A Beginner's Guide to Growing Sweet Peas from Seed

Why Sweet Peas Are Worth the Wait

Few flowers earn their keep like sweet peas. Sow them at the right moment and you will be cutting fragrant, ruffled blooms from early summer right through to the first sharp frosts of autumn. They are generous, forgiving plants, and they suit a small back garden, an allotment plot or even a decent-sized patio pot.

The catch is timing. Sweet peas are hardy annuals that love a cool, slow start, and the gardeners who get the best displays are almost always the ones who sowed early. Get the first few weeks right and the plants will reward you with longer stems, more flowers and far less trouble with pests and disease.

This guide walks through the whole process, from soaking the seed to pinching out the growing tips. It is aimed at beginners, so nothing here requires a greenhouse, a heated propagator or years of experience.

Choosing Seeds and Picking Your Moment

Sweet peas come in two broad moods: the tall climbing types grown as cordons or up obelisks, and the shorter, bushier varieties that are happier in containers. If you are new to this, pick a tall heritage variety with a strong scent, or a dwarf type if space is tight.

Sowing windows vary depending on where you garden and what protection you have:

• Autumn sowing (October to November): Best in mild areas or with a cold frame. Plants overwinter as small, sturdy seedlings and flower earliest the following summer.
• Late winter sowing (late January to March): The reliable option for most UK gardeners. Sow under cover in a cool, bright spot.
• Early spring sowing (April): Perfectly possible, but plants will flower later and may struggle if the weather turns hot and dry before they are established.

Sweet pea seeds are large and easy to handle, so they are a lovely choice for children or for anyone new to sowing. A packet of twenty seeds will give you far more plants than most gardens need.

To Soak or Not to Soak?

Sweet pea seeds have a hard coat, and soaking softens it so the root can break through more quickly. Fill a small jar with tepid water, drop the seeds in and leave them for twelve to twenty-four hours. Discard any that float and swell, as these are usually hollow and will not germinate.

The traditional alternative is chipping: nicking the seed coat with a knife or a nail file, just away from the little eye where the root emerges. It works, but you must be careful not to damage the growing point inside.

A word of caution, because this trips up plenty of beginners. If you sow into cold, wet compost, soaked seeds can rot before they sprout. In that case, sow them dry and let the natural moisture of the compost do the work. Soaking is a help, not a rule, and there is no shame in skipping it.

Sowing Into Deep Pots

Sweet peas develop a long taproot quickly, and if that root curls around the bottom of a shallow tray the plant will never grow quite as strongly. Use deep, narrow containers: root trainers, tall modules or even cardboard toilet roll tubes stood upright in a tray.

Fill them with a peat-free multipurpose compost, firm it gently and water before sowing. Push each seed about two centimetres deep, one per pot, and cover it over. Label the pot with the variety and the date, because in six weeks you will have forgotten both.

Keep the pots somewhere cool, bright and frost-free, ideally around twelve to fifteen degrees Celsius. A windowsill in an unheated room, a cold frame or a cool porch is ideal. Germination usually takes ten to twenty-one days. Too much warmth produces pale, leggy seedlings that are far more prone to problems later.

Pinching Out for Stronger Plants

This is the single most useful trick in sweet pea growing, and it takes seconds. Once a seedling has three or four pairs of true leaves, pinch out the growing tip with your thumb and fingernail, cutting just above a pair of leaves. Do not use scissors if you can avoid it, as the crushed stem heals more cleanly.

Pinching out removes the plant's urge to grow straight up as a single stem. Instead, it triggers side shoots from the base, giving you a bushier, sturdier plant with more flowering stems overall. Repeat the treatment on any side shoot that races ahead of the others.

Keep the plants cool and well lit after pinching, and water them when the compost surface begins to dry. If they are destined for the garden, harden them off over a week or so by putting them outside during the day and bringing them in at night.

Planting Out and Keeping the Blooms Coming

Wait until the risk of hard frost has passed, usually late May in most parts of the UK, though autumn-sown plants can go out earlier once they have been hardened off. Choose a sunny spot with reasonably fertile, well-drained soil and add plenty of garden compost or well-rotted manure beforehand.

Space plants about twenty to thirty centimetres apart and give them something to climb: an obelisk, a wigwam of canes, netting against a wall or a row of vertical strings. Tie in the stems as they grow and remove any side shoots from the base of cordon-grown plants to keep energy going upwards.

From midsummer, feed every ten to fourteen days with a high-potash liquid feed, and water thoroughly in dry spells. Pick flowers regularly, even if you have no vase to fill, because every bloom left to set seed signals the plant to slow down. Watch for slugs on young growth, aphids on the tips and powdery mildew in dry weather.

Do all of this and you will be cutting scented stems well into September, with enough left over to hand a few bunches to a neighbour. That, more than anything, is what makes sweet peas such an enduring favourite in British gardens.