Sowing cucumbers: choose between greenhouse and outdoors

Cucumbers can be started under cover and grown in a greenhouse or outside, but those routes do not share one timetable. The common mistake is to sow very early in a warm pot without knowing where the plant will go next. It will need enough light, a warm growing position and a move outdoors without a cold shock. Choose the destination first; choose the sowing time second.

Where will the plant actually grow?

A cucumber for a heated greenhouse can be started earlier because the protected space extends the season. An unheated greenhouse sits between greenhouse and open ground: it buffers some cold nights, but does not guarantee a stable temperature. Outdoors, wait until frost risk has passed and the site is warm enough. A greenhouse variety may also have different pollination needs from an outdoor type, so check the seed packet before removing flowers or planting near other varieties.

The SNHF cucumber guide recommends sowing in pots in a heated greenhouse in March or April at 18–22°C, and gives 10–12°C as a minimum temperature. It suggests planting outdoors in May once frost risk has passed. These are the horticultural society’s recommendations, not a substitute for a thermometer in your greenhouse or the weather in your garden.

Start in a pot without rushing the move

Sow in an individual pot deep enough to keep the rootball intact at transplanting. SNHF suggests two or three seeds per pot and keeping the strongest seedling once they are established. The RHS also recommends individual pots and germination at about 21°C. Its guide distinguishes heated greenhouse, unheated greenhouse and outdoor sowing, but its dates are for UK conditions and should not become a French calendar.

Keep the compost moist but not waterlogged, and put the seedlings somewhere bright. A warm but dark room does not make a sturdy plant. A cold windowsill at night can also undo the benefit of daytime warmth. Check the space you have available: a cucumber left indoors too long can become awkward to handle before the bed or greenhouse is ready.

Acclimatise before planting out

Before moving a plant outdoors, harden it off gradually. Start with a few hours in a sheltered spot, then lengthen its time outside over several days if conditions remain mild. Bring it back under cover if a cold night is forecast. This avoids exposing it all at once to wind, direct sun and a different moisture level.

Water the pot before transplanting so the rootball holds together. Handle the plant by its pot or rootball rather than its stem. Cucurbits do not appreciate root disturbance; plant at the same depth as before and water at the base. If the soil is still cold or waterlogged, wait instead of relying on a cover that does not warm the root zone.

In a greenhouse, ventilate and watch humidity. Constant condensation and wet foliage can encourage fungal problems; water at the base and avoid cold draughts. Outside, a sunny, sheltered position is more useful than an early sowing into an exposed bed.

Choose a type that fits a small garden

Cucumbers can be trained up a support or allowed to trail. The RHS describes both climbing and scrambling varieties and notes that vertical growing saves ground space. A sturdy trellis also makes fruits easier to pick and keeps them off the soil, but install it before the stems grow. In a pot, choose a container suited to the variety and expect to monitor watering more closely than in open ground.

The guide to raising young courgette plants covers a similar move from pot to bed for another cucurbit, though the two crops do not have identical needs. For a cucumber’s place in the succession, see the crop-rotation guide and the vegetable-garden plan for beginners.

Before sowing, record the final position, available protection and the conditions for moving the plant outside in your garden plan and journal. A few well-managed plants are more useful than a batch of pots ready while the greenhouse or bed is still unwelcoming.