Spring Seed Sowing in a Cold Greenhouse

Why a Cold Greenhouse Comes Into Its Own

By early March, an unheated greenhouse is doing quiet, valuable work. It will not give you tomatoes in April, but it will lift daytime temperatures several degrees above the garden, keep driving rain and nipping wind off young leaves, and dry the soil surface enough to sow into. That margin is often all a hardy salad or an early brassica needs to get going while the ground outside is still cold and claggy. Treat it as a shelter rather than a hothouse and you will get far more from it.

The trick is to work with the season rather than against it. Choose crops that shrug off a cold night, and you will be eating cut-and-come-again leaves weeks before your neighbours. Choose tender crops too early and you will spend April nursing sulking seedlings.

What to Sow First

Hardy salads are the natural starting point, and they are generous with their rewards. Sow lettuce such as 'Winter Density' or 'Arctic King', corn salad, land cress, mizuna, rocket, mustard and pea shoots. These will germinate at around 7–12°C and grow steadily rather than racing, which suits a cold house perfectly. A pinch of seed in a module tray every fortnight from late February gives you a continuous supply rather than a glut.

Brassicas come next, and they need a longer run-up than most people allow. Sow kale, cabbages, calabrese, Brussels sprouts and purple sprouting broccoli from late February into March. They are robust, they germinate reliably, and starting them under glass means they go into the ground as sturdy young plants that can outgrow the worst of the slug and flea beetle pressure.

Reliable choices to have on your list include:

• Lettuce – loose-leaf types for cutting, hearting types for spacing out later.
• Corn salad and land cress – slow but unfailingly hardy, superb winter-to-spring fillers.
• Rocket and mizuna – quick, peppery, ready to cut in five or six weeks.
• Kale and purple sprouting broccoli – sow now, crop for months.
• Cabbage and calabrese – sturdy plants that transplant well.
• Radish – fast, cheerful, and useful for marking rows.

Sowing and Caring for Seedlings Under Glass

Use fresh, peat-free seed compost and clean trays or modules. Old compost and dirty trays are the main cause of damping off, that frustrating collapse where seedlings keel over at the base. Sow thinly, cover lightly, and water from above with a fine rose, or stand the tray in a shallow bath of water until the surface darkens.

Always use water at room temperature. Cold tap water straight onto warm compost gives seedlings a shock they rarely forgive. Once germinated, move trays into the brightest spot you have. Light is the limiting factor in a cold greenhouse, not heat, so leggy, pale seedlings almost always mean too little light rather than too little warmth.

Prick out into individual modules as soon as the first true leaves appear, holding the seedling by a leaf rather than the stem. Water in gently and keep the compost just moist. Brassicas especially benefit from being moved on early, as it builds a strong root system before planting out.

Greenhouse Housekeeping as the Weather Wobbles

March and April are the months that catch growers out. A closed greenhouse on a bright day can easily reach 25°C by midday, then plunge to 2°C overnight. Ventilation is not optional: open vents and doors on mild mornings, and close them again by mid-afternoon so warmth is trapped for the evening.

Keep fleece or bubble wrap to hand for frosty nights. Draping fleece directly over a tray of seedlings is quick, effective and costs nothing. Water in the morning rather than the evening, so foliage dries before the temperature drops. Watch for greenfly on the soft new growth, and deal with them early by squashing or washing off with a gentle jet of water.

Hardening Off: The Step Everyone Rushes

This is where good intentions often come unstuck. A seedling raised under glass has thin leaves, soft tissue and no tolerance for wind or cold. Moving it straight outside shocks it, checking growth for a fortnight or killing it outright.

Harden your plants off over seven to fourteen days:

• Days 1–3: put plants outside in a sheltered spot for two or three hours, then bring them back in.
• Days 4–6: extend to half a day, still under cover at night.
• Days 7–10: leave them out all day, bringing them in only at dusk.
• Days 11–14: leave them out overnight under fleece or in a cold frame, then plant.

Reduce watering slightly during this period too. A plant that has had to work a little for its moisture grows tougher, waxier leaves that cope far better with the open garden.

Planting Out Into Prepared Beds

Prepare the ground a week or so before planting. Fork it over, but never dig soil that is wet enough to ball up in your hand. Spread a couple of inches of well-rotted compost on the surface, let the worms work it in, then firm and rake to a fine tilth.

Space your plants according to what they are. Hearting lettuces want 25–30cm; cut-and-come-again leaves can go closer. Brassicas need generous room: 45cm between plants and 60cm between rows for cabbages and sprouts, a little less for kale. Plant firmly, burying the stem to the first leaf, and water in well.

Two jobs make all the difference for brassicas. First, fit a collar of card or felt around each stem to deter cabbage root fly. Second, net the whole bed against pigeons and cabbage white butterflies, using a frame that keeps the mesh clear of the leaves. A cloche or fleece over early salads will speed them along for a fortnight, after which they can fend for themselves.

Get this rhythm right — sow under glass, grow them on steadily, harden them properly, plant into good ground — and your spring sowing will carry you right through to summer.