Grow light height and coverage

The inverse square law means doubling the distance quarters the intensity. How to find the right height for your light and what PPFD maps actually show you.

The most expensive grow light on the market is worthless at the wrong height. Too close and you burn the canopy or blast a small area while the edges get nothing. Too far and the light spreads thin, dropping below the PPFD threshold your crop needs to grow properly. The physics governing this are simple, but the consequences of ignoring them are significant.

The inverse square law

Light intensity from a point source decreases with the square of the distance. Double the distance, and the intensity drops to one quarter. Halve the distance, and intensity quadruples.

In practical terms: if your light delivers 800 micromoles of PPFD at 30 cm from the canopy, moving it to 60 cm doesn't give you 400 micromoles. It gives you roughly 200. That 30 cm change took you from strong flowering light to weak vegetative light.

Real grow lights aren't point sources (they're panels, bars, or arrays with optics), so the falloff isn't a perfect inverse square at close distances. But beyond about 20-30 cm from the fixture, the approximation holds well enough to be useful for planning. The takeaway: small height adjustments make large intensity changes. Moving a light 10 cm in either direction can be the difference between optimal growth and stress.

Why manufacturer specs are starting points

Most LED grow lights publish a PPFD map showing intensity readings at a specific mounting height, typically 45-60 cm (18-24 inches) above the canopy. These maps show the center reading (highest intensity) and readings at various distances from center, revealing the uniformity of coverage.

The problem: that map applies only to that exact height. Your grow space, reflective surfaces, plant heights, and growth stage all change the optimal height. Use the manufacturer's map as a starting point, then adjust based on what you observe.

Finding the right height by crop stage

Seedlings and clones: high and dim

Young plants need gentle light. 150-250 PPFD is enough. Mount the light higher than the manufacturer's recommended distance, or use a dimmer if available. High light on seedlings causes stretching (paradoxically, because the plant is trying to get away from the heat) or bleaching of delicate new tissue.

Vegetative growth: moderate

Leafy greens and herbs in veg do well at 300-500 PPFD. Fruiting crops in veg want 400-600 PPFD. Bring the light down to the manufacturer's recommended height or slightly above. Watch for signs of light stress: upward leaf curl, bleached patches on the top leaves, or brown crispy edges on the newest growth.

Fruiting and flowering: full intensity

Tomatoes, peppers, and other fruiting crops at peak production want 600-900 PPFD with 16-18 hours of photoperiod. This is where you run the light at or near its maximum output at the manufacturer's recommended height. Plants that can handle high DLI (daily light integral above 25 mol/m2/day) will produce more fruit with more light, up to their saturation point.

Coverage vs. intensity trade-off

Raising a light increases its coverage area (the footprint on the canopy that receives meaningful light) but decreases the peak intensity. Lowering the light does the opposite: smaller, more intense hot spot.

For a single fixture over a rectangular bed, you want the height that provides adequate PPFD across the entire growing area, not only under the center. If the center reads 800 PPFD and the edges read 200, the plants on the edges are getting less than half the light they need. Raising the light to 600 center / 350 edges gives more uniform growth across the bed.

Multi-fixture setups solve this differently. Overlapping the footprints of adjacent lights fills in the edge falloff. A 10-20% overlap between fixtures eliminates the dead zones. This is more efficient than raising a single light to cover a large area, because you maintain intensity while distributing it evenly.

Using the grow light calculator

The grow light calculator converts your fixture's specifications into a target height and photoperiod for your specific crop. Enter the fixture's PPF (total light output in micromoles per second, found on the spec sheet), your growing area, and the crop's DLI target. The calculator gives you the resulting average PPFD and how many hours of light per day you need to hit the target DLI.

If you have a PAR meter, verify the calculator's predictions with real measurements at canopy level. Take readings at center, edges, and corners. Adjust height until the lowest reading is above the minimum PPFD for your crop. A 50-dollar quantum sensor pays for itself by preventing crop underperformance from incorrect light placement.

Signs you have the height wrong

Plants tell you when the light is too close or too far. Learning to read these signals lets you adjust without a PAR meter, though a meter is faster and more precise.

Too close (excess intensity): The top leaves bleach to a pale green or white, especially at the center directly under the fixture. Leaf edges curl upward (taco-ing). Brown, crispy patches appear on the uppermost leaves while lower leaves look fine. In severe cases, the entire top canopy looks sunburned. Move the light up 5-10 cm and watch for improvement over 3-5 days.

Too far (insufficient intensity): Stems elongate rapidly with long gaps between leaf nodes (stretching or etiolation). The plant leans toward the light source. Growth is slow, leaves are pale green but not bleached, and fruiting crops produce few or no flowers. Lower the light 5-10 cm.

Uneven coverage: Plants on the edges are leggy while center plants are compact and dark green. The center is getting adequate PPFD while the edges are under-lit. Either lower the light slightly (which sacrifices some center intensity but widens the beam), add reflective material to the tent walls, or add supplemental edge lighting.

PAR meters: worth the investment

A quantum PAR meter (like the Apogee MQ-500 or the budget-friendly Photone phone app with its calibrated sensor card) reads PPFD directly at any point in your grow space. It removes the guesswork from height adjustment.

Take readings at canopy level (where the top of the plants are, not the pot surface) in a grid: center, four edges, and four corners. The difference between the highest and lowest reading is your uniformity spread. Aim for the lowest reading to be at least 60-70% of the center reading. If the edges are below 50% of center, the light is too low or the space is too wide for a single fixture.

For a rough calibration without a PAR meter: most manufacturers publish a PPFD map at a specific height. Start there and adjust based on plant response over 1-2 weeks. It's slower than using a meter, but it works for a single-fixture home setup where you can observe the plants daily.

Adjustable vs fixed fixtures

If you're buying a new light, get one with adjustable hanging height (ratchet hangers or adjustable wire, not fixed brackets) and built-in dimming. Dimming lets you lower the light for better coverage uniformity while reducing intensity to match the growth stage, which is often more effective than raising a fully-powered fixture that leaves the edges under-lit.

A 200W dimmable LED at 80% power positioned at 40 cm from the canopy can outperform the same light at 100% positioned at 60 cm, because the closer position gives more uniform coverage while dimming prevents center-point intensity from exceeding the plants' light saturation point.