly in a tray do the same job and can be planted whole.
2. Fill with multipurpose compost and water it before sowing.
3. Sow 1.5 cm deep, one or two seeds per cell.
4. Keep the compost just moist. Seed can rot in cold, waterlogged compost.
5. Germination usually takes one to three weeks, quicker in spring than in autumn.

Pinching out

When the seedlings are about 10 cm tall, pinch out the growing tip just above a pair of leaves. The plant then sends up two or three side shoots, which makes a bushier plant with more stems to flower on. It feels harsh, but the plant is better for it.

Planting out

Plant out from late March to late April, once the seedlings have been hardened off for a week or two by leaving them outside in the day and bringing them in or closing the frame at night. Autumn-sown plants that have spent winter in a cold frame are already well used to the cold.

Choose a sunny spot with rich soil, dug through with well-rotted compost or manure. Space plants 20 cm apart. If you have two rows either side of a support, keep the rows about 60 cm apart.

Support

Sweet peas climb to about 1.8 m, so put the support in before planting. A wigwam of canes, a row of canes, an obelisk or pea and bean netting all work. The tendrils will cling once the plants get going, but in the first couple of weeks guide the shoots onto the support and tie them in loosely with soft string.

Water well in dry weather. Sweet peas that dry out at the roots flower poorly and are more prone to mildew. A liquid feed every couple of weeks once flowering starts helps keep them going.

Picking to keep them flowering

Sweet peas flower from early June to mid-September, and the more you pick, the more you get. Cut stems every few days, ideally in the morning, and pick off any flowers that start to fade.

Above all, don’t let seed pods form. Once a plant sets pods, it stops making flowers. Check the plants every time you pick and remove any pods you missed.

Common problems

• Tall, thin, floppy seedlings: they’ve been kept too warm or too dark. Move them somewhere cooler and brighter, and pinch them out.
• Bud drop: flower buds turning yellow and falling off, usually after a cold night or a spell of dry weather. It tends to sort itself out once conditions settle.
• Powdery mildew in late summer: a white coating on the leaves, worst on dry, stressed plants. Keep them watered and keep picking.
• Seedlings eaten overnight: slugs or mice. Protect young plants and check under pots and trays.