Two questions decide whether a grow light does anything: how far it sits from the leaves, and how many hours it runs. Both are answered badly online, mostly because the specs printed on the box — watts, lumens, "equivalent to 1000W" — do not describe what reaches your plant.
Why distance matters more than the light you bought
Light intensity falls off roughly with the square of the distance. Double the gap between the fixture and the leaf and you deliver about a quarter of the light. That is not a small adjustment — it is the difference between a plant that grows and a plant that merely survives.
Which means a modest light placed close beats an expensive light placed far away, every time. The single most common mistake is buying a stronger fixture and hanging it three feet up because that is where the shelf is. You have paid for intensity and then thrown most of it away.
It also explains why a plant on a windowsill and the "same" plant six feet into the room are not in comparable conditions, even though both rooms look bright to you. Your eye adapts across an enormous range; the plant does not.
Starting distances
These are conservative starting points, not gospel — fixtures vary enormously and the only real test is what the plant does over two weeks.
| Fixture type | Starting distance above the canopy | Notes |
|---|---|---|
| Small LED bulb in a lamp (under ~20W actual draw) | 6–12 in | Weak. Fine for one herb pot or a low-light foliage plant, useless for a tray of seedlings |
| LED bar / strip light (~20–60W actual) | 12–18 in | The sweet spot for houseplants and seed starting. Even coverage, low heat |
| LED panel or quantum board (~100W+ actual) | 18–30 in | Overkill for houseplants; built for tents and fruiting crops |
| Fluorescent T5 tube | 6–12 in | Runs cool enough to sit close. Fading technology but still fine and cheap |
| Seedlings, any fixture | Closer than for mature plants | Seedlings want high light immediately or they stretch. Go as close as heat allows |
Heat check, once, with your hand: hold the back of your hand at leaf height under the light for thirty seconds. If it is uncomfortably warm on your skin, it is too close for the leaves. Modern LEDs rarely fail this — they push heat out the back, not down — but cheap fixtures with poor heat sinks and any HID lamp absolutely do.
Then let the plant tell you. Too far away and you see stretching, long bare gaps between leaves, and small pale new growth — the same leggy growth pattern as any light shortage. Too close and you see bleached or washed-out patches on the upper leaves nearest the fixture, sometimes with a bronzed or crispy look, while lower leaves stay fine. Bleaching on the top and only the top is the signature of light burn, and it means raise the fixture a few inches.
How many hours
Plants respond to total daily light — intensity multiplied by duration — so a weaker light run longer does partly compensate. Partly. There is a floor of intensity below which more hours does not help, and a ceiling of hours above which you are wasting electricity.
| What you are growing | Hours per day |
|---|---|
| Foliage houseplants (pothos, monstera, philodendron, snake plant) | 10–12 |
| Seedlings and young transplants | 14–16 |
| Herbs (basil, parsley, cilantro) and salad greens | 14–16 |
| Fruiting crops indoors (tomatoes, peppers) | 16–18, plus much higher intensity than a houseplant light provides |
| Succulents and cacti | 12–14 |
- Leave the dark period alone. Plants run necessary metabolic processes at night. Twenty-four-hour light does not double growth and stresses most species — the exception is some seedlings under low intensity, and even there the gain is small.
- Put it on a timer. Not a convenience. Erratic day length confuses flowering and dormancy cues, and "I'll remember" fails within a week. A basic mechanical timer costs a few dollars and is the highest-value purchase in this whole article.
- Run it during your daytime. Cheaper on most rate plans, and it means a lamp is not glaring in your living room all evening.
- Count window light too. A plant in a bright south window in July needs supplemental light for a couple of hours at most, or none. The same plant in the same window in January may need eight.
The specs that mean something
Ignore lumens. Lumens measure brightness weighted for the human eye, which peaks in green — the part of the spectrum plants use least efficiently. A light optimised for a lumen number is optimised for looking bright to you.
Ignore "equivalent to 1000W" claims entirely. There is no standard behind them. Find the actual power draw in the specifications, which is usually a small fraction of the headline number.
- PPFD (µmol/m²/s) — photosynthetically active light arriving at a given point, at a given distance. The number that matters. A reputable manufacturer publishes a PPFD map at several heights. If they publish nothing, that tells you something.
- DLI (mol/m²/day) — PPFD accumulated over the day. This is the number that ties intensity and hours together. Foliage houseplants do well around 6–12; herbs and greens want 12–20; fruiting crops want 20–30.
- Actual wattage at the wall — a rough sanity check on how much light there is to distribute. Not a quality measure, but a floor.
- Full spectrum / white LEDs are fine and easier to live with than blurple fixtures. The purple ones are not more effective; they just skip the wavelengths that make a room habitable.
Do this before spending anything: install a free light meter app on your phone, set it to lux, and measure at leaf height where your plants actually live. Phone sensors are not laboratory instruments and the lux-to-PPFD conversion is approximate, but the relative readings are honest and often startling. A spot that feels bright frequently reads at a tenth of what you assumed. Roughly: under 500 lux is decoration, not growing; 500–2,000 lux sustains low-light foliage; 2,000–10,000 lux is a genuinely bright indoor spot; a shaded outdoor patio is around 20,000.
When a light is not the answer
Worth saying plainly, because it saves people money. If a plant is declining and the soil is always wet, a grow light will not save it — you have a root problem and adding light adds demand the roots cannot meet. Fix the watering first.
If the plant is a genuine low-light species already sitting in decent ambient light, a lamp buys you very little. And if the pot is three sizes too big and staying wet in the middle, the pot is the problem, not the light.
Light is the right purchase when growth has stopped or gone leggy, new leaves come in smaller than old ones, and the soil dries at a reasonable rate. That combination is a real light shortage and a light genuinely fixes it.
If you would rather not engineer it
There is a legitimate case for buying a system where the light distance, run time and nutrient delivery are already solved, especially for edibles. You pay a lot more per plant, and what you buy is not having to think about any of the above.
Lettuce Grow Farmstand
Vertical hydroponic tower, 12–36 plant sizes. Optional glow rings for indoor use. Roughly $350–900 depending on size and lighting.
A self-watering vertical hydroponic system with matched lighting sold as an add-on. The pump timer, light height and nutrient schedule are pre-solved, which is genuinely the whole value proposition. We use one for salad greens and herbs — the turnover on lettuce and basil is fast enough that it earns its footprint, and greens are the crop where store-bought quality is worst relative to home-grown.
What works
- Distance and run time are handled — the glow rings are positioned for the plant sites
- Very fast growth on leafy greens and herbs; harvest in weeks, continuously
- Small footprint for the plant count, which suits an apartment or a kitchen corner
- Almost no failure modes — no soil, no overwatering, no repotting
What doesn't
- Expensive per plant. A shelf, a $40 LED bar and a $5 timer grows the same greens for a fraction of the cost if you are willing to set it up yourself
- Ongoing cost for nutrients and replacement seedlings, which the sticker price does not show
- Only suits leafy greens, herbs and small fruiting plants — not a route to growing your own tomatoes at scale
- The pump is audible. In an open-plan apartment you will notice it
- Indoor lighting is a separate purchase on top of the base unit
Affiliate link · LettuceGrow
Our honest position: if you are the kind of person who reads an article about PPFD, build it yourself and spend the difference on a better light. The Farmstand is for the household that wants salad, not a hobby. Both are fine reasons to buy something — but they are not the same purchase, and plenty of sites blur that on purpose.
Common questions
How far should a grow light be from plants?
As a starting point: 6–12 inches for a small LED bulb or fluorescent tube, 12–18 inches for an LED bar or strip, and 18–30 inches for a large panel. Go closer for seedlings. Then adjust based on the plant: stretching and small pale new leaves mean move it closer, bleached washed-out patches on the top leaves nearest the fixture mean raise it.
How many hours a day should I run a grow light?
10–12 hours for foliage houseplants, 14–16 for seedlings, herbs and salad greens, 16–18 for fruiting crops, 12–14 for succulents. Always leave a dark period — 24-hour light does not double growth and stresses most plants. Put the light on a timer; irregular day length disrupts flowering and dormancy cues.
Can a grow light be too close?
Yes. The sign is bleached, washed-out or bronzed patches on the upper leaves closest to the fixture while lower leaves look fine. Check heat by holding the back of your hand at leaf height for thirty seconds — if it is uncomfortable on your skin it is too close for the plant. Modern LEDs rarely burn plants; cheap fixtures with poor heat sinks do.
Do lumens matter for grow lights?
Not really. Lumens measure brightness as the human eye perceives it, weighted towards green — the wavelengths plants use least. Look for PPFD (light arriving at a stated distance) and DLI (that light totalled over a day), plus the actual wattage drawn at the wall. Ignore any 'equivalent to 1000W' claim; there is no standard behind it.
Do regular LED bulbs work as grow lights?
For low-light foliage plants, a bright white household LED close to the plant does help — plants respond to intensity and duration more than to a special spectrum. It will not support seedlings, herbs or anything fruiting, because a household bulb does not put out enough intensity at a usable distance. Cheap purpose-built LED bars are inexpensive enough that there is little reason to improvise.
How do I know if my plant needs a grow light?
Growth has slowed or stopped, stems are stretching with long bare gaps between leaves, new leaves come in smaller and paler than old ones, and the soil dries at a reasonable rate. If the soil is instead staying wet for days, you have a root problem and adding light makes it worse — fix the watering first.