Winter greenhouse crops in the UK

Plan for harvest, not rapid midwinter growth

The appeal of an unheated greenhouse in winter is an extension of the season, not a second summer. RHS winter-growing advice lists corn salad, land cress and oriental salad leaves among late-summer and autumn sowings. Its winter greenhouse inspiration describes rocket and mizuna as winter salad crops. Sow enough in time for plants to establish before day length and temperature sharply restrict growth. A tray sown in the darkest weeks may sit small for a long time even if it germinates.

Start with crops you will actually harvest. Cut-and-come-again mixes, rocket, mizuna, mustard leaves and corn salad offer different flavours and cold tolerance. Check each variety's packet for sowing and spacing. A few well-spaced rows are easier to ventilate, inspect and pick than a crowded carpet of seedlings. The UK growing calendar gives the annual sequence; use your region and actual weather to refine it.

Clear the summer crop and prepare the root zone

Remove spent tomatoes and cucumbers, old ties and diseased debris before planting winter crops. Inspect the border or container medium for drainage and pest problems. Healthy spent material can follow your usual composting practice, but do not spread suspect disease around the greenhouse. The hygiene guide covers cleaning tools and benches between crops.

If using a growbag as a shallow salad bed, the RHS greenhouse vegetable guide describes removing a long top panel to create planting space. Ensure there are drainage holes and do not assume old medium is suitable if it held a diseased crop. A border offers more root volume but requires ongoing soil management. The growbags and borders comparison helps choose the approach.

Manage cold, condensation and light

An unheated greenhouse can freeze. Put a max/min thermometer near the crop, reset it and watch the forecast. Bubble insulation and fleece may offer additional protection, but neither is a guarantee against severe or prolonged frost. Covering crops also reduces light and can trap moisture, so remove or ventilate protection when appropriate. The unheated greenhouse temperature guide explains why a mild afternoon is not a safe night-time assumption.

Keep glazing reasonably clean and avoid shading the low winter sun with redundant shelves or equipment. Ventilate on suitable mild days to reduce condensation, then close vents before conditions become too cold for the crop. Water the root zone only when needed, ideally early enough for foliage and surfaces to dry. Cold, saturated compost and still air can cause more trouble than a few dry-looking surface crumbs.

Harvest gently and plan the next changeover

Pick outer leaves rather than stripping a small plant bare. In low light, regrowth can be slow; do not expect summer salad yields. Check for slugs, aphids and decaying leaves during each visit. If a severe cold spell arrives, prioritise plants with an established root system and suitable cold tolerance, and accept that some crops may be lost without added heat. As days lengthen, clear exhausted winter rows and prepare a clean space for spring sowings. The what-to-grow guide helps map that rotation without crowding one season into the next.