How to Grow Sweet Peas from Seed (Lathyrus odoratus)

Sweet peas are the one flower where a date on the calendar changes the plant itself. Sown in late October and overwintered cold, Lathyrus odoratus spends the dark months building a taproot the length of your hand while the top growth barely moves — and then flowers three to four weeks earlier, on longer stems, than the identical seed sown in March. Gardeners have argued about soaking, chipping and chitting sweet peas since the Sicilian original reached Britain in 1699. Most of that argument is beside the point. Four things decide your crop: root depth, mice, pinching, and how ruthlessly you pick.

• Botanical name Lathyrus odoratus — the annual sweet pea
• Sow Oct–Nov under cover (best) · Feb–Mar under glass · Mar–Apr direct
• Germination 10–21 days at 12–15 °C — cool, never warm
• Flowering June–September, for exactly as long as you keep picking

Autumn or spring? The choice that sets your whole season

A sweet pea is a legume with a deep, brittle taproot, and almost everything good about an autumn-sown plant comes from that root. Sown in late October into a deep pot and kept cold and airy, the seedling puts on perhaps 8 cm of top growth by February — and 25–30 cm of root. Cold is not a hardship it endures; it is the signal that keeps the shoot short while the root runs. Come April that plant climbs away from a spring-sown one and never looks back: earlier flowers, longer stems, and more of them before late-summer heat calls a halt.

The honest counter-argument is that autumn sowing asks you to keep young plants alive for five months in the least forgiving season. If your only space is a cold, wet, mouse-ridden shed, a February sowing under glass is the safer route and puts you barely a fortnight behind. Sowing straight into the ground in April works too — it simply gives shorter stems and a shorter season. The technique is the same one behind autumn-sown hardy annuals, and sweet peas are the crop that repays it most.

One thing autumn sowing is not is a race. Late October to mid-November is the window; sowing in early September gives soft plants that get too big before the cold arrives, which is the same mistake that catches out overwintered pansies and spring cabbage.

How to sow sweet peas, step by step

1. Choose depth over width Root trainers, 9 cm pots at least 12 cm deep, or cardboard tubes stood in a tray. A standard seed tray is the commonest reason autumn-sown sweet peas disappoint — there is nowhere for the taproot to go. Two seeds per cell, thinned to the stronger one.
2. Sow dry, 2 cm deep — do not soak Overnight soaking is traditional and usually harmful: a seed that takes up water faster than it can breathe rots. Sow dry seed into moist compost, water once, and leave it. If nothing has swollen after 48 hours, nick the hard black coat opposite the eye with a craft knife and sow again — see how seeds germinate for what is actually happening.
3. Germinate cool: 12–15 °C A cold frame or unheated greenhouse. A heated propagator produces pale, drawn seedlings that collapse in the first frost — the classic leggy seedling failure. Keep the air moving to avoid damping off.
4. Mouse-proof the tray on day one Mice eat more autumn-sown sweet peas than frost ever does. A big starchy seed in a cold frame in November is a free meal. Raise trays off the floor and cover them with fine mesh or a sheet of glass until the shoots are up.
5. Pinch at three or four pairs of leaves Take the growing tip out between finger and thumb. One stem becomes four to six, and a pinched plant carries roughly twice the flowers of an unpinched one. Do it in autumn or in February — never after the plant has started to climb.
6. Plant out in March or April Two plants per cane, 20–25 cm apart, at the foot of the support, into ground enriched with compost the previous autumn. Harden them off over a fortnight first; overwintered plants are tough but still resent a sudden move.

Support, water, and the rule that keeps them flowering

Sweet peas climb with tendrils, so they need something thin enough to grip: netting, twiggy hazel, or vertical strings. A smooth pole defeats them entirely, which is worth knowing before you build anything — our guide to plant supports and trellising covers the structures, and sweet peas sit happily alongside the other twiners in our round-up of climbing plants to grow from seed.

Water is the other half of the job. Bud drop — fat buds that yellow and fall before opening — is almost always a dry root, and it is the single commonest complaint in a hot June. Prepare a deep, moisture-retentive root run the autumn before, mulch in spring, and soak thoroughly once or twice a week rather than sprinkling daily; our guide to watering explains why depth beats frequency. Feed with a high-potash liquid feed once the first buds show and stop the nitrogen, exactly as with any other flowering crop — see feeding and fertilising.

Then the rule that matters more than all of it: leave one seed pod on the plant and it stops. A sweet pea is an annual with a single goal, and a swelling pod tells it the year is over. Pick every second day, take the whole stem, and remove any pod you find. Ten minutes twice a week is the difference between six weeks of flowers and three months of them — the same discipline that keeps a cut flower garden productive.

Scent — and why some modern sweet peas have almost none

If you have ever bought sweet peas and found them beautiful but oddly silent, the packet was telling you and you did not know the code. Grandiflora or old-fashioned types — Painted Lady, dating to 1737, and the Sicilian originals — carry small, hooded flowers, three to a stem, and a scent that fills a room. Spencer types, the frilled, wavy-petalled forms bred from 1900 onwards, give bigger flowers, four or five to a long straight stem, and are what florists mean by a sweet pea; scent came second to form in that breeding and varies enormously between varieties. Neither is better. Sow Spencers for the vase, grandifloras for the doorway, and read the packet before you assume. Our spotlight on Painted Lady and the scented grandifloras follows that history from 1699 to the modern seed rack, and names the varieties worth growing for perfume alone.

Two more things worth knowing. The perennial pea, Lathyrus latifolius, is a different plant with a different job: it returns from the same rootstock for years and will smother a fence, but it has no scent at all. And sweet peas are strictly ornamental — Lathyrus seeds are toxic and are not a substitute for the edible garden pea, so keep the packets apart and out of reach of children.

Common mistakes

• Soaking the seed overnight. It drowns as many seeds as it starts. Sow dry into moist compost and only chip what refuses to swell.
• Sowing in a shallow tray. The taproot needs 12 cm at minimum. Depth of pot matters more than warmth, compost or feed.
• Germinating them warm. A heated propagator gives soft, drawn plants. Cool and bright wins every time.
• Forgetting mice. Cover autumn sowings from the day you sow, not after you find the empty cells.
• Not pinching. An unpinched plant makes one long stem and half the flowers.
• Letting pods form. One ripening pod ends the season. Pick relentlessly, holidays included.
• Planting into thin, dry soil. Sweet peas are hungry and thirsty; a compost-enriched trench pays for itself in stem length.

Frequently asked questions

When should I sow sweet peas?
Late October to mid-November under cover is the classic sowing and gives the best plants. February to March under glass is the safe alternative, and April is fine for direct sowing outdoors. Everything else you can still put in around now is in our September and October sowing collections.

Do I really need to soak sweet pea seed?
No. Modern advice is to sow dry seed into moist compost; soaking risks rotting the embryo before it can breathe. If a seed has not swollen after two days, nick the coat opposite the small pale eye with a knife and sow it again. Old seed is worth checking with a quick germination test — sweet peas stay viable for about three years.

Do sweet peas come back every year?
Lathyrus odoratus is a true annual and dies after flowering, so sow fresh each season. The everlasting pea, Lathyrus latifolius, is perennial and returns for years — but has no scent. If you want to save your own seed, let three or four pods ripen and brown at the very end of the season and follow our guide to seed saving; note that named varieties often will not come true.

Why have my sweet peas stopped flowering?
Pods, heat or drought, in that order. Check the whole plant for seed pods and remove every one; soak the roots deeply; and accept that a run of 28 °C days will shorten any sweet pea's season. A cool spell in September often brings a final flush from plants you had written off.

Sweet peas ask for very little that is difficult: a deep pot, a cold winter, a lid against the mice and a pair of scissors used often. Browse the Lathyrus odoratus range, the wider cut flower, annual and organic flower collections, or the whole flower seed library. Pair them with something for bees from our pollinator garden guide, start a pot of winter herbs on the windowsill while you wait, or dig into the rest of our growing guides and the full sowing calendar. Sow deep, keep them cold, and pick like you mean it.