The Ultimate Beginner’s Guide to Grow Lights – Part 1

If you are trying to figure out where to start and what to look for when trying to shop for a grow light, then you’ve come to the right place!

This week’s post is going to work hard to give you simple information to allow you to choose a grow light successfully that will work for you and your plants.

Looking for Part 2? Click here to go to that post

Table of Contents

How do you choose a light?

Disclaimer: This article will provide you the very basic information needed to understand grow lights while shopping for them. I would not call this guide comprehensive because there is SO MUCH information available about grow lights.

If you want a very comprehensive source for purchasing and using grow lights, I highly recommend the book Gardening Under Lights, by Leslie Halleck. (linked to Amazon)

This book goes way beyond what I am able to include in this post and even what I am able to fully understand about the science of grow lights.

When choosing a grow light we will consider 5 key factors and cover how to determine what we want each of them to be in our setup.

The 5 factors of choosing a grow light

  1. The color of the light
  2. The intensity of the light
  3. How large of a space that light can support
  4. How far away the light should be from your plants
  5. The length of time the light will be on

This week is part 1 of 2 in the beginner’s guide to grow lights.

In part 1 we will cover:

  • Why people want to use grow lights
  • Why light is so important for your plants
  • How to determine what color you want your grow light to be
  • How to determine what intensity you want your grow light to be

In part 2 (next week) we will cover:

  • How to determine how much space or how many plants your grow lights can support
  • How to determine how far your grow light should be from your plants
  • How to choose the length of time your grow lights should be on

Why do people want grow lights?

People decide to purchase grow lights for a few different reasons.

Some people need to supplement the light that their plants are receiving because it isn’t quite enough for the plants to grow and thrive at their best.

Others purchase grow lights to turn a dark corner or a dark room in their home into a place where plants can successfully live. These people need more intense lighting because the plants would be getting no light without artificial sources.

Others want to setup a station to grow seedlings and need more intense light than their windows offer to ensure the seedlings grow strong and healthy.

Why is light so important for your plants?

Plants eat and nourish their bodies through a process called photosynthesis.

What fuels the photosynthetic process? Light!

Photosynthesis comes from the Greek words, “photo” and “synthesis.”

Photo translates to ‘light’.

Synthesis translates to ‘put together.’

In other words, photosynthesis means to use the energy from light to create sugar the plant can use for energy to grow and thrive.

What does a plant use to create that sugar? Water and carbon dioxide.

Plants take up water through their roots.

They absorb carbon dioxide through tiny pores in their leaves called stomata or stoma.

Through the process of creating sugar/glucose, extra oxygen atoms are leftover and released, providing oxygen for us to breathe!

For those who like chemistry, here is an equation that shows how carbon dioxide (CO2) and water (H2O) combine to create glucose (C6H12O6) and oxygen (O2).

1. How to determine what color you want your grow light to be

All of the light colors we can see with our eyes are used by plants during photosynthesis.

What colors are those? Violet, Blue, Green, Yellow, Orange, and Red – as represented below.

The numbers in the diagram above are wavelength measurements using the unit, nanometer (nm).

Each of the colors are different wavelengths.

Light in the range from 400 nm (violet) to 700 nm (red) is referred to as PAR.

PAR (Photosynthetically Active Radiation) is the spectrum of light plants use for photosynthesis.

Full Spectrum Lights

When all of the colors in the visible spectrum are combined, the light appears white to our eyes.

White light, like the light from the sun, is called Full Spectrum, because it is made up of all the colors in the spectrum.

Note: There is a common misperception that plants do not use green light. This has been proven to be untrue. Scientists have found that plants are able to utilize green as well as the other colors.

File:Aspect Black 20W Growlight over Bonsai.jpg
Photo by: Hotmilk400, “Ficus plant grown under a white LED grow light.”, Source

Why do people use Red and Blue Lights?

While many growers use Full Spectrum lights because they offer all of the colors to plants while being a pleasing color for the grower, there are also many that use lights that only offer Red and Blue spectrums.

These Red and Blue Lights often appear purple or pinkish to our eyes and are not very pleasing to look at.

Photosynthesis can be optimized and still effective when using only red and blue light.

Red light supports foliage growth and flower production.

Blue light supports health growth without stretching.

File:Led grown lights useful.jpg
Photo by Sunshine 117, “LED grow lights with two potted plants”, Source

How we can use Kelvin to understand the color of a grow light

Grow lights often have a Kelvin rating on the package, which describes the color of the bulb.

Full Spectrum bulbs with a 5500 Kelvin rating or higher is considered blue light skewed bulb. It may appear white to us, but will be on the cooler, bluer side.

Full Spectrum bulbs with a 3000 Kelvin rating or lower is considered a red light skewed bulb. It may appear white to us, but will be on the warmer, more yellowy side.

Full Spectrum bulbs with a 4000 to 4800 Kelvin rating is a more balanced bulb, neither skewing blue or red dramatically.

Why does Kelvin matter?

Blue skewed bulbs will be more supportive of foliage growth and healthy, stocky plants.

Red skewed bulbs will be more supportive of flower productive, some foliage growth, but not as good at preventing stretching.

Depending on the types of plants you are growing, you can use this information to determine whether you want your bulb to skew in a certain direction.

For example, if I’m growing lettuce to eat, I want healthy foliage production and I don’t care about flower and seed production. As such, I can happily purchase a bulb that is skewed to the blue spectrum, or a bulb with a Kelvin rating of 5500+.

Super Simple Summary

– Plants use all the colors of light that we can see
– All the colors of light combined appear white to us and are referred to as Full Spectrum
– Grow Lights are available as Full Spectrum and as Blue and Red (which often looks like a pink or purple light to us)
– People sometimes use blue and red because those two colors are the most efficient wavelengths supporting photosynthesis
-Kelvin ratings are provided on grow lights to describe the color of the bulb
———- 5500+ Kelvin bulbs lean in the blue direction
———- 3000- Kelvin bulbs lean in the red direction
———- 4000 to 4800 Kelvin bulbs are well-balanced and do not skew much in either direction

2. How to determine what intensity you want your grow light to be

The next consideration when deciding on a grow light is how much light you need for your plants to thrive.

Is light brightness a way to know how much light a plant will receive?

Unfortunately how bright a lamp appears to our eyes is not a useful indicator for knowing how much useable light a plant will receive from a light source.

Lumens is a measurement used to understand the brightness of a bulb as humans perceive it.

Knowing the lumens of a specific bulb is not an accurate way to ensure your plants are receiving enough light.

Instead of brightness, plants are concerned with how many photons of light their leaves can absorb each day.

On page 18 in Gardening Under Lights, Leslie Halleck advises us to think of photons as calories that plants need to gain energy, “just as our bodies burn energy delivered from calories in food.”

One of our jobs is to ensure our plants are receiving enough photons/calories.

How to measure footcandles to help determine whether your plants are receiving enough light

Footcandle (fc) is a unit of measurement that provides us clues about whether or not our lamp is able to deliver enough photons of light to our plants.

We can use a light meter to determine how much light our plant is receiving in its current location.

I will link below an inexpensive option that is easy to use and can be set to measure footcandles.

You can use the guide below to determine how much light an area in your home is receiving.

Low Light: 75 to 200 footcandles
Medium Light: 200 – 500 footcandles
High Light: 500-1000 footcandles
Very High Light: at least 1000 footcandles

Even 1000 footcandles may not be enough for some plants. Full sun plants, like many of our vegetable garden plants, need closer to 10,000 footcandles to remain healthy.

Seedlings are also light-hungry plants. They need all that light to grow and develop in a healthy way. Think of these seedlings as human babies. Babies need to eat every couple hours to sustain their bodies because they are growing and developing so rapidly. So does your seedlings!

Dr. Meter Light Meter linked to Amazon

Knowing how many footcandles your plant is receiving is useful for deciding whether you need to add supplemental light or if your grow light is providing enough light. It may not be helpful when trying to decide on a light to purchase.

How to use the number of watts a light uses to understand light intensity

How many watts a light bulb uses tells you how much electricity/energy that light bulb needs to function. Higher wattage means more energy and, therefore, higher electricity costs.

It can also give you some info about how powerful your grow light is.

Knowing how many Watts a grow light needs can tell you 2 things:

  1. How much energy or electricity the grow light needs to work (how expensive the light will be to run)
  2. How much potential energy the grow light can convert into usable light for plants (how powerful the light may be)

Why do I say that watts only tells us the potential energy a grow light can convert into usable light?

Because the electricity a lamp draws isn’t only converted into usable light. It is also converted into heat.

The amount of heat your lamp will produce will depend on what type of lamp you choose.

What types of grow lights are there?

There are quite a few types of grow lights. We are going to cover 3 types here briefly. Those 3 are: fluorescent lights, compact fluorescent lights, and LED lights.

Fluorescent Lights

  • Relatively inexpensive
  • Pretty efficient: does produce some heat, but not a huge amount
  • Most efficient option of flourescent bulbs are those labeled “HO,” which means High Output
  • T5 HO bulbs are a great option for beginner gardeners
  • Is typically full spectrum, but often leans in the blue spectrum direction

Compact Fluorescent Lights

  • Very efficient and effective lights that are inexpensive
  • Does not cover a lot of plants or planted space; good for lighting small spaces or a single plant
  • Similar to fluorescent tube lights, compact fluorescent lights tend to skew in the blue direction
  • Unless the bulb has a very high wattage, it can be screwed into any standard light fixture

LED Lights

  • Highest upfront cost when purchasing lights for a large amount of plants and growing space
  • Lowest ongoing maintenance cost because they produce very little heat and require less energy to produce a good deal of light
  • Smaller LED lights are not as cost prohibitive upfront and still provide the energy savings
  • LEDs are debated among hobbyists. Some feel like they are not as effective at supporting plant life as the other options. Other feel they are equally capable of growing healthy plants.
File:Fluorescent-grow-lights.jpg
Fluorescent grow light, Photo by: Dennis Brown, Source Link
File:CFL-grow-light-dual-spectrum.jpg
Compact Fluorescent Bulb, Photo by: Adamantios, Source Link
File:Aspect Black 20W Growlight over Bonsai.jpg
LED Pendant Light, Photo by: Hotmilk400, “Ficus plant grown under a white LED grow light.”, Source

Are there more accurate measurements of usable light?

Yes, but it does get more complicated!

PPF, or Photosynthetic Photon Flux, is a measurement of usable light being produced by a grow light each second.

PPF is measured in mols per second per Watt or μmols/s/Watt.

PPFD (Photosynthetic Photon Flux Density) tells you how much usable light is delivered at a specific distance from the light source.

PPFD is measured in micromoles per square meter per second (μmol/m2/s).

It is telling us how much PPF is reaching a specific surface area (a square meter).

In other words, the farther your plants are from the light source, the less usable light they will receive.

PPFD allows you to calculate exactly how the quantity of light changes over specific distances and spaces.

Many serious and high quality grow light manufacturers will include a PPF rating in the product specifications.

For example, the GE BR30 LED Grow Lights for Indoor Plants, Full Spectrum, 9-Watt Grow Light Bulb has a rating of 16 μmols/s/Watt (linked to Amazon through the Affiliate Program)

Whereas, the GE Grow Light Bulb, PAR38 Grow Light Bulb for Indoor Plants, Full Spectrum, 32-Watt has a rating of 50 μmols/s/Watt. It is a more powerful bulb.

Important note: if you decide to try this bulb, please know it is EXTREMELY heavy and requires a weighted lamp or a ceiling-mounted lamp to support its weight. It will topple lightweight lamps right over.

How much PPFD do your plants need?

To answer this question, I’m going to share information that one of my favorite bloggers and youtubers put together for his orchids and houseplants, because I’m at the beginning stages of understanding PPFD myself!

Very Low-Light Houseplants (Begonia pavonina)
20–40 umol/m2/s PPFD (20 umol/ft2/s)

Low-Light Plants (African Violets, Tropical Begonias)
40–80 umol/m2/s PPFD (20 umol/ft2/s)

Moderate-Light Houseplants (Low-light Aroids like Anthuriums and Pothos)
80–150 umol/m2/s PPFD (30-40 umol/ft2/s)

High-Light Houseplants (Tropical Succulents like Christmas Cactus, Philodendrons, Monstera)
150–350 umol/m2/s PPFD (50-100 umol/ft2/s)

Very High-Light Houseplants (Tropical Trees like Chocolate, Citrus, etc)
350–600 umol/m2/s PPFD (50-100 umol/ft2/s)

Full-Sun Plants (Cannabis, Fruiting Trees/Shrubs, Eucalyptus, Desert Cacti)
600–2,200 umol/m2/s PPFD”
__________

Information shared in “Light Recommendations: PPFD (PAR) for Orchids and Houseplants,” by Dustin from herebutnot.com, Direct Link to Blog Post Here

Can you measure PPFD yourself?

Yes, you can use a quantum flux meter to measure PPFD.

I will link one below for you to check out. I don’t own one at this time because they are a bit of an investment that I would need to save up for. Maybe one day! 🙂

Important Note: A huge component of choosing a grow light is whether you need your grow light to supplement the amount of light your windows are providing or whether you need your grow light to provide enough light to support the plant completely via artificial light.

Providing all the light a plant needs to stay healthy and to grow is going to be a lot more than the amount needed to supplement an already-lit area.

If you are supplementing and aren’t sure how much light your plants are getting, check out the section above on footcandles and how to use an inexpensive tool to determine the amount of light your plants are already receiving. This will help to give you an idea of how much light you would need a grow light to emitt.

Super Simple Summary

– Light brightness (# of Lumens) is not an accurate way to judge a grow light

– Measuring footcandles is one way to understand how much light your plant is receiving
———- You can use an inexpensive light meter to measure the footcandles in a window or under a grow light

– Measuring wattage is another way to understand light intensity and efficiency
———- The # of Watts tells you how much electricity the bulb needs to function
———- This electricity / energy is converted into both light for your plants and heat
———- Grow Lights are considered more efficient when they minimize energy being converted into heat and maximize energy being converted into usable light
———- The most energy efficient lights are LEDS which convert most of their wattage into light and produce very little heat. They are also the most expensive upfront.

– The most accurate way to know how much light your plants will receive from a grow light is to find out the PPFD specs
———- PPFD tells us the amount of actual light that is reaching a plant and is within the usable spectrum
———- Quality grow light companies often include the PPF amount on the label of their products

Stay tuned for Part 2, coming next week!

In part 2, we will cover:

  • How to determine how much space or how many plants your grow lights can support
  • How to determine how far your grow light should be from your plants
  • How to choose the length of time your grow lights should be on

2 Comments

  1. Valda Kirwan

    Wonderful article on a ridiculously complex topic made more-so by seller’s lack of uniformity.

    Reply
    • Colleen

      Thank you so much. You are so right about the lack of consistency. It makes choosing a light so difficult because what you are provided and what you need are often mismatched!!

      Reply

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