Grow Light FAQs
Exploring the details and dispelling some myths. Your frequently asked questions about grow lights.

Table of Contents

Grow Light Safety

Safety is a good thing. Here we answer questions relating to grow light safety.

Yes. As safe as any other light — you don't want to look directly at any bright light source for too long.

The important exception is grow lights that put out UV (Ultraviolet) light. UV is invisible and will damage your eyes.

Most consumer LED and fluorescent grow lights don't put out UV light. However some are augmented specifically to provide UV — which may provide benefit to some plants. Less common grow lights, like Metal Halide and High Pressure Sodium, do put out meaningful amounts of UV.
Know your light and treat any potential UV source with care.

reference: University of Washington

Generally, yes grow lights are safe. Like any electrical device they should be properly designed and manufactured for safety. A poorly made grow light puts you at risk for fire or electric shock.

We recommend only using grow lights that are safety certified. Look for a UL or ETL certification mark. These marks mean the light is independently certified for safety.

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CE marked products can be self-certified. Meaning the manufacturer can declare that the product meets the proper standards.

If that sounds a little too trusting to you then look for UL or ETL. Those certifications require testing by an independent lab.

Sunburn is caused by ultraviolet light (UV). Most grow lights don't give off UV so they cannot give you a sunburn.

However some types or models of grow lights to give off UV — so be sure to follow the manufacturer's directions.

If you're using a grow light around cats or dogs you will want to be sure it's safe.

Grow lights are safe for pets as they are for humans. With the important note that UV augmented grow lights should be kept away from humans and pets as UV light is invisible and potentially harmful.

Grow Light Science

Knowing some basic plant-light science can remove a lot of the confusion that surrounds grow lights.

Many LED grow lights give out a purple light instead of white. Many lights are built this way because it's cheaper than using full spectrum white LEDs.

Some commercial growers use purple light by choice — it can be slightly more cost-effective for certain crops. In general though full-spectrum white grow lights are better for home use.

Blurple is a made up name to describe the pinkish-purple that some LED grow lights give off.

Full-spectrum lights give off all of the visible wavelengths (or colors) of light. A full-spectrum light will apear white to the human eye. 

Full-spectrum light is more appealing in living spaces and shows plants in their true color.

It's a common myth that plants cannot use green light for photosynthesis. In fact they do use green light.

There is a kernel of truth to the myth. Chlorophyll in leaves is essential to photosynthesis. And it absorbs blue and red wavelengths of light more efficiently than green. This means that the majority of reflected light from a leaf will be green. So we see plants appear green. However plenty of green light is still being absorbed and used in photosynthesis.

Reference: Greenhouse Production News

Measuring Grow Lights

If you can measure light you can harness it.

PAR is the range of light wavelengths that powers photosynthesis in leaves. PAR stands for Photosynthetic Active Radiation. So, PAR is the type of light a plant needs to grow.

PAR covers the range from 400nm (nanometers) to 700nm in the electromagnetic spectrum. 

PAR spectrum diagram

PPF (Photosynthetic Photon Flux) is a measure of the output of a grow light. It's a measurement of the number of photons in the photosynthesis-driving range (PAR) that the light gives off every second.

PPF is measured in µmol/s (micromoles per second).

A small screw in grow bulb might have a PPF of 15 µmol/s. A powerful, high end LED grow light could have a PPF of 300 µmol/s. These numbers tell us that the second light gives out 20x more plant-useful light.

PPFD stands for Photosynthetic Photon Flux Density. It's a measure of the number of plant-useful (PAR range) photons at a particular location.

PPFD is measured in µmol/m²/s (micromoles per metre squared per second).

Light disperses and becomes less intense as it travels away from the source. We we use PPFD to measure how much plant-useful light is available at a particular distance from a grow light.

A PPFD map diagrams the amount of PPFD across a surface at a certain distance from a particular grow light. They can be used to judge the qualities of a grow light.

How powerful is the grow light? How even is the spread of light? What size of growing area will it support? These can all be answered if you know how to read a PPFD map.

DLI (Daily Light Integral) is a measurement of plant-useful (PAR) photons per day.

Each plant has a DLI — think of it as the plant's appetite for light. If the plant receives enough light to cover it's DLI then it will thrive. If the light it receives falls short then it will grow slowly or even die.

DLI is measured in mol/m²/day (moles per metre squared per day).

The PPFD delivered by a light source can be converted to DLI. This tells you whether that light source will meet the needs of your plant.

A quantum meter is the ideal tool for measuring plant-useful (PAR) light levels.

A quantum meter measures individual photons of light in the PAR range of plant-useful light.

Lumens are often used to measure the output of lights. They're not the proper measurement for grow lights though.

Lumens measure brightness in the way humans perceive it. For plants the measurement you want is PPF.

Lux measurements go hand-in-hand with lumens. Lux is a measurement of the visible light arriving at a particular area. 

Like lumens, lux is not the proper measurement for plant-useful light (PAR). Lux is tuned to brightness as humans perceive it — and plants experience light very differently.

 

Foot Candles are another human-centric measure of light. They're just a different unit from Lux. One Foot Candle (FC) is equal to about 10 Lux.

Light meters measure lux or foot candles. Those measurements are about how humans perceive brightness. So they're not ideal.

But... a decent light meter might cost $30 while a proper PAR meter for measuring plant light will cost 10x as much. So lots of gardeners use a light meter as the best available option.

A light meter won't give you accurate measurements of the light your plants are receiving. It can be useful to take comparative measurements though — knowing one spot is twice as bright as another is useful information.

Watts are often used to indicate the power of a grow light. They're not a terribly useful measure though.

Watts (technically "consumed watts") are used to measure the electrical energy that a grow light uses. They don't tell you how much light it outputs.

Every light puts out a mixture of heat and light. An efficient light will put out a lot of useful light and a little waste heat. An inefficient light will do the opposite — wasting a lot of the electrical watts is consumes. So high consumed watts does not necessarily mean a grow light is effective — it could just be very inefficient.

 

K values (aka Kelvin temperature) describe the appearance white light. A lower value, like 3000K, would have a warmer appearance. Higher values, like 5000K have more of a blue-ish tint.

The kelvin value of a light can give you a hint about the wavelengths it contains. Generally a light with a lower K value will contain more red light — making it more suited for plants that fruit and flower. A higher K light will be more blue-white and better for leafy plants that don't need the extra red.

Kelvin scale of color temperature

 

Equivalent watts are used to compare light output to old-style incandescent bulbs. An 100 watt-equivalent light should give out about the amount of light a 100 watt incandescent bulb would have.

Equivalent watts are also used by sneaky marketing people to make their grow lights sound brighter than they are. Consumed watts are an imperfect but useful measurement. Equivalent watts are best ignored.

Grow Light Use

Common questions about effectively using grow lights.

It varies from plant to plant but 12 hours is a good starting point.

There are two reasons you'd change the on time for your grow lights:

  1. More hours of light means more photosynthesis. So boosting light hours lets you get more bang for your lighting buck.
  2. Some plants respond to different ratios of light/dark hours by beginning to flower — this is called photoperiodism.
    For plants like tomatoes you want to encourage flowering which is required for fruiting. For plants like lettuce on the other hand you want to prevent flowering which makes the leaves bitter and inedible.

Our plant guide gives individual light-schedule recommendations for all kinds of edible plants.

No, it's not a good idea to leave your grow lights on all the time. Plants have essential processes that only happen in the dark.

It depends on the plant and the grow light. Probably closer than you think though. As a general rule your grow light should be between a couple of inches and a foot from the upper leaves of your plants.

Check out our edibles under grow lights guide for specific distances.

As long as you make sure your plants are getting enough hours of darkness you can run your grow lights over night. Very few plants can thrive with 24 hours of light.

Also many plants (known as "long-day" plants) will begin to flower if they have too few hours of darkness.

Our grow light crops guide has information on the light cycles for different plants.

Plants don't actually care if their light is direct or indirect. They care that enough photons are landing on their leaves everyday to power photosynthesis. Direct sunlight is used as shorthand for the amount of light a particular plant needs.

Check our grow light/crop guide for specific lighting guidelines.

This is a foundational question: do grow lights make a difference? The answer is yes. A good grow light can make a huge difference to indoor plants.

In fact it's possible to grow plants in a totally dark room using only grow lights.

Yes, all grow lights emit some heat. Heat is generated as an unwanted side effect — more efficient lights give out more light and less heat.

Yes. As soon as a seedling breaks through the soil it begins searching for light. Providing a bright light for 14-16 hours will help your seedlings thrive.

Generally using grow lights will give you a better crop of microgreens. Your microgreens will grow taller, more colorful and tastier with light.

See our guide to selecting grow lights for microgreens.

Grow lights can be very helpful for supplementing weak winter sunlight.

A sunny windowsill might support your plants in the summertime — when days are long and the sun is bright. Once winter comes around though your plants will be getting only a fraction of what they need.

Adding a simple grow light on a 12 hour timer to boost your light can do wonders.

They are. LEDs are the most advanced form of lighting. So a good quality LED grow light will be more efficient than any other grow light.

Types of Grow Lights

Grow lights may seem like a swirl of jargon. We cut through that to give you answers on what kinds of grow lights are what.

LEDs are probably the best overall bet. Fluorescent grow lights have their strengths though.

LEDs are more energy-efficient which means less waste heat and lower power bills. Plus they can be tuned to put out a specific of wavelengths of light — for example they can be red-heavy to support flower and fruiting.

Fluorescent lights are generally cheaper to buy but put out less intense light. They can be a very good choice for seedlings or microgreens — where you want a large area of even, moderately bright light.

T5 describes the physical form of a grow light. It's the classic fluorescent tube style of light. Sometimes known as a shop light.

Usually T5 lights are fluorescent but LED retrofit tubes are also available — giving the efficiency and long-life of an LED light that can be fitted into existing T5 fixtures.

T8 and T10 are the same kind of bulb in larger diameter — they're less efficient than T5 though. High Output T5 is the bulb technology that works well for grow lights.

For nearly all purposes white lights (aka full spectrum) are best. Contrary to popular belief plants use all colors of visible light — not just red and blue. Plus a white light is much more attractive and lets you spot any problems with your plants more easily.

High Output fluorescent lights (aka HO T5) work well for seedlings. They are relatively cheap to buy and cover a large area with the even, moderate light most seedlings need.

Tomatoes need a red-heavy light source. The red wavelengths are essential to help the plants flower and fruit. Fluorescent lights don't give off much light in the red wavelengths — so you'll need an LED grow light.

The grow light should be a pretty powerful one too. Tomatoes are light-hungry and as they're quite tall plants you need a grow light that can intense light at a distance.

Read our guide to growing tomatoes with grow lights for tried-and-tested light recommendations.

An exceptionally strong grow light can burn leaves — but this isn't a common problem. More common is excess heat from grow lights damaging your plants.

You can avoid heat problems by using energy-effecient LEDs which generate little heat. You can avoid heat build up by having an electric fan blowing across your plants.

Buying Grow Lights

Our guide to common questions about grow light purchases.

You can definitely get good grow lights on Amazon. Beware though, there's a lot of junk on Amazon that doesn't live up to the hype.

For small lights a screw-in LED grow light bulb by GE or Sansi is a good option. 

GE also make some mid-sized LED fixtures.

Sunblaster are good for fluorescent grow lights.

HLG make efficient and high quality LED grow lights.

It's possible to find a bargain. Mostly with grow lights you get what you pay for though. Many no-name brands promise great things but don't deliver.

One sign of a trustworthy brand is publishing plant-specific measurements. Look for brands that share their PPF and PPFD data — that's the data that experts look for and measure. If all the marketing info talks about lumens and watts the brand is probably trying to hide something.

And, we recommend you always buy grow lights with a proper safety certification.

Most grow lights are built for function not for looks. There are some good options for grow lights that fit into your decor though. Check out our guide to grow lights that aren't ugly.

Grow Light Costs

How much does it all cost?

At average US power costs, a high-intensity screw-in LED grow bulb would cost less than $20/year to run.

Consumed Watts x Hours/day x KWh Cost x 365

You can get a good screw-in grow light bulb for $25. Cheaper lights tend to be too low power for growing vegetables. 

One of our favourites is GE's PAR38 Grow Bulb. It has a standard screw-in base so you can put it in any convenient lamp or hanging fixture.

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