Choosing a grow light without wasting £200

A plant builds itself out of light. Everything else you supply — water, nutrients, a warm room — lets it use the light it gets, and none of it substitutes for light it does not get. That is why a nutrient problem and a light problem look so different once you know the difference: a starved plant is small and pale, and a light-starved plant is stretched, reaching for more, with long gaps between leaves.

The reason this matters more in Britain than the internet assumes is latitude. Between about October and March, an indoor windowsill here receives a fraction of what a plant wants, and a north-facing one receives almost nothing. Summer salad on a bright sill genuinely works. Winter anything, or fruiting anything, does not — and this is the honest reason most first attempts disappoint.

A grow light is therefore not an upgrade. For year-round indoor growing it is the thing that makes the hobby work at all, and it is worth buying before a better system rather than after.

The one figure to learn is PPFD, and the important thing about it is that it is meaningless without a distance. A light quoting a big number at 15cm and a modest one at 60cm is telling you it is intense and small — fine over a seed tray, useless over a shelf. Ask what the figure is at the distance you would actually hang it, and if a listing does not say, that is information too.

On spectrum: the purple lights are a hangover. The reasoning was that plants use red and blue most, so emitting only those is efficient — which is true and turns out not to be the point. Plants use the rest as well, white light is now efficient enough that the saving has largely gone, and there is a practical argument that outweighs both: under purple light you cannot see what colour your plants are, which removes your best diagnostic tool. Buy white.

Roughly, and in ascending order of what it will cost you:

• Herbs and salad leaves — The least demanding things worth growing and the reason to start with them. A modest panel over a small area is genuinely enough, and this is the tier where a £40–£60 light does real work.
• Leafy greens and brassicas — A step up but still comfortably within a domestic fixture. Pak choi, chard and kale all do well indoors.
• Fruiting plants — tomatoes, chillies, strawberries — A large step, and where budgets escalate. A fruiting plant needs both more intensity and more hours, and this is the tier where people who bought a salad light discover it. If this is the goal, size the light for it from the start rather than upgrading twice.

On hours: most growers run leafy crops for something like fourteen to sixteen hours a day and put the light on a timer, because consistency matters more than the exact figure and a timer costs a few pounds. Plants also need a dark period — running a light twenty-four hours a day is not more growth, it is a stressed plant and a larger bill.

One thing worth doing before buying anything: work out what the light will cost to run. Multiply its actual wattage by the hours you intend and by your unit rate. For most people this is the largest recurring cost in the hobby by a wide margin, it is entirely predictable, and knowing it in advance is what stops the disappointment being about money as well as lettuce.

• Buy: a white full-spectrum LED panel or bar from a named maker — With a PPFD figure quoted at a stated distance, and a driver you can dim if possible. Dimming is more useful than it sounds because it lets one fixture serve seedlings and mature plants.
• Buy: a timer — A few pounds and it supplies the consistency that does more than any spectrum decision.
• Consider: a plain LED shop light — For herbs and salad, a good cool-white linear LED fitting is a legitimate and much cheaper option. It is less efficient per watt for growing than a purpose-built fixture and for a small windowsill setup that rarely matters.
• Avoid: anything quoting only "equivalent" wattage — It is a figure with no standard behind it, and its presence usually indicates the real numbers would not flatter the product.
• Avoid: purple blurple panels — Outdated reasoning, and they make it impossible to see the colour of your own plants, which is the first thing you would use to diagnose a problem.
• Avoid: buying for the plant you might grow later — A fruiting-scale light over a tray of basil is money spent on nothing. Buy for what you will actually grow this year.