Why You Don't Need a Heated Greenhouse

Walk into any garden centre in February and you'll see heated propagators, warming mats and thermostatically controlled greenhouses lined up like promises. They're lovely bits of kit, but the truth is that most of the seeds British gardeners sow in late winter and early spring don't need them. Tomatoes, chillies, peppers, aubergines and brassicas will all germinate quite happily on a warm windowsill, provided you get a few basics right.

The trick isn't heat for its own sake — it's consistent warmth, good light and clean conditions. A south-facing windowsill above a radiator gives you all three. What you're really doing is stealing a six-week head start on the season, so that by the time the soil outdoors has warmed enough for direct sowing, you've already got sturdy little plants ready to harden off and go out.

Pick Your Spot Before You Pick Your Seeds

Light is the limiting factor indoors, not heat. A windowsill that faces south or south-west is worth more than any gadget. East-facing will do at a push, but you'll get leggy, pale seedlings that never quite catch up. North-facing is a non-starter for anything tender.

• Move pots to the brightest spot you have, and rotate them a quarter-turn every couple of days so stems grow straight.
• Watch for draughts from old sash windows — a sheet of bubble wrap taped across the glass at night keeps the chill off without blocking much light.
• Keep seedlings just above a radiator if you can, but lift them onto a waterproof tray so they don't cook when the heating comes on.
• If your sill is narrow, a cheap wire shelf unit that clips over the radiator gives you two or three tiers of growing space.

Don't underestimate the difference a few degrees makes. Most of the seeds we sow indoors in the UK want a soil temperature of around 18–21°C to germinate. Once they're up, they'll usually do fine at 15–18°C, which is roughly what a normal heated living room gives you anyway.

Clean Trays, Fresh Compost, Gentle Bottom Heat

Damping off — that horrible collapse of seedlings at soil level — is the number one reason indoor sowing fails. It's almost always a hygiene problem. Wash trays and modules in hot soapy water, rinse them well, and if you've had trouble before, give them a wipe with a weak bleach solution and let them dry completely. Reused trays with a crust of old compost are asking for trouble.

Use a fresh, fine seed compost rather than last year's grow-bag contents. It's low in nutrients, which is exactly what germinating seeds want, and it drains freely. Fill modules loosely, firm gently with the base of another tray, and water from below by standing the tray in a centimetre or two of tepid water until the surface darkens.

As for bottom heat, you don't need a mat. A few options that cost nothing:

• Stand trays on the top of a fridge or freezer — they give off a gentle, constant warmth.
• Use the shelf above a boiler or a warm airing cupboard for germination only, then move seedlings to the light the moment they show.
• Place trays on a thick layer of newspaper over a radiator shelf so the heat is buffered rather than direct.

The golden rule is simple: warmth to germinate, light to grow on. Leaving newly sprouted seedlings in the dark warmth of an airing cupboard for even two days will give you white, spindly stems that never recover.

What to Sow When

Timing matters more than technique. Sow too early and you'll be nursing huge, root-bound plants through March with nowhere to put them. Here's a rough guide for a typical British season, adjusted for how far north you are:

• Late January to mid-February: onions, leeks, early chillies and aubergines — these need the longest run-up.
• Late February to early March: tomatoes, peppers, celery, early brassicas.
• Mid to late March: courgettes, cucumbers, squash, sweetcorn, French beans — anything that grows fast and hates the cold.
• April: hardy herbs, lettuce, and a second sowing of anything that failed.

If you garden in Scotland or the far north of England, push everything back two or three weeks. If you're in Cornwall or the Channel Islands, you can nudge it forward. The seed packet dates assume a heated greenhouse, so treat them as a guide rather than gospel.

Pricking Out and Growing On

Once seedlings have their first true leaves — the second pair, not the two rounded seed leaves — it's time to prick them out into individual pots or larger modules. Handle them by a leaf, never the stem, and set them slightly deeper than they were growing. Tomatoes in particular will happily grow roots along a buried stem, giving you a sturdier plant.

Water from below, keep the compost just moist rather than soggy, and give them as much light as you possibly can. If stems still stretch, it's a light problem, not a feeding problem — don't reach for the plant food. Seedlings raised in fresh compost have enough nutrients for three or four weeks.

About a week to ten days before you plan to plant out, start hardening them off. Put them outside in a sheltered spot for an hour on the first day, two the next, building up gradually. Bring them in at night at first, then leave them out once the forecast is kind. It's a faff, but skipping it shock-stunts plants that were perfectly healthy on the windowsill.