You've hung the light, now you're second-guessing the height
Most growers start by searching for a single number — "hang your light 60 cm above the canopy" — and then worry when their plants don't behave accordingly. The honest answer is more useful than that, and it starts with understanding why distance matters in the first place.
Why height matters
Light intensity drops sharply as distance increases. Double the distance from the source and you get roughly a quarter of the intensity — that is the inverse square law working against you. Too close and the plant receives more energy than it can use, causing heat stress and bleaching. Too far and it stretches upward searching for light it isn't finding, putting energy into stem length rather than leaves and roots.
Starting distances by growth stage
These are sensible starting points for a mid-range LED panel in the 75–100W range. Treat them as a place to begin, not a rule to follow blindly.
- Seedlings and early cuttings: 50–70 cm. Young plants have no established root system to support recovery from stress. Keep intensity low.
- Vegetative growth: 35–50 cm. Plants are building structure and can handle more light, but still benefit from some headroom.
- Flowering and fruiting: 25–40 cm. Demand for light energy is at its highest. Close enough to deliver intensity, far enough to avoid burning.
These distances assume a light running at or near full power. If you are running a dimmable unit at 50%, you can reduce the height accordingly without changing a thing about the fixture's position.
Watch the plant, not the tape measure
This is the part most guides skip. A plant grown in a warm room, in a small pot, in dry air will respond differently to the same light at the same distance as one grown in cooler, more humid conditions. The tape measure cannot know any of that. The plant can show you.
Check your plants every day, ideally at the same time. Look at the newest growth — the top leaves and any growing tips. They are the most exposed and will show problems first.
Signs the light is too close
- The top leaves curl upward at the edges, like a taco shell
- Leaf tips turn brown or crisp without any sign of nutrient deficiency lower down
- Upper leaves look washed out, pale yellow or almost white, while lower leaves stay green — this is light bleaching
- The growing tip appears to stall or harden off prematurely
If you see any of these, raise the light by 5–10 cm and give the plant 48 hours to respond before adjusting again.
Signs the light is too far
- Stems between nodes are long and thin — the plant is stretching toward the light
- The plant leans toward the light source rather than growing straight up
- Lower leaves stay healthy but upper growth looks pale and weak despite being closest to the light
- Overall growth is slow when temperature, watering and nutrition are all in order
Lower the light by 5–10 cm and again, give it a couple of days before deciding whether to go further.
What dimming lets you do that moving the light cannot
This is the practical advantage of a dimmable LED that rarely gets explained clearly. When you raise or lower a light, you change the intensity across the whole canopy — but you also change the spread. A light raised high covers a wider area but delivers less intensity to any given point. A light lowered concentrates intensity but can create hot spots directly beneath the panel and leave edges under-lit.
Dimming changes output without changing geometry. You can keep the light at the ideal height for even coverage and simply turn it down for seedlings, then increase it gradually as the plant matures. You are not compromising spread to manage intensity, or managing intensity by sacrificing spread.
It also makes transitions gentler. Moving from 30% to 60% over two weeks is far less of a shock than suddenly dropping a light from 60 cm to 30 cm.
The LM301H EVO 85W Quantum Board LED Grow Light – Dimmable Full Spectrum is the fixture we stock that makes this approach straightforward — the dimmer lets you set position once for good coverage and adjust intensity as plants develop, rather than constantly repositioning the light as growth stages change.
A practical routine that works
Set your starting height based on the stage guide above. Run the light at around 70% for the first week. Each time you check the plant, look at the newest leaves at the top of the canopy. If they look healthy and growth is steady, leave it. If you see stress in either direction, adjust one variable at a time — height or intensity, not both together — and wait before adjusting again. Small, patient changes tell you far more than large sudden ones.
The one thing worth doing next
Before you move the light or touch the dimmer, spend five minutes looking closely at your top leaves right now. Note what you see. Everything from here follows from that observation, not from a number on a chart.
Read next
- Why is my seedling tall, thin and falling over?
- What Does It Actually Cost to Run a Grow Light?
- Do I need a grow light timer? Hours of light by stage
Looking for the kit? Start here, or browse the full grow range.