How do I grow sweet peas that keep flowering?

Autumn against spring sowing, the deep root run they need, the pinching that produces a bushy plant, and the one habit that decides whether they flower for six weeks or four months.

Difficulty: beginner
Time: 1 hr
Read: 5 min

Short answer

Sow in deep pots or root trainers in October or in February, pinch the tips out at three or four pairs of leaves, plant out into rich soil with a tall support, and then pick the flowers every two or three days without fail. A sweet pea that sets a single seed pod starts shutting down.

Sweet peas fail in two predictable ways: they run out of root room and they are allowed to set seed. Everything worth knowing about them addresses one of those two problems. Get both right and they flower from June until September; get either wrong and you get a short burst in early July and then a yellowing tangle.

What you'll need

Sweet pea seed
Root trainers, deep pots or loo-roll tubes
Peat-free seed or multipurpose compost
A tall support — canes, netting, hazel or an obelisk
Well-rotted manure or compost
Cold frame or unheated greenhouse for an autumn sowing (optional)
Slug protection

Step by step

Choose autumn or spring sowing.October sowing, overwintered in a cold frame or unheated greenhouse, gives stronger plants that flower earlier and for longer. February or March sowing under cover is easier and perfectly good. A direct sowing outdoors in April works but flowers latest and least.
Sow into something deep.Sweet peas make a long root and hate being cramped. Root trainers, deep 90mm pots, or cardboard tubes stood in a tray are all fine. Shallow seed trays produce weak plants that never recover.
Do not bother soaking or chipping the seed.The traditional advice to soak overnight or nick the seed coat causes more rotting than it prevents with modern seed. Sow two seeds per cell about 20mm deep and thin to the stronger seedling.
Keep them cool and airy.Sweet peas want cool, bright, well-ventilated conditions, not warmth. A cold frame, an unheated greenhouse or a cold porch is ideal. Warm indoor conditions produce drawn, floppy seedlings.
Pinch the tip out at three or four pairs of leaves.Take the growing point out with finger and thumb. The plant then produces several side shoots from the base instead of one long stem, which is the difference between a thin plant and a full one.
Prepare the planting position generously.Dig in plenty of well-rotted manure or compost. Sweet peas are hungry and thirsty and the traditional trench of manure dug the previous autumn is not sentimentality — it feeds and holds water.
Plant out after hardening off, once the worst frost has passed.Autumn-sown plants go out in March or April, spring-sown in April or May. Harden them off over a week or two first. Protect from slugs, which take young sweet peas readily.
Give them a support they can actually grip.Sweet peas climb with tendrils, so they need something narrow — netting, twine, canes with string between them, or twiggy hazel. Tie the plants in at the base to start them off; once they find the support they climb themselves.
Water deeply and feed once flowering starts.A thorough soak two or three times a week in dry weather rather than a daily sprinkle. A high-potash liquid feed every fortnight from the start of flowering.
Pick every two or three days, without exception.This is the whole thing. A sweet pea's purpose is to set seed, and the moment a pod starts to form the plant reduces flowering. Pick every flower, whether you want it in the house or not, and remove any pods you find.
Keep going into late summer.Plants often look tired in August. Cut off any pods, feed, water thoroughly, and many will produce a further flush into September.

Tips

Old-fashioned and grandiflora varieties have far more scent and shorter stems; modern Spencer types have long straight stems for cutting and less fragrance. Choose on which matters to you.
An autumn sowing pinched in spring gives the strongest plants of all, but they must be kept cool and airy through winter or they go soft and mouldy.
Sweet peas grown up an obelisk in a border need the same picking discipline as ones grown for cutting. Nobody deadheads them and they stop by mid-July.
The tendrils grab everything nearby including each other. Netting or twine gives a much better result than a solid trellis.

Common mistakes

Not picking regularly — One ripening seed pod tells the plant it has succeeded and flowering falls away. Picking every two or three days is what extends the season by months.
Sowing into shallow trays or small modules — The long root has nowhere to go. The plants are permanently checked and never grow away strongly.
Growing them warm indoors — They come up drawn, pale and floppy. Cool and bright is what they want, even at seedling stage.
Planting them in poor dry soil — They are hungry and thirsty plants. In thin dry ground they mildew, yellow and finish early.

If it doesn't work

Flowering stopped in July

Cause: Seed pods left on the plant — Fix: Remove every pod, feed with high potash, water deeply. Many plants produce another flush.

Short flower stems

Cause: Under-fed, under-watered or a short-stemmed variety — Fix: Feed and water more generously; check whether you are growing a grandiflora, which is naturally short-stemmed.

Buds forming and then dropping off

Cause: Sudden temperature swings or irregular watering — Fix: Water consistently and mulch. Bud drop usually passes as conditions even out.

White powdery coating on the leaves in late summer

Cause: Mildew, from dry roots — Fix: Water deeply and mulch. Late-season mildew is very common and rarely worth spraying for.

Questions people ask

When should I sow sweet peas?

October for the strongest, earliest plants, overwintered somewhere cold and airy, or February to March under cover for an easier route to a good display.

Why have my sweet peas stopped flowering?

Almost always because seed pods have been allowed to form. Remove every pod, feed and water, and pick every flower from now on.

Do I need to pinch them out?

Yes — at three or four pairs of leaves. It converts a single tall stem into several, which is the difference between a sparse plant and a full one.

Can I sow sweet peas straight into the ground?

Yes, in April, though they flower later and less than pot-raised plants. Protect the seedlings from slugs and mice, which take direct-sown seed readily.