Grow Light Spectrum Explained: What Each Colour Does for Your Plants

Every grow light box talks about spectrum: full spectrum, 3000K, 660 nm, UV, far red. It is easy to assume the colour of the light is the secret to good growth, and just as easy to ignore it completely. The truth sits in between. Spectrum changes the shape of a plant more than it changes how much it grows, and the amount of light matters far more. This guide explains what each colour does, what the numbers on the box mean, which spectrum suits seedlings, leafy crops and fruiting crops, and when extras like UV and far red are worth adding. It applies to salads, herbs, tomatoes, chillies, houseplants and anything else you grow under lights.

What spectrum actually means

Light is made of different wavelengths, measured in nanometres (nm). Our eyes see roughly 400 to 700 nm as a rainbow running from violet and blue, through green and yellow, to orange and red. Plants happen to use almost exactly the same band to photosynthesise, and growers call it PAR: photosynthetically active radiation.

A light's spectrum is simply the mix of wavelengths it gives out. Sunlight contains all of them in fairly even amounts, plus ultraviolet below 400 nm and far red and infrared above 700 nm. A grow light gives out a chosen mix, and that mix decides how efficient the light is, how the plants grow under it, and what the room looks like to you.

What each colour does

Blue (400 to 500 nm): the shape setter

Blue light is a signal as much as a food. Plants read plenty of blue as "I am in open sunlight" and respond by growing short and sturdy, with thicker, darker leaves and shorter gaps between them. Blue also helps the tiny pores on the leaves, the stomata, open so the plant can take in carbon dioxide. Too little blue and seedlings stretch towards the light, going tall, pale and floppy. That is why propagation lights are often cool white or blue-heavy.

Green (500 to 600 nm): not as wasted as people think

Leaves look green because they reflect more green than any other colour, which led to the old idea that green light is useless. It is not. Leaves still absorb most of the green that hits them, and the green that passes through the top leaves keeps going to feed the leaves underneath, where red and blue have already been soaked up. A light with some green in it also shows your plants in their true colours, which makes it far easier to spot a pest, a deficiency or a spot of mildew early.

Red (600 to 700 nm): the engine

Red light drives photosynthesis very efficiently, and deep red LEDs at around 660 nm are among the most energy-efficient LEDs made. That is why almost every modern grow light leans warm and adds a band of deep red to its white chips. Red on its own grows plants that are leggy and soft, so it always needs some blue alongside it.

Far red (700 to 780 nm): the stretch signal

Far red sits just past the edge of what we can see and is not counted in PAR. Plants read lots of far red as "something is shading me", because leaves above them absorb red and let far red through. The response is to stretch: longer stems, bigger leaves and a more open shape. In small doses alongside red that can be useful, because bigger leaves catch more light and far red helps red light do more work. Too much and plants go leggy.

Ultraviolet (below 400 nm): an optional extra

UV is outside PAR and does very little for growth directly. In small amounts it acts as a mild stress that some plants answer by making more protective pigments and tougher leaves, which is why red lettuces and herbs often colour up more under UV. It is also harmful to eyes and skin, so it is a specialist add-on rather than something every grower needs.

Reading the numbers on the box

Colour temperature (Kelvin)

White LEDs are described by colour temperature, written in kelvin (K). Lower numbers look warmer and more orange, higher numbers cooler and more blue. It describes how the light looks to us, not exactly what the plant gets, but it is a useful guide:

"Full spectrum"

Full spectrum has no legal definition. It usually means a white light that covers the whole of PAR with no big gaps, often with extra deep red. It does not mean the light matches sunlight, and it does not guarantee it includes UV or far red. Check the spectrum chart in the product images if you want to know what is really in there.

Efficiency (µmol/J)

The number that tells you most about running cost is photon efficacy, written in µmol/J: how much plant-usable light comes out for each watt that goes in. Spectrum feeds into it, because a red photon takes less energy to make than a blue one. Our guide to LED vs HPS running costs puts that into pounds and pence.

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White LEDs vs purple "blurple" lights

Early LED grow lights used only red and blue chips, which made the room glow purple. They worked, because red and blue are the two colours plants absorb most strongly, but they had real drawbacks: you could not see your plants properly, so pests, deficiencies and mould were easy to miss; the light was hard on the eyes; and the missing green meant lower leaves got less.

Modern white LEDs, usually with extra deep red, have largely replaced them. They are now just as efficient or better, they grow a more natural shape, and they let you judge leaf colour by eye. Our nutrient deficiencies guide relies on reading leaf colour, which is hard to do under purple light. If you still run a blurple light, switch on a normal white torch or room light whenever you check your plants.

The right spectrum for each stage

You do not need a different light for every stage. A good full-spectrum white will grow almost anything from seed to finish. But if you are choosing a light for one job, or running a separate propagation area, this is how spectrum lines up with what you are growing.

HPS, metal halide and CMH: spectrum in older lights

Before LEDs, growers chose between bulbs with very different spectrums, and many still run them.

• HPS (high pressure sodium) gives a strong orange-yellow light with lots of red and orange and very little blue. It grows a lot of plant per watt for an older technology, but plants under HPS alone tend to stretch, and the orange glow makes leaf colour hard to judge. Dual spectrum HPS bulbs add a little blue to soften that.
• MH (metal halide) is bluer and whiter, traditionally used for young plants to keep them compact, then swapped for HPS later on.
• CMH or CDM (ceramic metal halide) gives a broad, daylight-like spectrum with good colour rendering. A 3000K-ish lamp leans warm, a 4000K-plus lamp leans cool. It is the closest a bulb gets to a full-spectrum LED, though it runs hotter and less efficiently.

All three put out far more heat than an LED of the same output, which matters in a small tent. Our summer heat guide covers that, and CMH and HPS need the right ballast, so check compatibility before buying a lamp on its own.

Adding UV and far red: when it is worth it

Supplementary UV and far red bars are the newest part of the grow light market. They can be worthwhile, but they are finishing touches, not foundations.

Far red

Far red is most useful when your main light has little or none, which is true of many white LEDs. Run with the main light, a modest amount helps leaves spread and catch more light. Add it gradually and watch the plants: if gaps between leaves get long and stems thin, cut it back.

UV

UV is the one to be careful with. Run it at a low level, for part of the day, and never look at UV lamps or work under them without UV-rated eye protection; switch them off when you are in the tent. Start with short periods and increase slowly, because too much UV scorches and curls leaves. For most growers of vegetables and herbs, UV is optional; it is most noticeable on red-leaved salads and herbs.

Many modern lights also dim, and a few let you adjust the spectrum. A controller that ramps the lights up and down at the start and end of the day is gentler on plants than snapping them on and off, and makes it easy to match output to the stage. Our timers and contactors guide covers switching lights safely.

Spectrum matters less than intensity

This is the most important point in the guide. A light with a perfect spectrum hung too high, or simply too weak for the space, will always grow worse plants than a slightly imperfect spectrum at the right strength. Think of intensity as how much food the plant gets and spectrum as the recipe: the recipe matters, but not if the plate is half empty.

So when choosing or setting up a light, work in this order:

1. Enough light for the space. Match the light's output to your tent size; our LED grow lights by tent size buying guide does the matching for you.
2. The right height and dimmer setting for the stage. Our guide to light height and PPFD gives the numbers to aim for.
3. A sensible spectrum. A good full-spectrum white with some deep red covers this for nearly everyone.
4. Extras last. Far red, UV and adjustable spectrum are worth considering only once the first three are right.

The same logic applies to other upgrades. Extra CO2, for example, only pays off once light levels are already high; our CO2 enrichment guide explains why.

Why lux meters and phone apps mislead

A lux meter measures brightness as the human eye sees it, and our eyes are most sensitive to green and yellow. Plants are not. So a lux reading under a red-heavy grow light, or an old purple one, can badly understate the light your plants are actually getting, while a green-tinted light can read higher than it should. Phone apps have the same problem, plus a camera that was never designed for the job.

For a real number you need a PAR or quantum sensor, which counts photons across 400 to 700 nm equally and gives you PPFD in µmol/m²/s. Ideally choose a full-spectrum sensor that reads accurately under LEDs. If you do not have one, follow the maker's height chart for your light and watch the plants: bleaching or curled top leaves mean too much, stretching and pale growth mean too little.

Seven common spectrum mistakes

1. Blaming spectrum for a light that is too weak or too high. Fix intensity and height first.
2. Buying by wattage claims and "full spectrum" stickers. Look for real wattage drawn, µmol/J and a published spectrum chart.
3. Thinking green light is wasted. It feeds the lower leaves and lets you see what is going on.
4. Adding far red without watching for stretch. Start small and cut back if stems thin and gaps lengthen.
5. Running UV without eye protection. Always switch it off before working in the tent.
6. Judging plant health under purple or orange light. Check leaves under white light.
7. Trusting a lux meter or phone app. Use a PAR sensor or the maker's height chart.

Quick answers

What is the best spectrum for a grow light?

For most growers a full-spectrum white LED with some extra deep red is the best all-round choice. It covers the whole of PAR, grows a natural, compact plant from seedling to finish, and lets you see true leaf colour. Spectrum fine-tunes plant shape, but getting the amount of light right matters far more.

Is blue or red light better for plants?

Plants need both. Red light drives photosynthesis very efficiently, while blue light keeps growth short and sturdy with thicker leaves. Red on its own grows leggy plants and blue on its own is less efficient, which is why good grow lights combine them, usually inside a white spectrum.

Do plants use green light?

Yes. Leaves reflect more green than other colours, but they still absorb most of the green that hits them, and green light passes through the top leaves to feed the ones below. A light with some green also shows plants in their true colours, which helps you spot problems early.

What colour temperature grow light do I need?

Cool or daylight white (5000K to 6500K) suits seedlings, cuttings and leafy greens. Neutral white (around 4000K) is a good all-rounder. Warm white (2700K to 3000K), usually with added deep red, suits fruiting crops such as tomatoes and chillies. Most modern full-spectrum LEDs grow everything well.

Is a full-spectrum LED the same as sunlight?

No. Full spectrum usually means a white light that covers 400 to 700 nm without big gaps, often with extra red. It does not copy sunlight exactly and may contain little or no UV or far red. Check the spectrum chart in the product images to see what a light really gives out.

Are purple LED grow lights worse than white ones?

Purple red-and-blue lights grow plants, but white LEDs have largely replaced them. Modern whites are as efficient or better, grow a more natural shape and let you judge leaf colour and spot pests by eye. If you use a purple light, check plants under ordinary white light.

Do I need UV or far red?

Not to grow healthy plants. They are optional extras. A little far red helps leaves spread and red light work harder, and small doses of UV can deepen colour in red-leaved salads and herbs. Get intensity, height and a good white spectrum right first, and always protect your eyes from UV.

Can I measure grow light spectrum or strength with my phone?

Not reliably. Phone apps and lux meters are tuned to the human eye, which is most sensitive to green and yellow, so they misread red-heavy and purple grow lights. A PAR or quantum sensor measures PPFD properly. Without one, follow the maker's height chart and watch how the plants respond.