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Grow tents · 7 minute read · Updated October 15, 2026

How high should an LED grow light be at each stage?

Height charts that give one number in inches are answering the wrong question. Two lights at 18 inches can put twice as much light on your plants as each other. The real question is how much light lands on the leaves, so what does your light's own chart say it delivers at the height you hung it?

Why inches are the wrong unit

Plants do not care how far away the light is. They care how many photons hit each leaf every second. That is PPFD: photosynthetic photon flux density, measured in micromoles per square meter per second (µmol/m²/s).

Different lights put out very different amounts of light. A 150 W board and a 650 W bar at the same height are nowhere near each other at the canopy. So the useful targets are in PPFD, and the height is just how you get there with your light.

The other number worth knowing is DLI, the daily light integral: the total light a plant gets in a day, in moles per square meter. It joins intensity and hours together:

DLI = PPFD x hours of light x 0.0036

The 0.0036 just converts seconds to hours and micromoles to moles. It matters because 18 hours of veg light at a given PPFD delivers 50% more light per day than 12 hours of flower light at the same PPFD.

Light targets by stage

These are working ranges, not hard limits, and the research says plants can use more than many growers think:

• Veg. In a University of Guelph trial, cannabis clones grown for three weeks at 135 to 1,430 µmol/m²/s got more compact and sturdier as light went up. The authors pointed to about 900 for compact, robust plants, and about 600 for a more open plant with better airflow.
• Flower. In a 2021 Guelph study, dry flower yield rose in a straight line with canopy PPFD all the way from 120 to 1,800 µmol/m²/s, at normal CO2 levels, with no effect on cannabinoid potency. That room was held at about 77°F (25°C) and 60% RH with tight control.

So why stop at 1,000 in a tent? Because a home tent rarely has that level of climate control. More light means more watts, more heat and faster drinking, and the plant has to be able to keep up. Push past 1,000 only if your temperature, humidity, watering and feeding are already dialed in.

How to read your light's chart

Most LED makers publish a PPFD map: a grid of readings at one or more hanging heights. Here is one maker's published footprint for a popular 450 W bar light rated for a 4x4 tent:

Three things jump out:

1. Close is a lot. At 12 inches the center reads 1,821, nearly twice a common flower target. Hanging this light at "12 to 18 inches for flower", as some generic charts say, would put the center well above a typical home target at full power.
2. Height is a weak lever. Going from 12 to 22 inches only drops the center by about 37%. A bulb-like point source would lose about 70% over that distance, but a wide bar spreads its light, so the intensity falls slowly. That is why the dimmer, not the height, is your main control.
3. The center is not the tent. Edges read about a third less. If you set the height by the center reading, the outer plants are getting less than you think.

There is also a ceiling no height can beat. That light is rated at 1,171 µmol per second total. Spread over a 4x4 tent (about 1.49 m²), the average across the whole floor can only get close to about 790, and only if almost no light is lost. Lowering the light concentrates light in the middle. It does not create more.

A practical starting point by stage

If you have a mid-size bar light and its chart, this is how to use it:

• Seedlings: Raise the light near the top of the tent and dim it hard, then find the dimmer setting that puts the center at about 200. If the chart only shows full power, start at the lowest dimmer setting and raise it over days, not hours. Watch the first week: stretching seedlings want more light, pale tips want less.
• Veg: Bring it up to about 400 to 600 at the top leaves over a week or two. With a 450 W class light that usually means mid-height with the dimmer partly up.
• Flower: Raise intensity over the first two weeks of flower to about 700 to 1,000 at the canopy. For the example light above at full power, that lands around 22 inches or a little higher at the center. Plants stretch in early flower, so check the gap every couple of days.

Without a meter, the chart is your best guide, but remember it was measured by the maker in their own setup. Tent walls, reflectivity and dimmer settings all change real numbers. A rented or borrowed quantum PAR meter for one afternoon is the honest way to check.

Signs the light is too close

• Bleaching: the top leaves or bud tips closest to the light go pale yellow or white, while lower growth stays green.
• Cupping up: leaf edges near the top curl upward like a taco.
• Crispy tops only: damage confined to the top of the plant, under the brightest spot.

Do not trust the old back-of-the-hand test with LEDs. It was made for HPS lamps, which throw a lot of radiant heat. Many LEDs feel cool at the canopy while still delivering enough light to bleach the top. Light stress and heat stress look alike, so check temperature too. See grow tent too hot. Yellowing that starts on the lower leaves is a different story, usually feeding: start with nitrogen deficiency.

Signs the light is too far

• Stretch: long gaps between nodes and thin stems. The Guelph veg trial found the gap between nodes got shorter as light went up.
• Leaning: plants bending toward the brightest spot.
• Big, thin leaves and slow growth for the stage.
• Airy buds in flower on otherwise healthy plants.

Seedlings that stretch tall and floppy in the first week are the classic case. More light, a gentle fan and burying the stem a little deeper at transplant are the usual fixes. More in germinating cannabis seeds.

What if you run HPS instead?

The same PPFD targets apply, because the plant counts photons, not fixtures. Two things change.

• More heat at the canopy. HPS lamps throw strong radiant heat downward, so they usually hang higher than an LED of similar output, and leaf temperature can run above air temperature. Check the leaves with an infrared thermometer, not just the air.
• Fewer photons per watt. In a Utah State University comparison, the best double ended HPS fixtures made about 1.7 µmol of light per joule, and common mogul base HPS fixtures about 1.0. Many current LED bars are rated near 2.7. To hit the same PPFD, an HPS tent burns more watts and has more heat to remove.

The hand test was built for HPS and still has some use there. With LEDs, go by the chart and the leaves.

Keep the canopy flat

A light can only be one height. If one plant towers over the others, that top gets bleached while the short plants starve. An even canopy lets you set one height that suits every plant. Topping, bending and screens all help, covered in training plants to fill a tent.

Check the gap every few days, and every other day in the first three weeks of flower, when plants can stretch a lot. Note the height and dimmer setting each time you change them. In Canna GLAP you can log that against each plant's phase, so next grow you know exactly where you ended up.

Questions growers ask

Read next

• How many watts of LED does a grow tent need?
• Grow tent too hot? How to bring it down
• How to train cannabis plants to fill a grow tent
• Light leaks and light schedules: protect the dark period

Education only, for adults 21+ where home cultivation is legal. Updated October 15, 2026.