Sowing Sweet Peas in Autumn Under Glass: A UK Guide
Sow sweet peas in autumn from October to November in the south and Midlands. Use deep pots and keep them at 12C to 15C for germination. Keep seedlings cool and bright under glass through winter. Pinch out the tips at 3 to 4 pairs of leaves. Plant out from April to May after hardening off. Autumn-sown plants usually flower from late May or June.
Key takeaways
- In northern England and Scotland, many growers wait until January to February before sowing.
- Deep pots suit sweet peas because their roots grow long.
- Germination usually takes 10 to 21 days at about 12C to 15C.
- Water sparingly through winter, only when the compost starts to dry.
- Plant hardened-off seedlings 20cm to 25cm apart beside supports in April or May.
Expert tip
Write the sowing date on each label. Next summer, you can easily compare when your autumn and spring sowings flower.
When should you sow sweet peas in autumn?
Sow sweet peas in October to November in the south and Midlands.
In northern England and Scotland, many growers wait until January to February instead. Autumn sowing is therefore a regional choice, rather than a date everyone needs to follow.
Young autumn-sown plants spend winter building a deep root system. They usually flower a few weeks ahead of spring sowings, often with longer stems. Expect flowers from late May or June.
If you are wondering when to sow sweet peas in the UK, there are several opportunities. Spring sowing under cover runs from late January to March. Direct sowing outdoors follows in April.
Is it too late to sow sweet peas once November has passed? No, you can sow under cover from late January to March for flowers from early summer. Missing the autumn window does not mean missing sweet peas next year.
Our month-by-month seed sowing guide puts these dates alongside other sowing jobs. You can also plan autumn tasks with our October garden jobs guide.
Why sow sweet peas under glass rather than outside?
Under glass, you can protect autumn sowings from heavy rain and manage protection against mice and hard frost.
The aim is cool, bright shelter through winter. Sweet peas are hardy annuals, so they do not need warm indoor conditions. Too much warmth produces weak, leggy plants.
A greenhouse or cold frame gives you control over ventilation and frost protection. Open vents or lids on mild days. Close them on hard frost nights.
Remember that glass alone does not keep mice away from seeds. Cover the pots until shoots appear, or set mouse traps in the greenhouse.
Our guide to what to grow in an unheated greenhouse covers other crops for the space. For sweet peas, keep the focus on full light and cool conditions.
Do you need to soak sweet pea seeds before sowing?
No, soaking sweet pea seeds is optional, and many germinate without it.
An overnight soak can speed up germination, mainly for dark, hard seeds. Do not leave seeds soaking for longer, as they can rot.
Another option is nicking the hard seed coat with nail clippers. Make the nick on the side away from the seed's "eye". You can also sow without either treatment.
Sweet peas germinate best at about 12C to 15C. Shoots usually appear within 10 to 21 days. A cool porch, windowsill, unheated greenhouse or cold frame is suitable.
Do not add heat simply because shoots have not appeared after a few days. The normal germination window extends to 21 days. Our seed germination temperatures guide explains temperature requirements for other seeds.
How do you sow sweet peas in pots?
Sow sweet peas in deep containers, about 1cm to 2cm below the compost surface.
Root trainers, deep 9cm pots and cardboard toilet roll tubes stood in a tray all work. Choose depth because sweet peas grow long roots.
- Fill deep pots, root trainers or tubes with peat-free multipurpose or seed compost. The compost should be moist but not wet.
- Sow 2 seeds per deep cell or tube, about 1cm to 2cm deep. In a 12cm pot, sow 5 to 6 seeds around the edge.
- Water after sowing. Keep the compost moist rather than wet.
- Label each pot or tray with the sowing date.
- Cover pots with glass or a lid against mice until shoots appear. Alternatively, set mouse traps in the greenhouse.
- Place the containers somewhere cool, ideally about 12C to 15C. No heat is needed.
Once shoots appear, move the seedlings to the coolest bright spot with full light. Our greenhouse propagation guide covers seed sowing alongside cuttings and division.
Where should sweet pea seedlings go over winter?
Keep sweet pea seedlings on unheated greenhouse staging or in a cold frame, with full light.
After germination, choose the coolest bright position available. Avoid keeping seedlings somewhere warm, as excess warmth gives weak, leggy growth.
Water sparingly over winter, only when the compost starts to dry. Check the compost before watering rather than following a fixed schedule. Ventilate on mild days.
Young plants cope with light frosts. On hard frost nights, close greenhouse vents and cold frame lids. Our guide explains how warm an unheated greenhouse stays in winter.
Our greenhouse staging collection has options for keeping seed trays up in the light. The Vitavia staging below is powder-coated aluminium and 1200mm wide. It has slatted top and bottom shelves.
View and buy the Vitavia 2 Tier Green Staging £109
A smaller option is the Palram Canopia 2 Tier Staging £79, at 800mm wide and 400mm deep.
It is polypropylene and has perforated shelves for drainage.
Check overnight temperatures with the Vitavia Max Min Thermometer £30. It records the highest and lowest temperatures and has a reset button. The minimum reading shows how cold the greenhouse or cold frame became overnight.
Can you grow sweet peas in a cold frame?
Yes, a cold frame is suitable for autumn sowing and overwintering sweet pea seedlings.
Open the lids on mild days for ventilation. Close them on cold nights, especially when a hard frost is expected. Keep the seedlings in full light and water only when the compost starts to dry.
A cold frame also doubles as the hardening-off space in spring. You can gradually increase daytime exposure before planting out.
Browse our cold frames collection or read our cold frame gardening guide for seasonal growing advice. The Elite model below is extruded aluminium and has 2 roof vents.
View and buy the Elite Min E Lite 4x2 Cold Frame £249
The budget 3x2 Alderley Cold Frame £69 has grey-washed hardwood, twin-wall polycarbonate, two independent lids, 1000mm width and 400mm height.
When should you pinch out sweet peas?
Pinch out sweet peas when each seedling has 3 to 4 pairs of leaves.
Remove the growing tip just above a leaf joint. This makes the plant branch and gives more flowering stems.
Use the leaf count as your guide, rather than a fixed date after sowing. Look for pairs of leaves, rather than counting individual leaves. Seedlings are ready for pinching out once they reach that stage.
After pinching, keep the plants cool and bright. Continue watering sparingly through winter, only when the compost starts to dry.
What does the sweet pea year look like from autumn sowing?
Autumn-sown sweet peas spend winter under cover before hardening off in March and planting out in April or May.
| Month | Job | Where |
|---|---|---|
| October to November | Sow in deep pots | Cool porch, windowsill, unheated greenhouse or cold frame |
| November to December | Germination and first leaves | Cool, bright position under cover |
| December to February | Pinch out, keep cool, water sparingly | Unheated greenhouse or cold frame |
| March | Start hardening off | Open cold frame or outdoors by day, under cover at night |
| April to May | Plant out with supports | Workable ground or a large outdoor pot |
| Late May to September | Pick flowers often | Outdoor growing position |
When should you plant out autumn-sown sweet peas?
Plant out hardened-off sweet peas from mid April in the south, or May further north, once the ground is workable.
Start hardening off from March over 2 to 3 weeks. Open cold frame lids during the day, or move plants outside. Bring plants back under cover at night.
This gradual routine comes before planting out, even though sweet peas are hardy annuals. Our guide to hardening off seedlings explains the process step by step.
Plant seedlings 20cm to 25cm apart at the base of their supports. A cane wigwam or netting about 1.8m to 2.4m tall is suitable. Tie in the young stems at first.
If you are deciding when to plant sweet peas, check both the regional timing and the ground. Wait until seedlings are hardened off and the ground is workable.
Can you grow sweet peas in pots all summer?
Yes, sweet peas can grow in large pots with supports and regular summer watering and feeding.
Choose a pot at least 30cm to 45cm across. Plant 6 to 8 plants around a wigwam.
The sowing dates are the same whether plants will eventually grow in pots or the ground. Start with deep containers, then follow the hardening-off routine before planting outside.
Pick flowers every few days through the flowering season. Leaving seed pods to form stops new flowers.
Our team at Greenhouse Stores can help you choose a greenhouse, staging or cold frame for winter sowing.
Frequently asked questions
Is it too late to sow sweet peas in the UK?
You can sow under cover from late January to March if you miss autumn. Direct sowing outdoors is another option in April. Spring sowings produce flowers from early summer.
Do sweet peas come back every year?
No, sweet peas, Lathyrus odoratus, are hardy annuals. The perennial everlasting pea, Lathyrus latifolius, comes back each year. It has no scent.
Can you sow sweet peas indoors on a windowsill?
Yes, a cool windowsill is suitable for sowing sweet peas. Aim for about 12C to 15C during germination. Once shoots appear, choose the coolest bright spot, as excess warmth produces weak, leggy plants.
How many sweet pea seeds per pot?
Sow 2 seeds per deep cell or tube. For a 12cm pot, sow 5 to 6 seeds around the edge. Sow about 1cm to 2cm deep.
Do autumn-sown sweet peas need heat?
No, autumn-sown sweet peas do not need heat. Germination is best at about 12C to 15C and usually takes 10 to 21 days. Keep seedlings cool and bright afterwards.
Can you eat sweet peas?
No, sweet pea seeds and pods are not edible. They can be harmful if eaten. Keep seeds away from children and pets.
