How High Should Your Grow Light Be? Light Height and PPFD by Stage

Almost every lighting problem in a tent is a distance problem, not a wattage problem. The same fixture that scorches a seedling will grow a perfect crop three weeks later, and nothing about the light has changed — only how far away it is. Get the height right and a modest LED outperforms an expensive one hung carelessly. This guide covers what PPFD actually means, the numbers to aim for at each stage, sensible starting heights for every size of fixture, and how to check you have it right without spending £500 on a sensor.

Why height matters more than wattage

Buy a bigger light and you buy more photons. Where those photons land is decided entirely by how you hang it. Light spreads as it travels, so the further your canopy sits from the fixture, the more thinly the same output is spread across it — and the falloff is much steeper than most people expect.

For a small, concentrated source the relationship is the inverse square law: double the distance and you get a quarter of the intensity. Move a light from 30cm to 60cm above the canopy and you have not halved the light reaching your plants, you have quartered it. That is the single most useful fact in this guide.

There is one honest caveat. The inverse square law describes a point source. A modern LED bar fixture is a wide area source, and close underneath it the falloff is gentler than the law predicts, because as you move away you also start catching light from the far ends of the bars. It tightens up to the true inverse square once you are a reasonable distance below. So treat the quartering rule as the shape of the problem rather than a precise calculator — it is right about the direction and roughly right about the scale.

PPFD, PAR and DLI, in plain English

Three abbreviations get thrown around, and only one of them is a number you can act on.

PAR

Photosynthetically Active Radiation is simply the band of light plants can use for photosynthesis, roughly 400 to 700 nanometres — visible light, near enough. PAR is a category, not a measurement. A fixture advertised as having "great PAR" is telling you nothing at all.

PPFD

Photosynthetic Photon Flux Density is the one that matters. It counts how many usable photons land on a square metre of your canopy every second, in units of µmol/m²/s. This is the number that changes when you raise or lower the light, and it is the number every target in this guide refers to.

Be careful with the related figure PPF (no D). PPF is the total photon output of the fixture, measured in µmol/s, with no reference to area or distance. It tells you how good the light is. PPFD tells you what your plants are actually receiving. A 720W fixture with excellent PPF hung two metres up delivers a miserable PPFD.

DLI

Daily Light Integral is PPFD added up over the whole photoperiod, in mol/m²/day. It matters because plants respond to the day's total, not to any single instant. The arithmetic is straightforward:

DLI explains something that confuses a lot of growers: a modest light running a long day can deliver the same daily total as a powerful light running a short one. If your fixture cannot reach the intensity you want, adding hours during the vegetative stage is a legitimate substitute. It is also why cutting the photoperiod without raising intensity quietly cuts your daily light by a third.

The numbers to aim for at each stage

These are broad, well-established horticultural targets and they apply to anything you grow under a light — tomatoes, chillies, herbs, leafy salad or ornamentals. A seedling wants gentle light because it has almost no root system to support the water loss that bright light causes. A mature plant in full fruit wants as much as you can give it without cooking it.

The final row deserves a caution. Pushing past roughly 900 PPFD only pays if everything else is already perfect: CO2 enrichment, tight temperature control, ample feed and healthy roots. Without those, the extra light is wasted at best and damaging at worst. For the overwhelming majority of tent growers, 600–800 PPFD in flower is the sensible ceiling, and getting there evenly across the whole footprint beats hitting 1000 in a hot spot in the middle.

Sensible starting heights

Every manufacturer publishes a distance chart for their own fixture and that chart always beats a general table — it is measured, and it accounts for the beam angle and the lens. Check the box or the product page first. What follows is a reasonable starting point when you have no chart to hand.

Two things stand out in that table. First, HID fixtures hang further away than LEDs of comparable output, and that is about heat, not light. A 600W HPS radiates a great deal of infrared straight down; the limit on how close you can hang it is the temperature at the leaf, not the photon count. LEDs run cooler at the canopy, which is precisely why they can sit closer and why people so often burn plants with them — the usual warning sign, radiant heat on the back of your hand, simply is not there.

Second, the numbers move a long way between stages. A light that sits at 75cm through veg and stays there through flowering is throwing away a third of its useful intensity at exactly the point in the cycle when the plant wants the most. Height is not a set-and-forget adjustment.

Dim first, raise second

If your fixture has a dimmer — and most modern LEDs do, either as a dial on the driver or through a 0-10V controller — use it before you reach for the ratchets.

The reason is uniformity. Raising a light does reduce intensity, but it also spreads the beam wider than your tent, so a growing share of your output ends up on the walls rather than the canopy. Dimming reduces intensity while the light keeps covering the same footprint in the same pattern. You get a weaker but evenly distributed canopy, which is what you want, and you use less electricity into the bargain.

This is also the argument for buying a fixture with headroom. A 480W light run at 60% is quieter, cooler, longer-lived and more efficient per photon than a 300W light run flat out, and it leaves you somewhere to go later in the cycle. Dimmable drivers are no longer a premium feature.

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How to check you have it right without a meter

A proper quantum sensor is the only way to know your PPFD for certain, and until recently that meant several hundred pounds for a tool you would use twice a year. It is worth saying plainly: for a single tent, you do not need one. There are three checks that between them will keep you out of trouble.

1. The back-of-the-hand test

Hold the back of your hand at canopy height, palm down, for about thirty seconds. If it becomes uncomfortably warm, the light is too close — but note this only works for HID. Under a modern LED your hand will feel almost nothing at a distance that is still bleaching your plants, which is exactly the trap described above. Use this test for HPS and CDM, and ignore it for LED.

2. Read the plants, every day

Plants are excellent light meters if you know what to look for, and the two failure modes look completely different.

• Too far away: stems elongate and the gaps between leaf nodes stretch out, the plant leans or reaches toward the light, growth is pale and soft, and the whole thing looks leggy. This appears over a week or two.
• Too close: the leaves nearest the light lose colour and go yellow or white while the rest of the plant stays deep green, leaf edges curl or the whole leaf cups upward like a taco, and tips crisp. This can appear in a couple of days.

The diagnostic detail worth remembering is where the discolouration sits. Light stress hits the tops — the parts closest to the fixture — and leaves the lower canopy healthy. A plant that is yellowing from the bottom up, or all over, has a feeding or root problem, and moving the light will not fix it. Our nutrient burn guide covers that side of it.

3. A phone lux app, used carefully

Your phone's ambient light sensor measures lux, which is weighted for human eye sensitivity, not for photosynthesis. Converting lux to PPFD needs a factor specific to the light's spectrum, so the absolute number a phone gives you under a magenta-tinted LED can be badly wrong. What it is genuinely good for is comparison: walk the sensor across your canopy at a fixed height and you will find your hot spots and your dead corners in about a minute. Uniformity is something a phone can measure honestly, and uniformity is half the battle.

If you do want to measure properly — because you are running several rooms, or dialling in a commercial space — a full-spectrum PPFD sensor that logs to a controller is the tool for the job.

The gear that makes height adjustable

None of this is any use if moving the light is a two-person job with a stepladder. The difference between growers who keep their light at the right height and growers who do not is almost entirely whether adjusting it takes ten seconds or ten minutes.

Rope ratchets are the standard answer and cost less than a bag of coco. Squeeze the release, slide to height, let go, done — and they hold several kilos each without slipping. Jack chain with S hooks is cheaper still and completely reliable, but it only adjusts in fixed link-sized steps, which is usually fine for a light and less good for fine-tuning. Whatever you use, hang from the tent's top bars rather than a side rail, check the manufacturer's stated load limit for your tent poles before hanging a heavy HID fixture, and always use two hangers so the fixture stays level.

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Matching the fixture to the footprint

Height only works if the light suits the space to begin with. A fixture that is too powerful for a small tent has to be hung so high or dimmed so far that you lose the benefit; one that is too weak cannot reach flowering intensity at any height. As a rough guide, budget around 200–300W of quality LED per square metre for a full flowering canopy, less if you are only vegging.

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Five mistakes that cost people a crop

1. Hanging by the manual and never touching it again. The chart gives you a starting height for a stage, not for the whole cycle. Check the gap weekly — plants can put on 5cm in a week during a growth spurt, and the light does not move on its own.
2. Treating bleached tops as a nutrient problem. Whitened, crispy leaves right under the fixture with a healthy green plant below is light stress, every time. Adding more feed to a light-stressed plant makes it worse. Raise the light or dim it and the new growth comes back normal.
3. Measuring to the wrong place. Distance is measured from the fixture to the top of the canopy, not to the pot rim or the floor. As plants grow, the gap closes even when nothing has moved.
4. Forgetting that the corners are darker. Growers dial in the centre of the tent perfectly and wonder why the outside plants are leggy. Rotate pots, or accept that the middle position is the premium one.
5. Running the light closer to compensate for a weak fixture. Dropping an underpowered light very low gives you a bright circle in the middle and darkness at the edges. Total light is set by the fixture; height only redistributes it. If the light is too small for the space, height cannot rescue it.

Quick answers

How high should my LED be from seedlings?

Further than most people think. Seedlings and fresh cuttings want only 100–300 PPFD, which for a typical 200W LED means around 60–70cm, and for a 720W bar light more like 75–90cm. If you have a dimmer, an easier approach is to hang the light where it will live for the rest of the grow and turn it down to 20–30% instead. Seedlings have almost no roots, so intense light drives water loss they cannot replace, and the plant wilts even though the growing medium is wet.

What is a good PPFD for flowering and fruiting?

600–900 µmol/m²/s across the canopy, and for most tent growers without CO2 enrichment the useful working range is 600–800. Going higher only pays off if temperature, feed and CO2 are all dialled in; without those, the extra intensity is wasted and the tops will bleach. Getting 700 PPFD evenly across the entire footprint is worth far more than 1000 in a bright patch under the middle of the fixture with dim corners around it.

Do I really need a PAR meter?

For one tent, no. The manufacturer's distance chart plus a weekly look at your plants will keep you in the right range. Stretching and pale, leggy growth means the light is too far away; bleached, cupping tips right under the fixture mean it is too close. A phone lux app is useful for finding hot spots and dark corners even though its absolute numbers are unreliable under coloured LED light. A true quantum sensor is worth buying when you are running multiple rooms or need to prove your numbers.

Why are my leaves cupping upward?

Leaves that curl or cup upward like a taco, usually on the tops nearest the light, are the classic sign of too much light or too much radiant heat. It is the plant reducing the surface area it presents to the fixture. Check the temperature at canopy height first — if that is fine and the cupping is still only on the top growth, raise the light 10cm or drop the dimmer 10% and watch the new growth over the next few days. Cupping downward is a different problem, usually overwatering or root trouble.

Can I use height instead of a dimmer?

You can, and for a fixture with no dimmer it is your only lever. But dimming is the better tool where you have it, because raising the light spreads the beam wider than the tent and sends a growing share of your output onto the walls rather than the plants. Dimming cuts intensity while keeping the same coverage pattern, and it saves electricity. Use the dimmer for stage-to-stage adjustment and reserve moving the fixture for when you run out of dimmer range.

Does the light need to be higher for HPS than for LED?

Yes, and the reason is heat rather than light. A 600W HPS radiates a lot of infrared downward, so the limit is the temperature at leaf level, not the photon count — typically 50–65cm in flower, further in a warm room. LEDs put much less radiant heat on the canopy and can sit closer. That is exactly why LEDs catch people out: the back-of-the-hand test that works well for HID feels perfectly comfortable under an LED that is still close enough to bleach the tops.

How does DLI change things if my light is not very powerful?

DLI is intensity multiplied by hours, so a longer day can make up for a weaker light. At 300 PPFD for 18 hours you get about 19 mol/m²/day, which is a respectable vegetative figure even though the instantaneous intensity is modest. The limit is the photoperiod your crop needs — you cannot extend a 12-hour flowering day without changing what the plant does — so during flowering, intensity is the only lever you have and DLI is fixed by it.