Winter seed starting is the gardener's way of getting spring started early. While the garden outside is dormant, seeds started indoors under lights grow into sturdy young plants that are ready to go out as soon as the weather allows, weeks ahead of seeds sown directly outdoors. It is also one of the most satisfying winter jobs: green growth on a windowsill in January is a promise that spring is coming. But it is also one of the easiest jobs to get wrong. Seedlings started too early grow leggy and weak, seedlings without enough light stretch and flop, and seedlings started in cold, damp conditions damp off and die. This guide explains what to start in winter, when to start it, and how to keep seedlings healthy until they can go outside.
The first rule of winter seed starting is to work backwards from your last frost date, not forwards from today. Every seed packet gives a number of weeks before the last frost, and that number is the plan: count back from your average last frost date, and sow on that date. Starting earlier does not give you a head start — it gives you an overgrown, root-bound plant that has been indoors too long. Frost dates are averages and ranges, not exact dates, and they vary even within a zone, so use the range for your zone and check your local extension service for the dates that fit your area. The seed starting calculator will work out sowing and transplant dates from your last frost date, and the frost date calculator gives the average dates by zone.
What to start in winter
In midwinter, the seeds to start are the slow growers that need a long season: onions and leeks from seed, celery, peppers and chillies, aubergines, and some slow perennials and flowers. These need ten to twelve weeks or more indoors before they are ready to go out, so a January or February sowing is right for them. Tomatoes, which many gardeners are tempted to start in January, are better started later — six to eight weeks before the last frost is enough, and earlier sowing usually produces leggy plants that are hard to transplant. Lettuce, spinach and other fast cool-season crops are better sown directly outdoors or under cover in late winter, not started indoors in midwinter. Herbs such as basil are tender and fast-growing, so they should wait until closer to spring; hardy herbs such as parsley and chives can be started earlier. Our seed starting mix guide covers the mix to sow into, and the growing tomatoes and growing lettuce guides cover the crops themselves.
In zones 11 to 13, which are frost-free, winter seed starting works differently: there is no last frost to count back from, and many seeds can be sown directly outdoors through winter. In zones 9 and 10, winter is a good season for starting cool-season crops indoors or outdoors. In zones 1 to 4, the indoor season is long, but the outdoor season is short, so timing matters most — start slow crops early, but do not start tender crops until the count-back says so, however tempting the January sunshine looks.
Setting up: containers, mix and warmth
You do not need expensive equipment to start seeds in winter, but you do need the basics done well. Use clean containers with drainage holes — seed trays, small pots, or reused yoghurt pots with holes punched in the bottom. Fill them with a fine, free-draining seed starting mix, not garden soil, which is too heavy, may carry disease, and can contain weed seeds. Our seed starting mix guide gives a simple recipe. Sow at the depth on the packet — as a general rule, about twice the seed's own width — and water gently from below or with a fine spray so the seeds are not washed away.
Most seeds germinate best at ordinary room temperature, roughly 18 to 22°C. A warm windowsill, the top of a fridge, or a heated propagator all work. Some seeds, such as peppers and aubergines, like it warmer and germinate slowly in a cool room, so a heat mat or a warm spot helps. Once seeds have germinated, move them to the brightest, coolest spot you have — warmth without light produces leggy seedlings, while bright light with moderate warmth produces stocky ones. Do not keep seedlings in a hot, dark room: they will stretch towards any light they can find and flop over.
Light: the make-or-break factor
Light is the single most important factor in winter seed starting, and the one most gardeners underestimate. A windowsill in winter gives short days and weak light, and even a south-facing window is often not enough for strong seedlings. If seedlings are tall, pale and leaning towards the window, they need more light. A simple fluorescent or LED grow light, hung 5 to 10 cm above the seedlings and left on for 12 to 14 hours a day, makes a dramatic difference. Raise the light as the seedlings grow, keeping it close — light that is too far away is almost as useless as no light at all. The grow light calculator will help you work out hours and hanging distance.
A timer on the light is worth having: seedlings need a dark period too, and a light left on all day and night stresses them. Turn the light off at night, and give the seedlings a gentle breeze from a small fan on a low setting for a few hours a day if you can — moving air strengthens stems and reduces disease. Rotate trays every day or two so seedlings do not lean, and keep the light close even if it feels fussy: the difference between seedlings grown under a close light and seedlings grown on a windowsill is visible within a week.
Watering and feeding seedlings
Seedlings need steady moisture, not wet and not dry. Water from below by standing the tray in a shallow dish of water for a few minutes, then letting it drain — this wets the mix evenly without disturbing seeds or encouraging disease on the surface. Water in the morning so the surface dries during the day, and never leave trays sitting in water. Overwatering is the most common cause of damping off, the fungal disease that makes seedlings keel over at the base and die. Good air movement, clean containers, fresh mix and not sowing too thickly all reduce the risk. Our damping off guide covers the disease and how to prevent it, and the watering calculator gives a starting point for how much water young plants need.
Seed starting mix contains little food, so seedlings will need feeding once they have their first true leaves — the second pair of leaves, which look like the adult plant, not the first smooth seed leaves. Feed at half the strength on the label, once a week, with a balanced liquid feed. Do not feed before true leaves appear, and do not overfeed: too much fertiliser burns tender roots and produces soft, leggy growth. Our fertilizing vegetables guide covers feeding once plants are growing strongly.
Hardening off and transplanting
Seedlings raised indoors are soft, and they will be damaged if they are put straight outside. Hardening off — gradually exposing them to outdoor conditions over a week to ten days — is essential. Start with a few hours outside in a sheltered spot on a mild day, bring them in at night, and increase the time and exposure gradually until they are outside all day and night. Do not harden off during a cold snap, and do not leave tender seedlings out overnight until the risk of frost has passed. Our winter transplanting guide covers the transplanting side in winter conditions, and the garden tasks in February guide shows where seed starting sits in the late-winter calendar. The zone 5 January page shows what a cold-zone January looks like for indoor sowing.
Common mistakes
The most common mistake is starting seeds too early, which produces leggy, root-bound plants that struggle after transplanting. The second is not enough light, which produces the same result. The third is overwatering, which causes damping off. The fourth is sowing too thickly, so seedlings compete and disease spreads. The fifth is using garden soil instead of seed mix. The sixth is skipping hardening off, so seedlings are damaged by their first day outside. And the seventh is starting tender crops in midwinter because the packets are in the shops — shops sell seeds months before they should be sown. Work from your last frost date, not from the shop display. If you are unsure what to sow and when in your area, check your local extension service for advice.
Frequently asked questions
What seeds should I start indoors in winter?
Slow growers that need a long season: onions and leeks from seed, celery, peppers, chillies and aubergines. Start them ten to twelve weeks before your average last frost. Tomatoes and basil should wait — six to eight weeks before the last frost is enough for them.
Do I need a grow light for winter seed starting?
In most cases, yes. Winter daylight is short and weak, and even a sunny windowsill is often not enough for strong seedlings. A simple LED or fluorescent light hung close above the seedlings for 12 to 14 hours a day makes a large difference.
Why are my seedlings tall and floppy?
Almost always not enough light, or too much warmth with too little light. Move the light closer, give them 12 to 14 hours of light a day, and move them to a cooler, brighter spot once they have germinated. A gentle breeze from a fan also strengthens stems.
How often should I water seedlings?
Keep the mix evenly moist, not wet and not dry. Water from below in the morning, let trays drain, and never leave them sitting in water. Overwatering is the most common cause of damping off.
When can winter-started seedlings go outside?
After hardening off over a week to ten days, and after the risk of frost has passed for tender crops. Hardy crops can go out earlier, while the ground is workable and frosts are light. Work from your average last frost date, not from the calendar alone.