Plant Light Spectrum Explained
What the plant light spectrum means - full spectrum versus red/blue, PPFD and DLI versus lumens, how long to run a light and how far away to hang it, with the figures university extension services give.
Walk down a grow-light aisle and every box says something different. “Full spectrum!” “Red and blue for maximum yield!” “5000K daylight!” The terms are real, but they describe different things, and few product pages explain how to choose. This page untangles the jargon with the help of university extension services.
We have not measured any lamp ourselves. The figures below come from the extension publications linked beside them.
What “Spectrum” Means
Light is a mix of wavelengths, and a plant uses them differently. Iowa State University Extension sums up the two that matter most:
- Blue wavelengths help regulate chlorophyll production, control excessive stem elongation, and regulate the stomata through which a leaf exchanges gases. In practice: compact, sturdy growth.
- Red wavelengths, in the same source’s words, are used extensively for photosynthesis, cause stretching and elongation, and promote more flowering and fruiting.
Leaves reflect more of the green in between than of the red or the blue, which is why they look green to us.
The light a plant can use for photosynthesis is called PAR, photosynthetically active radiation. Every useful grow-light specification is a way of counting it.
Full Spectrum or Red/Blue?
Red/blue (“purple”) lights put their output where photosynthesis uses it best. The University of New Hampshire’s extension service says as much: photosynthesis does not prefer daylight full spectrum - it best utilizes a red/blue combination. The drawbacks are practical: the pink glow is hard to live with, and under coloured light you cannot see the true colour of a leaf, so problems are easy to miss.
Full-spectrum white lights look like daylight. Iowa State calls the ideal home fixture “full spectrum” (sometimes called “daylight”), with a colour temperature in the middle of the range so that it carries both red and blue.
Which should you buy? For a living room, white. And do not agonize over it: New Hampshire’s guide says that LED shop lamps will work fine, and that failure with plants under lamps is more often about light quantity - intensity and duration - than about spectrum.
Not sure whether you need a light at all? Our light-level checker helps you gauge how bright a spot really is.
The Specifications That Matter
PPFD
Iowa State defines photosynthetic photon flux density as the amount of PAR provided over a surface area - essentially, the amount of light useful to plants that actually reaches the leaves. It is measured in micromoles per square metre per second (µmol/m²/s), and it falls as the lamp moves away. A maker who is serious about plants publishes PPFD at stated distances.
DLI
The daily light integral is PPFD added up over the day: the same source calls it the amount of PAR delivered over an area in 24 hours, and gives the sum:
DLI = PPFD × hours of light × 0.0036
So a lamp giving 120 µmol/m²/s at the leaves, run for 12 hours, delivers 120 × 12 × 0.0036 = about 5.2 moles per square metre per day.
How much do plants want? New Hampshire’s extension service gives these daily targets, in moles per square metre:
| Plants | DLI target |
|---|---|
| Houseplants, largely | 4-6 |
| Shade plants | 6-8 |
| Part-shade plants | 10-12 |
| Full-sun plants | 14-20 |
| Seedlings before transplanting | 8-14, even for full-sun plants |
The specifications that tell you less
- Lumens and lux. Iowa State: lumens are a measure of the amount of light you see and do not indicate the amount available for the plant to use. There is no universal way to convert one into the other, because every fixture emits a different mix of wavelengths.
- Watts. They tell you what the lamp draws from the socket, not what reaches the leaf.
- Kelvin. Colour temperature describes the tint of white light: lower numbers are redder, higher numbers bluer. It is a description of appearance, not a measure of quantity.
How Long to Run the Light
Iowa State’s guidance for home growers: it is best to supply light 12 to 14 hours a day; the lights can be on for as little as 10 hours and no more than 16. Plug the lamp into a timer and stop thinking about it.
Whether a plant needs darkness depends on the plant. New Hampshire’s guide notes, for example, that tomatoes need about six hours of dark to fruit and peppers do not. For flowering houseplants, check the species.
How Far Away
Distance changes everything, because PPFD drops quickly as the lamp rises. Iowa State’s rule of thumb: fluorescent fixtures 6 to 12 inches from the foliage; LEDs 12 to 24 inches in most cases. For seedlings under shop lamps, New Hampshire’s guide suggests hanging the lamp 4-6 inches above the tops. If the maker publishes a distance chart, use that.
Then watch the plant. The same Iowa State page lists the signs:
- Too little light: stretching, spindly growth, leaf drop, yellowing foliage, no flowers where there should be some.
- Too much: leaf scorch, bleached leaves, excessive reddening, extremely compact growth.
Choosing a Light
- A few houseplants or a tray of seedlings. Iowa State recommends nothing more exotic than a standard 4-foot plug-in shop fixture with full-spectrum (“daylight”) light. A clip-on white LED does the same job for a single pot.
- Herbs and salad leaves. The same kind of fixture, closer and for longer - work out the DLI with the formula above. These are the edible crops in our gardening section.
- Fruiting crops. Full-sun plants need roughly three times the daily light of a houseplant, by the table above. Look for a fixture whose maker publishes PPFD at stated distances, and do the sum before you buy.
We name no brands, because we have tested none.
Mistakes to Watch For
- Hanging the light too high. A lamp on the ceiling lights the room, not the plant.
- Judging by brightness. What looks bright to you is measured in lumens; the plant counts something else.
- Running it round the clock for everything. Some plants need their night.
- One light for very different plants. A succulent and a fern want different amounts. Group plants by need.
Frequently Asked Questions
Is full-spectrum white better than red/blue purple grow lights?
For a home, white is easier to live with and lets you see the plant properly. For photosynthesis itself, a red/blue mix is used at least as well, by New Hampshire’s extension guide. Quantity of light matters more than either.
What colour temperature should a grow light be?
Iowa State suggests a full-spectrum or “daylight” fixture with a Kelvin value in the middle of the range, so that it has both red and blue. Lower numbers are redder, higher numbers bluer.
Can I use a regular LED bulb to grow plants?
Often, yes. New Hampshire’s extension service says LED shop lamps will work fine; what usually goes wrong is too little light for too short a time. A desk lamp far from a plant will not do much.
How close should a grow light be to plants?
Iowa State’s rule of thumb is 12 to 24 inches for LEDs and 6 to 12 inches for fluorescent tubes; seedlings under shop lamps are kept closer. Follow the maker’s chart where there is one, and watch for bleaching or stretching.
Do plants need UV light?
Not from a grow light. None of the extension guides we read lists ultraviolet as something a home fixture must supply; the specifications that matter count light in the range plants use for photosynthesis.
Recommended Gear on Amazon
- Full-spectrum LED grow light - browse current options and reviews on Amazon.
- Plant light meter - browse current options and reviews on Amazon.
- E26 grow light bulbs - browse current options and reviews on Amazon.
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