Grow Lights for Seedlings: Do You Need One, and How to Use It

The clocks go back at the end of October, and from then until February a British windowsill gets too little light for seedlings to grow well. Sow chillies, onions or lisianthus in that window without help and you get pale, stretched seedlings that flop over — then are overtaken by plants sown six weeks later. A grow light fixes that, and it does not need to be expensive or complicated. This guide explains when seedlings actually need a grow light and when a windowsill is enough, which kind of LED to buy, how high to hang it, how many hours to run it, and which crops are worth starting under lights this winter.

• Needed For sowings from November to mid-February · rarely after mid-March
• Type Full-spectrum white LED panel or bar · judge by real watts, not "equivalent"
• Height 5–15 cm for low-power strips · 20–45 cm for panels · raise as plants grow
• Hours 14–16 hours a day on a plug-in timer · always at least 8 hours of darkness

Do seedlings need a grow light?

Most do not — most of the year. A seedling needs strong light the moment it emerges, and between mid-March and September a bright, south-facing windowsill or a greenhouse gives it that. The problem is winter. At British latitudes there are fewer than ten hours of daylight from early November to early February, and the light that does come through a window is weak and comes from one side. Below that threshold growth slows almost to a stop, which we explain in the 10-hour daylight rule. Seedlings respond by stretching towards the glass, and the result is the thin, pale stems described in our guide to leggy seedlings.

So the honest answer is: you need a grow light if you want to sow between November and mid-February, or if your only spot is a north-facing or shaded window. If you sow from mid-March onwards on a sunny sill, save your money. And microgreens and sprouts are an exception all year — microgreens grow well on a bright windowsill, and sprouts need no light at all.

One common confusion: germination and growth are different. Most seeds germinate in darkness or ordinary room light, and the few that need light to germinate — lisianthus, lettuce, snapdragons, begonias — need only a little. The grow light matters from the moment the first leaves appear.

Which grow light to buy

Full-spectrum white LEDs are the best choice for seedlings. They run cool, use little electricity and let you see whether your plants look healthy. Red-and-blue "purple" LEDs also grow plants, but the colour makes it hard to spot yellowing or disease. Old-style T5 fluorescent tubes still work well if you already have them. Avoid ordinary incandescent and halogen bulbs: they give more heat than useful light.

Judge a lamp by its real power draw in watts, which is printed on the label or in the specification, not by an "equivalent" figure in the product name. As a rough guide, a good LED panel drawing 30–50 W covers a 60 × 60 cm area of seedlings, and a slim 10–20 W LED bar covers one standard seed tray. Look for a fixture you can raise as the plants grow, either on chains or an adjustable stand.

How to set up a grow light, step by step

1. Germinate first, then move under the light Start the seed in a heated propagator or a warm room as usual — see how to germinate seeds. The moment the first seedlings show, move the tray under the lamp. A day or two in the dark at this stage is when stretching starts.
2. Hang it close Low-power LED strips and fluorescent tubes work best 5–15 cm above the leaves; stronger LED panels 20–45 cm, depending on their power. Start with the maker's recommended height. Then put the back of your hand at leaf level: if it feels warm after a minute, raise the lamp.
3. Put it on a timer Run the light 14–16 hours a day, for example 6 am to 10 pm, on a cheap plug-in timer. Seedlings need a dark period too: lights left on around the clock can cause yellowing leaves on tomatoes and peppers, and waste electricity.
4. Bounce the light back Line the shelf or the sides of the area with white card or kitchen foil. It reflects light back onto the edges of the tray, where seedlings otherwise lean towards the centre.
5. Keep it cool and moving Grow seedlings at 16–20 °C under the light, not on the heat mat. Brush your hand over the tops of the seedlings once or twice a day, or run a small fan on its lowest setting. It gives sturdier stems and drier air, which helps prevent damping off.
6. Raise it as they grow, then wean them off Keep the gap the same as the plants get taller. From late March, move them to a bright windowsill or greenhouse, then harden them off before planting out.

Windowsill or grow light?

At typical British electricity prices of around 25p per kWh, that 30 W panel costs roughly 12p a day to run, or a few pounds for the whole six to eight weeks seedlings spend under it.

What to sow under a grow light this winter

A grow light is most useful for slow crops that need a long season. Chillies, especially Habaneros and other Capsicum chinense, are sown in January — our chilli growing guide explains why a January sowing only beats a February one with a lamp. Aubergines follow the same rule. Onions from seed are sown in January for large bulbs, as described in how to grow onions from seed. Among flowers, lisianthus needs a December or January start to flower in summer, and snapdragons, begonias and geraniums all benefit from an early start. In the kitchen, basil is the one herb that needs a lamp in winter; see growing herbs indoors in winter.

What a grow light does not do is make early sowing right for every crop. Tomatoes sown in January under lights become too big for their pots long before they can go outside in late May. Sow them in March as usual.

Common mistakes

• Hanging the lamp too high — light weakens sharply with distance. Stretching means the lamp is too far away or too weak.
• Hanging it too close — pale, bleached or crisp leaf tips mean the lamp is too close or too strong. Raise it a few centimetres.
• Running it 24 hours a day — plants need darkness. Use a timer.
• Buying on "equivalent" watts — a "1000 W" LED may draw only 100 W. Check the real figure.
• Keeping seedlings on the heat mat — warmth plus light gives soft growth. Heat for germination, then cooler.
• Sowing everything early — use the lamp for the slow crops only; most things are better sown in spring.

Frequently asked questions

How far should a grow light be from seedlings?
It depends on the lamp. Low-power LED strips and fluorescent tubes work best 5–15 cm above the leaves, while stronger LED panels usually sit 20–45 cm above. Follow the maker's guidance, check with the back of your hand for heat, and watch the seedlings: stretching means move it closer, bleaching means move it away.

How many hours a day should seedlings be under a grow light?
Fourteen to sixteen hours, with at least eight hours of darkness. A plug-in timer makes it automatic. Winter herbs on a windowsill need less — 12–14 hours is enough.

Can I use an ordinary LED bulb as a grow light?
A bright, daylight-white LED bulb in a desk lamp, placed about 10–15 cm above a few pots, will help a small number of seedlings. For a full tray, a purpose-made LED panel or bar spreads the light far more evenly.

Does a grow light replace a heated propagator?
No. They do different jobs: the propagator provides warmth for germination, and the light provides energy for growth once the seedlings are up. Most winter setups use both, one after the other.

One lamp, one timer and a sheet of white card turn the darkest months into a head start on next summer. Browse our chilli pepper, aubergine, onion and basil seeds, early flowers such as lisianthus, snapdragons and begonias, the full vegetable seed range or all our seeds, and keep our growing guides to hand for the season ahead.