How to sow chilli seeds in the UK
Updated: 21 October 2026
You already have a packet, and a cold May sowing on a dry windowsill will not finish a chilli that needs the whole warm season. This is how to sow chilli seeds in the UK: indoors from late winter, at about 21°C until they emerge, then kept inside until nights stay at 12°C. The twelve packets are the [Chili seeds comparison](/hub/herb-seeds/chili-seeds). This page does not rank them. Peter F. used the RHS grow guide, Kew, and Which?, plus the germination lines those listings print. He did not run a germination trial. A qualifying purchase can earn a commission at no extra cost to you.
You already have a packet, and a cold May sowing on a dry windowsill will not finish a chilli that needs the whole warm season.
You already have a packet, and a cold May sowing on a dry windowsill will not finish a chilli that needs the whole warm season. This is how to sow chilli seeds in the UK: indoors from late winter, at about 21°C until they emerge, then kept inside until nights stay at 12°C. The twelve packets are the Chili seeds comparison. This page does not rank them. Peter F. used the RHS grow guide, Kew, and Which?, plus the germination lines those listings print. He did not run a germination trial. A qualifying purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Germination window | Sow temperature | Outdoor minimum listed | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  PRIMERASEMILLAS.COM Piri Piri | 8–21 days | 22–28°C | Above 12°C | 7.4 | about £3.45 | View offer |
 JustSeed JustSeed Jalapeno | n/a | n/a | n/a | 8.2 | about £4.35 | View offer | |
| Alternative |  SPECIALITYSEEDS.COM Thai Dragon | 12–21 days | 22–28°C | Above 15°C | 5.0 | about £3.45 | View offer |
On this page
When to sow in the UK
Sow indoors from late winter to mid-spring. The RHS gives that window and, from mid-February, groups chilli with tomato and aubergine in peat-free compost. Kew sows in February at 21°C and notes frost into late May or June. Which? says mid-February to March. A January line on the JustSeed description is earlier, and it still needs warmth, not a cold sill. The RHS harvests Carolina Reaper from mid-August to October, so a May sowing misses a superhot. If the seed arrives in May, sow a faster kitchen type under glass. No UK statutory pepper germination percentage was retrieved. A 12 month Real Seed Guarantee is seller text, not a percentage.
Heat and moisture until they emerge
The RHS germinates seed at about 21°C, then grows seedlings at 16–18°C. Which? states a minimum of 18°C and compost kept just moist. Do not average those figures. Sellers mostly print 22–28°C, with 12–21 days on Thai Dragon, Basket of Fire, and Jamaican Red, and 8–21 days on the Scorpions, Bolivian Rainbow, Scotch Bonnet Red, Piri Piri, both Reapers, and Tabasco. The jalapeño window is not stored. Its description says up to 35 days at 30°C. Thompson & Morgan’s 7–10 days at 18–25°C is another commercial calendar. One Tabasco review was slow, then 8 of 10 came up. One Reaper Yellow review saw nothing at 30°C. Do not dry compost on a radiator. A poly bag holds moisture. It does not heat a cold room.
Pot on before they get tall
Prick seedlings out at about 2–3 cm into peat-free multi-purpose compost, then move them to 13 cm pots when roots show. Which?’s 25–30 cm container is a later pot. Keep plants indoors until at least late May. A warm sunny sill can hold them. A north window is a poor place for the season. If nothing is up after the seller’s window, check moisture and whether the tray was near 21°C. The listing has not promised a percentage. A plant that is plainly the wrong variety is a Consumer Rights Act 2015 complaint to the retailer.
Harden off, then wait for the nights
Harden off for two to three weeks, out by day and in if the night will be cold. Plant out only when nights are reliably at least 12°C, usually late May or early June. Plants grow better above 15°C. Kew’s frost can last into June, so one mild afternoon is not the signal. Several sellers print outdoors only above 15°C. Piri Piri and Tabasco print above 12°C. Follow the RHS night line. An unheated greenhouse in early May can still drop below 12°C, and the RHS wants vents open above 30°C. A plant passport for Capsicum seed is the supplier’s paperwork, not yours.
Common questions
When should I sow chilli seeds in the UK?
Sow indoors from late winter to mid-spring. The RHS and Kew point to February, and Which? says mid-February to March. A seller line that says January is that listing's advice, and it still needs warmth. A May sowing is late, especially for a superhot that the RHS harvests from mid-August.
What temperature do the seeds need?
The RHS germinates them at about 21°C, then grows seedlings at 16–18°C. Which? states a minimum of 18°C. Sellers in this set mostly print 22–28°C. One jalapeño description says 30°C. Do not average those into one number. Hold about 21°C until they emerge if you are following the RHS, and do not bake dry compost.
Can I sow them on a windowsill without a propagator?
Yes, if the sill stays warm and the compost stays just moist. Which? allows a warm windowsill as well as a heated propagator. A cold glass, a north window, or compost dried by a radiator is the failure. One review in this set suggested a large poly bag as a stand-in propagator. It holds humidity. It does not heat a cold room.
How long do they take to come up?
Seller windows here are 8–21 days or 12–21 days. The RHS does not turn those into a guarantee. Thompson & Morgan says usually 7–10 days in its own guide. The JustSeed description says up to 35 days at 30°C. Reviews in this comparison include both a slow-then-successful tray and trays with no growth. Treat every day-count as a range, not a promise.
When do I pot the seedlings on?
The RHS pricks them out at about 2–3 cm into peat-free multi-purpose compost, then moves them to 13 cm pots when roots show. A final pot around 25–30 cm wide is the later Which? container, not the first prick-out. Keep them indoors through that sequence until at least late May.
When can they go outside overnight?
After two to three weeks of hardening off, and only when nights are reliably at least 12°C, usually late May or early June. They grow better above 15°C. Frost into late May or June is still possible, which is why a mild afternoon is not the signal. An unheated greenhouse can be too cold at night in early May.
Is there a legal germination percentage for pepper seed in the UK?
No UK statutory pepper germination percentage was retrieved for this guide. Do not use a United States figure in its place. Seller guarantees in this set are marketing text where they appear. A failed expectation about variety is a consumer-rights point with the retailer, not a percentage on the packet.
Related pages
Sources
Public UK sources named below. amazon.co.uk prices, stars and review counts on these twelve listings were read on 19 October 2026. Page updated 21 October 2026.
- RHS: grow your own chilli pepper (retrieved 19 October 2026)
- RHS: Capsicum chinense Carolina Reaper (retrieved 19 October 2026)
- Kew: chillies (retrieved 19 October 2026)
- Which?: how to grow chillies (retrieved 19 October 2026)
- GOV.UK: plants that need a plant passport in Great Britain (retrieved 19 October 2026)
- GOV.UK: issuing plant passports in Great Britain (retrieved 19 October 2026)
- The Seed Marketing Regulations 2011, Schedule 3 (retrieved 19 October 2026)
- Guinness World Records archive: hottest chilli, 2017 (retrieved 19 October 2026)