Independent comparisons. We may earn from shop links. More info

How to sow cucumber seeds in the UK

Updated: October 2026

You sowed in February because the packet mentioned that month, and the compost is still bare and cold. How to sow cucumber seeds in the UK starts with the place the plants will finish in, then with warmth of about 20–21°C, not with the earliest date on a greenhouse variety. A cold windowsill is the usual reason nothing comes up. Our [cucumber seeds comparison](/hub/seeds-and-plants/cucumber-seeds) is where the packet type is chosen. This page is the week and the method once you already hold seed. Dates come from RHS, Thompson & Morgan, Gardeners' World, and Which? guidance, not from a sowing trial of our own. A qualifying purchase can earn a commission at no extra cost to you.

You sowed in February because the packet mentioned that month, and the compost is still bare and cold.

Our top picks

Mr Fothergill's Mr Fothergill's Socrates F1

Best overall

Mr Fothergill's Mr Fothergill's Socrates F1
8.8/10about £4.99
View offer
Mr Fothergill's Mr Fothergill's Marketmore 76

Best value

Mr Fothergill's Mr Fothergill's Marketmore 76
8.6/10about £2.39
View offer
Mr Fothergill's Mr Fothergill's Telepathy F1

Alternative

Mr Fothergill's Mr Fothergill's Telepathy F1
7.6/10about £3.49
View offer
On this page
  1. 1.Choose the week from the place
  2. 2.Sow one seed warm, on its side
  3. 3.Keep the seedlings brighter than they are hungry
  4. 4.Plant out only when the nights allow

Choose the week from the place

Pick the final place first, then sow late enough that you can keep the seedlings warm. The RHS sows for a heated greenhouse from mid-February to mid-March, for unheated cover in April, and for outdoor planting from a late-April sowing. Direct sowing of outdoor varieties, in warmer spots, waits until late May or early June. Which? puts sowing under cover at mid to late April. Use the later end if you cannot heat the pots. A February sowing without that heat fails even when the packet allows February. Telepathy F1 says February to May at 20–25°C, and it still demands the temperature. Marketmore 76 is not a reason to start on a cold February sill. If you have not chosen between a greenhouse type and a ridge type, do that on the buying guide before you fill pots. The method below is the same for both. The week is not.

Sow one seed warm, on its side

Use a small pot, about 10 cm, and sow one seed rather than a pinch. The RHS suggests one seed per 10 cm pot, 1–2 cm deep, laid on its side. A range of about 0.5–2 cm, on the side or edge, covers the packet and the RHS. Laying the seed flat in wet compost is a common way to lose it to rot. Do not overwater. The temperature is about 20–21°C. Suttons has separated ordinary types at about 15°C from all-female types at about 21°C, so give a greenhouse packet the warmer end. A window that drops to the mid-teens at night is not enough for a February pot. We are not stating a germination percentage.
Mr Fothergill's Mr Fothergill's Socrates F1

Testsieger

Mr Fothergill's Mr Fothergill's Socrates F1
8.8/10about £4.99
View offer

Keep the seedlings brighter than they are hungry

Once they are up, light matters more than feed. A warm, dim windowsill produces a tall, pale stem that flops when you move it. Separate a cold or dark pot from a packet that failed, and do not blame the variety for a February sill you could not heat. Keep the compost moist rather than wet. Gardeners' World says not to grow a ridge variety in the same greenhouse as an all-female type. That decision is on the greenhouse versus outdoor page. If nothing has moved after a fortnight and the compost never reached about 20°C, sow again warmer. If the bag had no variety name, do not plant it. UK reports go to planthealth.info@apha.gov.uk.

Plant out only when the nights allow

A rule of thumb is to wait until nights in the greenhouse stay above about 12°C before you plant into the border, a large pot, or a grow bag. That figure is a practical threshold, not a result from this comparison. The RHS treats these cultivars as tender summer plants. Frost in a mini greenhouse is a recorded way to lose a Socrates crop. Outdoors, wait until frost has passed, often late May or June. Marketmore belongs in that sunny bed. Socrates or Telepathy belongs under cover. When flowers appear, remove males on a greenhouse type that says so, and leave them on a ridge type. That distinction is on the all-female cucumber seeds page.

Common questions

What temperature do cucumber seeds need?
About 20–21°C. The RHS uses about 21°C, Gardeners' World says at least 20°C, and Thompson & Morgan says about 20°C. Telepathy's packet says 20–25°C. Suttons has suggested about 15°C for ordinary types and about 21°C for all-female types, so give greenhouse and all-female packets the warmer end. A cold February windowsill below that range is the usual reason a pot stays empty.
Should the seed lie flat?
No. Sow it on its side or on its edge. The RHS says on its side at 1–2 cm in a 10 cm pot. Thompson & Morgan says on the side at about 1 cm. Telepathy says on edge at 0.5 cm. Flat seed in wet compost is more likely to rot. Keep the compost moist, and do not flood the pot. Which? specifically warns against overwatering.
When do I sow for an unheated greenhouse?
April is the RHS timing for unheated cover. Which? says mid to late April under cover, and up to mid-May in the north. You still need about 20°C or more for germination, which may mean a propagator in the house and a move to the unheated greenhouse only after the seedlings exist and the nights are kinder. Sowing in February for an unheated house repeats the cold-pot failure.
Can I sow ridge cucumbers straight outside?
Yes, in warmer spots, late in the season. The RHS says late May or early June for direct sowing of outdoor varieties. Thompson & Morgan says May to June. You can also raise them in warmth from a late-April sowing and plant them out after frost. Marketmore is the outdoor ridge packet in this set. Do not put it in the same greenhouse as an all-female type.
When do seedlings go into the greenhouse border?
When the nights in that greenhouse stay warm enough, as a rule of thumb above about 12°C, and after the plants have been hardened if they were raised in a propagator. These are tender plants. A frost in a small greenhouse can finish a crop that germinated well. Outdoors, wait until frost has gone, often late May or June. Then follow the packet on male flowers rather than treating every cucumber the same.

Related pages

Best cucumber seeds

Cucumber seeds

All-female cucumber seeds: pinch out the male flowers

Best cucumber seeds UK: match the packet to the place

Greenhouse vs outdoor cucumber seeds: pick the place

Sources

Public UK sources used for sowing, flower rules and seed marketing. Last checked on 16 October 2026.

  1. RHS, How to grow cucumbers (retrieved 16 October 2026)
  2. Gardeners' World, How to grow cucumbers (retrieved 16 October 2026)
  3. The Seed Marketing Regulations 2011 (retrieved 16 October 2026)
  4. GOV.UK, Apply to have seeds certified for marketing in England and Wales (retrieved 16 October 2026)
  5. RHS, Cucumis sativus Socrates (retrieved 16 October 2026)
  6. RHS, Cucumis sativus Marketmore (retrieved 16 October 2026)
  7. Suttons, How to avoid bitter cucumbers (retrieved 16 October 2026)
  8. RHS, Cucumber mosaic virus (retrieved 16 October 2026)