When to sow cucumbers UK: indoor and outdoor dates

The seed is in a small envelope, the propagator is on the spare bedroom floor, and a single date copied from a video would put an outdoor bed and a heated glasshouse on the same morning. When to sow cucumbers UK gardeners is three dates, set by the Royal Horticultural Society according to the place the plants will grow. Heated glass starts earliest. An unheated tunnel waits until April. Outdoor seed is later again. The variety on the packet has to belong to that place.

In short

Sow cucumbers for a heated greenhouse from mid-February to mid-March, for an unheated greenhouse or polytunnel in April, and for an outdoor crop either indoors in late April or direct in late May or early June in milder spots. The RHS cucumber page gives about 21°C for germination and one seed sown 1–2 cm deep. Plant without burying the stem. A greenhouse variety and an outdoor ridge variety do not share one calendar.

Three start dates

The RHS page on growing cucumbers splits the year by cover. A heated greenhouse is mid-February to mid-March. An unheated greenhouse or polytunnel is April. An outdoor crop is sown under cover in late April, then planted when the weather is reliably warm, or it is sown direct outdoors in late May or early June where the spot is milder. Those are the dates to write on the packet. A blog that offers one Saturday in March for every postcode has skipped the site.

Mid-February only pays if you can keep the heat on after germination. A seedling that then sits on a cold landing for six weeks is drawn before the tunnel is warm. If the glass is heated only when you remember, use the April date. The earlier date is for a house that stays warm enough for this crop.

April covers the unheated greenhouse and the late-April indoor sowing for plants that will go outside. Label the pots. A Socrates plant for a tunnel and a Marketmore for a grow bag look alike at the seed-leaf stage. Mix them up and you put a greenhouse type on a windy patio.

Our cucumber seeds comparison holds both kinds of packet. Mr Fothergill's Socrates F1, about five seeds at £4.99 in the captured file, is an all-female mini for a greenhouse or tunnel, rated 4.4 from 102 ratings. Mr Fothergill's Marketmore 76, about 25 seeds at £2.39, is an outdoor ridge type, rated 4.3 from 235 ratings. Socrates is the top pick, and the listing in the box, because the usual mistake is to give a tunnel variety an outdoor date. It is a poor buy if your only space is an open bed.

Heat and the pot

Germination wants about 21°C, which is the figure on the RHS cucumber page. The same charity's general guide to sowing indoors says many seeds are content nearer 18°C. Use 21°C for cucumbers. A kitchen that feels warm to you can still be 17°C at pot level overnight. A soil thermometer in the compost tells you more than a room thermometer on the wall. If you cannot hold the compost near 21°C, wait. An extra fortnight in April beats a February sowing that rots.

Sow one seed about 1–2 cm deep, as the RHS describes. Socrates is about five seeds, so three seeds in one module spend most of the packet on one plant. Fill a small pot with moist seed compost, set one seed on its side, and cover to that depth. Soggy compost excludes air, and dry compost at the right temperature still fails. Water with a fine rose, then cover the pot until the hook of the stem shows. Light matters after emergence. Before that, steady warmth and moisture matter more.

A windowsill is bright by day and cold against the glass at night. Turn the pot once the seed leaves open, and move it off the pane on a frost. Vent a propagator lid once true leaves show. Five seeds is a short second chance: sow two now and keep two in the envelope. If the first pot is empty after a warm, moist fortnight, sow a spare. A pot that never reached 21°C has not given the packet a fair test. We did not germinate this seed.

Outdoor sowing is later

Direct sowing is the route that gets rushed. The RHS puts it in late May or early June, and only for milder spots. Cold, wet clay around a cucumber seed is how a short packet disappears. In a frost pocket or a colder garden, sow under cover in late April and plant out when the weather is reliably warm.

Reliably warm is the RHS phrase for planting. Our source note does not add a soil figure such as 12°C. Frost is the clear boundary. Fleece over a freshly planted cucumber helps for a chilly night. It does not turn a March bed into a June bed.

Direct sowing is still one seed at 1–2 cm, into damp workable soil. A grow bag or a bed with compost dug in drains better than wet allotment clay. A July seedling rarely pays before the weather turns.

Marketmore is the outdoor ridge type with the larger seed count, about 25 seeds. That count gives you a second sowing if pigeons, slugs or a cold night take the first. The mildew line on its bullet is a seller claim, not a result we measured. An outdoor variety still wants the warmest bed you have. A north wall is a greenhouse problem without the greenhouse.

Planting without burying the stem

When you plant a cucumber that was sown in a pot, set it at the same depth it was growing. The RHS advice on tender crops in a greenhouse is to avoid burying the stem. Cucumbers are prone to neck rot. A collar of wet compost against the stem is a common way to lose a plant that germinated perfectly. Tease the rootball out, sit it so the compost surface matches the bed or the grow-bag surface, and water around the plant rather than into a buried hollow at the neck.

Hardening takes about a week. Start with a few hours in the tunnel or a sheltered spot, and bring plants in if the night is cold. A plant that goes from a 21°C lid to a breezy allotment in one afternoon stalls. You have skipped the step between pot and bed.

A mini cucumber in a tunnel needs a string or a cane at planting. An outdoor ridge type can sprawl, so keep the stem base clear of soaked mulch. Close the vents only against a cold night. A sealed tunnel on a sunny April day cooks seedlings.

Water after planting, then let the surface dry slightly before you soak again. Constant wet at the neck undoes the careful depth. Past the middle of June, a new plant is a gamble on a short season.

Match the packet to the site

The calendar is useless if the variety is for a different building. Socrates is the greenhouse or tunnel packet in our file. Use the heated dates only if that glass is actually heated, and the April date if the tunnel is unheated. Marketmore is the outdoor ridge packet. Use late April under cover, or direct sowing in late May or early June in a milder spot. Listings that omit the place, including Unwins Cucumber Merlin at £1.99 with a single rating and no site line in the captured fields, ask you to supply the instructions from somewhere else. That is a fair buy only if you already know the variety.

All-female types are there to avoid the bitter fruit that can follow pollination. An outdoor ridge cucumber is a different choice, and insects will visit it. This page is about shop packets with a variety name, sown once, in the RHS window for their site.

Socrates at £4.99 and Marketmore at £2.39 are the captured figures, in GBP, and they move on the live amazon.co.uk page. We compared those listings with the RHS cucumber, indoor-sowing and greenhouse pages. We did not run a sowing trial.