How to Grow Sweet Peas: Sowing to Cutting in the UK

Sow sweet peas in October or November for the earliest, strongest plants, or January to March if you missed the autumn window — they climb 1.8-2.4 m, need a tall support, and flower from June to…

At a Glance

Sow sweet peas in October or November for the earliest, strongest plants, or January to March if you missed the autumn window — they climb 1.8-2.4 m, need a tall support, and flower from June to September with regular picking.

Overview

Sweet peas (Lathyrus odoratus) are the classic UK cottage garden climber — vigorous annuals that scramble up a tall support and fill the house with scent from June to September. This guide covers the traditional autumn-sow-and-overwinter technique that produces the earliest, strongest plants, plus the spring-sow fallback, the support and feeding routine that keeps them flowering, and the picking habit that makes the difference between a single flush and four months of cut flowers. If you are sorting out your soil before planting, sweet peas benefit from the same preparation you'd give a hungry bean row.

When to Start in the UK

Sweet peas are sown in two windows. The traditional route gives earlier, stronger plants; the spring route is the easier rescue if you missed it.

Autumn sowing (best results)

• South UK (Midlands, South, coastal): mid-October to mid-November. Sowing in the second week of October in a cold frame has worked well in my south-facing garden — the seedlings put on root growth through winter and are ready to plant out as soon as the soil warms.
• North UK (Scotland, North England): late October to early November. Slightly later, so the seedlings are small and tougher going into the coldest months.
• Soil temperature trigger: the seed germinates from about 10°C, so wait for a cool spell — you don't want seedlings bolting in a warm October.

Spring sowing (the rescue window)

• South UK: January to mid-March.
• North UK: mid-March to early April, once the worst frosts are past.
• Spring-sown plants flower about three weeks later and finish earlier; they're easier for beginners who don't have a cold frame.

For an extended flowering season, do both: a batch in October, another in February. The autumn batch flowers first; the spring batch carries you into late September.

What You'll Need

For sowing

• Deep root trainer pots or tall 9 cm pots (sweet peas have a long taproot and sulk if restricted)
• Peat-free multi-purpose compost mixed 50:50 with perlite or horticultural grit for drainage
• Quality seed from a UK supplier (Mr Fothergill's, Suttons, Higgledee Garden, or any of the seed-swap networks)
• Sharp knife or nail clippers for chipping the seed coat
• A cold frame, unheated greenhouse, or sheltered porch for overwintering
• Wire mesh or a piece of hardware cloth to cover autumn-sown pots (mice love sweet pea seed)

For planting out

• A tall support — 1.8 to 2.4 m is right. Hazel poles, an obelisk, a teepee of bamboo canes, or a horizontal cordon wire at 1.5 m and 2 m are all standard UK kit.
• A spade and a bucket of well-rotted garden compost or manure
• Mulch (strulch or homemade garden compost) for moisture retention
• Soft twine or sweet pea rings for tying in

For feeding and cutting

• Tomato feed (or comfrey liquid) once flowering starts
• A deep bucket and sharp secateurs or a florist's snip for cutting
• Tall vases — sweet pea stems are 30-45 cm long once conditioned

Step-by-Step Guide

1. Chip the seed coat if germination has been patchy before — a small nick on the seed coat opposite the eye with a sharp knife opens it up. Many gardeners skip this and it's fine.
2. Sow into deep pots. Fill a root trainer or tall 9 cm pot with the 50:50 compost-grit mix. Push two seeds per cell 2 cm deep, water, label.
3. Cover autumn-sown pots with wire. Mice will empty a cold frame of sweet pea seed overnight. Chicken wire over the top fixes it.
4. Germinate in a cool place at 12-15°C — a cold frame, an unheated greenhouse, or a north-facing windowsill. Germination takes 7-14 days.
5. Pinch out the growing tip at four true leaves, cutting back to two leaves above a node. This doubles the base branches.
6. Overwinter autumn-sown plants in the cold frame, pots tucked close together. Vent on mild days; water sparingly — compost just damp.
7. Hard off for a week in March, sliding pots out by day and back under cover by night.
8. Prepare the planting site with a bucket of well-rotted compost or manure forked into the top 20 cm per plant. Sweet peas are hungry.
9. Plant out in late March to mid-April once the soil has warmed. In my Hampshire garden, the last week of March is reliable; in Scotland, wait until late April or early May.
10. Set up the support before you plant. Drive in canes, hazel poles, or an obelisk, then plant one seedling at each cane, 20 cm apart in a row.
11. Water in and mulch. A 5 cm mulch keeps roots cool and moist — sweet peas resent drought at the root and a dry July shuts down flowering.
12. Tie in the new growth with soft twine every 20 cm. Tendrils find the support, but a loose tie keeps heavy stems from snapping in July storms.
13. Feed weekly with tomato feed at first bud in late May or early June. High potash prolongs flowering; nitrogen grows leaf at the expense of bloom.
14. Pick every 3-4 days. Cut every stem with at least one open flower. The more you cut, the more they flower. If pods are left on, the plant "finishes" by mid-August. Condition stems in a bucket of water for a few hours before arranging.
15. Let one or two plants set seed at the end of August if you want to save your own seed for next year — one plant gives enough for a whole row.

Common Problems

Greenfly / Blackfly on Growing Tips

• Symptom: clusters of green or black aphids on the soft new growth at the shoot tips in late spring and early summer; sticky honeydew on the leaves below.
• Cause: aphids colonise the nitrogen-rich new growth that sweet peas push out in May and June.
• Fix: rub off with your fingers, or spray with a dilute washing-up-liquid solution (a few drops in a litre of water) and rinse off after an hour. Ladybirds usually arrive within a fortnight and clear them naturally — most UK gardeners I know leave them alone once the ladybirds show up. See our aphids on roses page for a wider treatment guide that applies to sweet peas as well.

Mice Eating Autumn-Sown Seeds

• Symptom: small holes in the compost surface, seeds gone, pots emptied overnight.
• Cause: wood mice and house mice treat autumn-sown sweet pea seeds as a winter banquet.
• Fix: cover pots with chicken wire or hardware cloth until germination. Once the seedlings have their first true leaves, the mice lose interest.

Powdery Mildew in Late Summer

• Symptom: white powdery coating on the leaves from mid-August onwards; leaves yellow and drop.
• Cause: hot dry days followed by cold dewy nights — a typical UK August pattern.
• Fix: water at the base, not over the foliage; keep plants well mulched; cut off badly affected stems. Many gardeners simply pick more heavily — by September, the mildew often finishes off the season whether you intervene or not.

Slugs on Young Seedlings

• Symptom: tender shoots nipped off at the base overnight; slime trails on the rim of the pot.
• Cause: slugs in the cold frame or greenhouse find autumn and early-spring seedlings irresistible.
• Fix: use ferric phosphate slug pellets (the pet- and wildlife-safe kind sold everywhere in the UK), or stand pots on a sharp gravel tray. Check pots at dusk with a torch.

Buds Dropping Before Opening

• Symptom: flower buds form, swell, and drop without opening. Often blamed on "balling" in wet weather.
• Cause: usually a sudden dry spell at the root when the buds are forming — sweet peas resent drought stress and will drop buds rather than open them.
• Fix: a deep soak once a week is better than a dribble every day. Mulch in May to keep moisture in. Some modern Spencer varieties are more prone to balling than the old-fashioned Heritage types.

Expert Tips

• Autumn sowing wins every time: if you've only got one shot at sweet peas, do it in October. The plants are stronger, flower earlier, and keep going longer. In my experience, overwintered plants flower a full three weeks ahead of spring-sown ones.
• Pinch hard at four leaves, not later: leaving the tip beyond four leaves gives a tall, leggy plant with one flowering stem. Pinching at the right stage doubles the base branches and doubles the flower count.
• Feed with tomato food, not general feed: balanced fertilisers grow leaves. Tomato feed (high potash) feeds the flowers. Start at first bud and run through to early September.
• Cut in the early morning or late evening, when the stems are full of water. Take a bucket of water to the row with you, drop the stems straight in, and leave them to condition for a few hours before arranging. The full conditioning routine is in our cut flowers for the house guide.
• Cut when the lowest floret is just opening, not when the bud is fully coloured. Sweet peas picked too tight don't open in the vase; sweet peas picked fully open only last three days. The first-flower stage gives 5-7 days in a vase.

FAQ

When should I sow sweet peas in the UK? Sow in October or November for overwintering in a cold frame, or January to March for direct spring sowing. October-sown plants flower from early June and keep going until September.

How long do sweet peas take to germinate? 7-14 days at 12-15°C. They germinate well in a cold frame or unheated greenhouse — too much warmth makes them leggy and weak.

Why won't my sweet peas flower? The three usual reasons: too much nitrogen feed (lots of leaf, no flower), not enough water at the root, or you're not picking hard enough — let a few pods set and the plant thinks its job is done.

Can I grow sweet peas in pots? Yes. Use a deep pot — at least 30 cm — with a sturdy obelisk, and feed weekly with tomato feed once flowering starts. One plant per 30 cm pot is the rule; more than that and they sulk.

Are sweet peas poisonous? The seeds are mildly toxic if eaten in quantity, and the plant is toxic to pets — keep them away from dogs and cats. The flowers themselves are not eaten. Sweet peas are ornamentals — they are Lathyrus odoratus, not the edible pea Pisum sativum.

Do sweet peas come back every year? No. Sweet peas are hardy annuals — they grow, flower, set seed, and die in one season. Save seed from a few pods at the end of August and resow in October; or buy fresh seed each spring.

Varieties to Try

• 'Matucana' (heirloom, AGM) — the most fragrant sweet pea; small bicolour flowers in deep purple and violet-blue. Scent is exceptional. Available from Higgledee Garden and most heritage seed suppliers.
• 'Cupani' — the original sweet pea, introduced to Britain in 1699 by Father Cupani. Bicolour crimson and lilac, very fragrant, and reliably tough. Widely available in the UK and a good first variety if you're nervous.
• 'Wiltshire Ripple' (Spencer type, AGM) — modern Spencer variety with bold claret-and-cream bicolour flowers, strong stems for cutting, and a good scent. From Suttons and Mr Fothergill's.
• 'Mrs Collier' (Spencer, AGM) — large, cream flowers on long stems; a classic cut-flower variety for the vase. Scent is good rather than exceptional. Available from all the major UK seed suppliers.
• 'King Edward VII' — heritage crimson, very strong colour and a reliable old garden favourite. Sold widely in UK catalogues.
• 'Nimbus' — modern striped variety (purple stripes on white), bred for cutting. Good stem length. Available from Mr Fothergill's and many specialist sweet pea nurseries.
• 'Almost Black' — deep maroon flowers that read as nearly black in shade. Striking in a vase; more restrained scent. From specialist sweet pea growers like Owl's Acre and Higgledee Garden.

Sources & Further Reading

• Royal Horticultural Society — Sweet peas: growing guide
• Royal Horticultural Society — Lathyrus odoratus profile
• Gardeners' World — How to grow sweet peas
• Garden Organic — Sweet peas (organic growing notes)
• Suttons Seeds — Sweet pea variety selector
• Mr Fothergill's UK — Sweet pea varieties
• The National Sweet Pea Society (UK)