How Much Light Do Seedlings Need When Started Indoors?

Light is the resource most gardeners underestimate when starting seeds indoors. You can have perfect compost, perfect warmth, and perfect watering — but without enough light, seedlings stretch and weaken within days of germination. Understanding light requirements helps you place seedlings correctly and decide whether supplemental lighting is worth the investment.

How Much Light Do Seedlings Actually Need?

Most vegetable seedlings need 14–16 hours of good light per day to grow stockily and compactly. In early spring in northern latitudes, natural daylight provides only 10–12 hours, and much of that light is filtered through cloud, glass, and curtains. The usable light intensity on a windowsill in February in the UK or northern US is roughly 30–50% of what seedlings receive outdoors in May. That gap is why windowsill seedlings so often become leggy.

South-Facing Windows: The Best Natural Option

A south-facing window (in the northern hemisphere) receives the most light through the day. In late winter and early spring this is the best natural location for seedling trays. Place trays as close to the glass as possible — light intensity drops rapidly with distance from the window. The problem is that even a south-facing sill provides only around 6–8 hours of useful light in February, and temperatures near glass can drop sharply at night. Move trays back from the glass after dark.

The Windowsill Light Problem

Light from a window comes from one direction only. Seedlings grow toward it, leaning noticeably within 24–48 hours. Rotate trays 180° every morning to keep growth upright. Even so, the one-directional nature of window light means seedlings grown on sills are rarely as stocky as those grown under overhead artificial lighting.

Using Reflective Surfaces

Placing a sheet of white card, aluminium foil, or a mirror on the opposite side of the tray reflects light back onto the seedlings. This cheap trick can increase effective light by 30–40% and visibly reduces legginess without any extra electricity cost.

When Natural Light Is Not Enough

If you are sowing before mid-March (UK) or before the days reach 14 hours of daylight in your location, artificial grow lights will give much better results than any window. Full-spectrum LED grow lights set to run 16 hours a day transform seedling quality — stems stay short and thick, leaves are a deep green, and transplant success rates improve noticeably.