Grow Lights Explained: Intensity, Distance and Hours

Young plants growing indoors under a bright pink LED grow light — Indoors, your lamp and timer play the role of the sun — and the seasons.

Indoors, you are the weather. Nothing you control matters more than light: it powers photosynthesis, sets the pace of growth, and decides whether a seedling turns stocky and green or pale and stretched.

This guide explains the three numbers that actually matter — intensity, distance and duration — plus the light problems beginners run into and how to read them off the plant itself.

A windowsill is rarely as bright as it looks

Human eyes adapt so well that a room that feels bright to you can be nearly dark to a plant. Outdoor midday sun is enormously more intense than the light a few feet inside a window, and light falls off sharply with distance from the glass.

That's why herbs on a windowsill often survive but never thrive, and why a modest LED grow light usually outperforms the sunniest sill in a home. Leafy greens and herbs are the most forgiving; fruiting plants like tomatoes, peppers and strawberries need considerably more light to actually set fruit.

Intensity: how much light your plants get

Intensity is how much usable light lands on the leaves. Watts on the box tell you what the lamp consumes, not what your plant receives — two lamps with the same wattage can deliver very different results.

As a practical rule of thumb for beginners: leafy greens and herbs are happy with modest light, while fruiting crops want as much as you can reasonably give them. Rather than chasing numbers, watch the plant — it reports the result of your setup better than any spec sheet.

Rows of lettuce and leafy greens growing indoors on vertical racks under artificial light — Compact, deep-green growth is the sign that intensity and distance are about right.

Distance: the setting people get wrong most often

Light intensity drops sharply as the lamp moves away from the canopy — halve the distance and the plant receives dramatically more light. That makes lamp height the fastest, cheapest adjustment in your whole setup.

Too far and plants stretch: long gaps between leaves, thin stems, seedlings that lean and fall over. Too close and you see bleaching, curled or taco-shaped leaves, and crispy edges on the tips nearest the lamp. Start with the manufacturer's recommended distance, then let the plant's response move the lamp up or down an inch at a time.

Duration: how many hours to run the light

Most common indoor crops do well with roughly 12–16 hours of light per day, and they genuinely need the dark period — plants use the night to move sugars around and grow. Running lights 24 hours a day is not a shortcut; for many plants it causes stress rather than faster growth.

A cheap mechanical or smart timer is one of the highest-value purchases in indoor growing. Consistency matters more than perfection: the same hours, every day, beats an irregular schedule you keep adjusting by hand.

Seedlings and leafy greens: around 14–16 hours is a comfortable starting point.
Herbs: 12–16 hours, depending on how fast you want them to fill in.
Fruiting plants: long, bright days plus a genuine dark period every night.
Flowering houseplants: some are day-length sensitive — check the specific species before changing their schedule.

Reading light problems off the plant

Your plants tell you what's wrong with the light long before they die of it. Watch for these:

Long, bare stems with widely spaced leaves — not enough light, or the lamp is too far away.
Seedlings leaning hard toward a window — light is arriving from one side only; rotate the pot or add a lamp.
Pale, washed-out or bleached patches on the top leaves — too much intensity, or the lamp is too close.
Leaves curling upward or edges going crispy right under the lamp — heat and light stress; raise the fixture.
Healthy growth that suddenly stalls — check whether the timer failed before you start changing nutrients.

Light is one piece of a bigger system

Light interacts with everything else in your indoor garden: more light means more water use, higher nutrient demand, and more heat to manage with airflow. Turning one dial without watching the others is how beginners chase problems in circles.

Understanding each piece in sequence — rather than patching knowledge together from forum threads — is the difference between a stressful first grow and a confident one.