When to sow cucumbers

Match the sowing date to where they will grow

The RHS gives three indoor sowing dates, depending on where your cucumbers will grow:

- Heated greenhouse: mid February to mid March. - Unheated greenhouse, polytunnel or large cold frame: April. - Outdoors: late April.

It says sowing indoors is best, because the plants start to fruit sooner.

Garden Organic gives one window: sow indoors from mid March to mid April and plant out in mid May. That is a little earlier than the RHS date for outdoor plants and later than its date for a heated greenhouse. If you have no heated greenhouse, April is the date both sources share.

Warmth: what cucumber seeds need

Use 10 cm pots of peat-free seed compost, watered well before sowing, the RHS says. Sow one seed per pot, 1–2 cm deep, ideally on its side. Garden Organic sows at 1.5 cm, also on the side.

Cucumbers need warmth to germinate. The RHS uses a heated propagator at 21 °C, or covers the pot with a clear plastic bag and stands it on a warm, sunny windowsill indoors. Seedlings appear after a week or two. Garden Organic gives a germination time of 3 to 5 days.

Growing on

As soon as the seedlings appear, take off the cover. The RHS keeps them warm, in bright light and watered regularly. Garden Organic moves seedlings into larger pots as they grow.

Cucumber plants raised indoors are ready for their final position after about a month, once they are growing strongly, the RHS says. Water them well beforehand and avoid disturbing the rootball when you plant them.

When seedlings collapse: damping off

If seedlings fail to come up or suddenly collapse, often under a mass of whitish fungal growth, the cause may be damping off. The RHS describes it as a disease of seedlings caused by several soil-borne fungi and fungus-like organisms. It is mainly a problem when you sow indoors or under glass.

High humidity, poor air movement and sowing too thickly all encourage it, the RHS says, and so do low light and low temperatures in spring, when seedlings grow slowly. To prevent it, use fresh commercial compost and new or cleaned pots, and never reuse pots where damping off has been a problem. Use mains water where you can, and keep a rainwater butt covered. Do not overwater, and ventilate well. The RHS says no fungicides are available to gardeners against damping off.

Slugs and snails are the other threat to young plants, especially in damp weather, the RHS warns.

Planting out and the risk of frost

Cucumbers need warmth outside as well as in. The RHS plants them in an unheated greenhouse from late May, once the temperature stays above 12 °C. In a heated greenhouse planting can start in late March. Plants for outdoors are hardened off first and planted from early June, once local temperatures are reliably above 12 °C. Space them 30 cm apart in the ground, or use a container at least 30 cm wide and deep.

Your local date depends on where you live. The Met Office says the average last air frost in the south and on the coast often comes before the end of April. In the north and on higher ground it often runs well into May, and in a few places into June. It also notes that in May ground frost is more common than air frost.

Sowing straight into the ground

You can also sow outdoors in late May or early June, especially in warmer parts of the UK or in mild weather, the RHS says. Sow up to three seeds together, 1–2 cm deep, then thin to the strongest seedling. Cover with cloches or plastic-free fleece to keep the soil warm, and leave the cover on after the seeds come up.

Garden Organic gives an average of about 12 weeks from sowing to harvest and picks from July to September. Training, watering and picking are covered in How to grow cucumbers.