When to sow sweet peas

Sweet peas are hardy annuals, so you have two windows. The main one is late winter to spring: sow in pots under cover from February to April. In the UK you can also sow in October or November and keep the young plants in a cold frame or unheated greenhouse over winter. Autumn-sown plants flower earlier than spring sowings.

Outdoors, sow straight into the ground from April into early May, once the soil has started to warm. Direct sowings are simpler but flower a few weeks later than plants raised in pots.

Pot-grown plants go out in April in mild areas and in the UK. In colder parts of Germany, Austria and Switzerland, wait until mid-May, after the Ice Saints, especially for plants raised on a warm windowsill.

Sweet peas grow long roots, so use deep pots: root trainers, 4 in pots with one or two seeds each, or cardboard tubes that go into the ground with the plant. Sow 0.4–0.8 in deep in seed compost and keep the pots at about 59°F. Seedlings appear after one to three weeks, often unevenly.

The seeds have a hard coat. Many gardeners soak them overnight, but this can make them rot. A gentler trick is to lay them on damp kitchen paper in a closed box until they swell. Seeds that stay hard can be nicked with a knife on the side away from the eye.

Once the plants are about 2.8–4 in tall, or have three pairs of leaves, pinch out the tip. They then branch from the base and make more flowering stems.

Harden the plants off before planting them out 6–10 in apart at the foot of their support. Handle the roots gently: they are white, fleshy and break easily.

Climbing sweet peas usually reach 59–98 in, so give them a support about 79 in high before you plant: a wigwam of canes, pea sticks, or netting or trellis. They climb with thin tendrils, so fine netting or twiggy sticks suit them better than thick posts. Dwarf varieties stay at 8–16 in and suit pots.

Choose a sunny spot sheltered from strong wind, in humus-rich, fertile soil that holds moisture but never stands wet. Sweet peas like soil with some lime in it.

Water well in dry spells: plants in the ground may need a drink every three or four days. Once flowering starts, a liquid feed high in potassium, such as tomato feed, keeps the flowers coming. Too little water is a common reason for buds dropping before they open.

Sweet peas flower from June into September, sometimes longer. The rule is simple: the more you pick, the more they flower. Cut stems every few days, before flowers fade. Once a plant is allowed to set pods, it puts its energy into seed and soon stops flowering.

Pick in the morning and put the stems straight into water.

To save seed, stop picking a few stems of your favourite plant late in summer and let the pods turn brown and dry on the plant. Sweet peas mostly pollinate themselves, so named, open-pollinated varieties usually come true; hybrids may not. Store the dry seed cool and dry in a paper bag.

The seeds and pods are toxic and must never be confused with garden peas. Keep them away from children and pets.

Powdery mildew is the most common problem, usually from midsummer, when plants are dry at the roots and crowded. A white coating spreads over the leaves. Water in dry weather, give the plants air, and remove badly affected growth.

Aphids gather on shoot tips and buds and can spread viruses that mottle the leaves and stunt the plants. Wipe or hose them off early. Slugs and snails like the young shoots, so protect seedlings after planting out.

Sweet peas rot in waterlogged soil, so avoid heavy, wet ground or improve it before planting.

Sweet peas are legumes, like garden peas and beans. Grow them in a different spot each year rather than on the same ground, and treat them as part of the pea and bean group in your rotation.

Sweet peas are often planted among vegetables to bring in pollinators, and some growers put low flowers at their feet to keep the roots cool. Both are sensible habits rather than proven effects; we found no trials measuring them.

Two practical points: tall sweet peas cast shade, so put them on the north side of a bed, and keep them clearly apart from edible peas so nobody picks the wrong pods.