Hydroponic lighting: how much light your plants actually need

Light is usually the real limit on indoor growth, not nutrients. Here is how to tell whether yours is enough, and what to do if it is not.

A flat white LED grow light panel suspended on ratchet hangers above a tray of young lettuce and herb seedlings, casting an even pool of light across them. — Height and coverage matter more than headline wattage. Even light beats intense light.

When an indoor grow underperforms, light is the cause far more often than nutrients. Human eyes adjust so well that a room we call bright can be a tenth of what a plant considers usable.

How much do different crops need?

Crop	Hours per day	Intensity needed
Lettuce and leafy greens	10 to 14	Low. A bright windowsill is often enough.
Herbs	12 to 16	Low to medium.
Strawberries	12 to 16	Medium.
Tomatoes and peppers	14 to 18	High. A dedicated grow light is effectively required.

Is your windowsill bright enough?

Two quick tests, no meter needed:

The shadow test. At midday, hold your hand a foot above the spot. A sharp, well-defined shadow means good direct light. A soft, fuzzy shadow means indirect light, which is enough for herbs and greens but not for fruiting crops. No visible shadow means you need a lamp.
The direction test. South-facing windows (in the northern hemisphere) give the most hours. East and west give a useful half day. North-facing windows are not enough on their own for anything productive.

Getting the distance right

For a typical small LED panel, keep it 6 to 12 inches above the canopy and raise it as the plants grow. Too far and plants stretch, going tall, pale and floppy. Too close and you get bleached, crisped patches on the top leaves directly under the light.

Use a timer, always

Plants need a dark period to respire properly, and running a light around the clock wastes electricity while stressing the plant. A $5 mechanical timer removes the decision permanently. Set it once and stop thinking about it.

Wondering what those hours actually cost? Our grow light running cost calculator works it out per day, month, year and harvest using average electricity rates for your state.

See our grow light recommendations