Cucumbers sown in April are cropping by midsummer

Sow cucumbers indoors from late March to late April. They grow fast, so there’s no prize for sowing early: a late April sowing often catches up with one from March, without weeks of a pot-bound plant waiting on the windowsill.

Plants go into a greenhouse from early May, and outside from late May once the nights stay mild. Either way, the first cucumbers are ready from late June or July.

Greenhouse or outdoor types

Greenhouse cucumbers give the long, smooth fruit you see in shops. Most modern ones are all-female, so every flower can become a cucumber. They need warmth and do best under glass or in a polytunnel.

Outdoor or ridge cucumbers are shorter and often a little bumpy or spiny. They’re tougher, crop well in a sunny, sheltered spot, and need insects to pollinate them.

Sowing

Cucumber seed needs about 20 °C to germinate.

1. Sow one seed per 8 cm pot, 2 cm deep. Some growers sow the flat seed on its edge so water doesn’t sit on it.
2. Keep it warm, on a windowsill above a radiator or in a propagator.
3. Seedlings come up in about a week. Give them plenty of light and keep them warm at night.
4. Pot on into a 12 cm pot if roots fill the first one before it’s time to plant.

From early May to early June you can also sow outdoor varieties straight into the ground, two seeds per spot, under a cloche.

Planting out

Cucumbers are killed by frost and stall below about 10 °C at night. These dates are for a typical lowland garden in England or Wales, with a last frost around 20 April.

• Greenhouse: from early May, into a border, a large pot or a growbag, two plants per bag.
• Outdoors: from late May to early June, after a week of hardening off. Further north, wait until June.

Space plants 30 cm apart, in rows 90 cm apart, and give them something to climb. Dill, nasturtiums, radishes, peas and French beans grow well nearby. Keep cucumbers away from potatoes.

Training and care

Tie greenhouse plants to a cane or string up to the roof, and pinch out the side shoots two leaves past each fruit. Outdoor plants can climb a trellis or sprawl over the ground.

Water little and often so the compost never dries out. Once the first fruits swell, feed weekly with a high-potash liquid feed. Damp down the greenhouse floor on hot days to keep the air humid, which helps keep red spider mite away.

Picking

Cucumbers are ready about 40 to 55 days after planting out. Pick them while they’re still firm and before they yellow, cutting the stalk with a knife. Pick often, as a plant left with a big cucumber on it slows down. A healthy plant keeps cropping for around ten weeks.

Common problems

Powdery mildew. White patches on the leaves from late summer. Water regularly and remove the worst leaves.

Red spider mite. Fine speckling and webbing under glass. Keep the air humid.

Fruit withering at the tip. Usually dry compost or poor pollination on outdoor types. Water steadily.

For more summer crops under glass, see when to sow tomato seeds and when to sow chillies. Set your own frost dates on the cucumber page.