083-Gardening Indoors: The Science of Light, with Leslie Halleck

| Care, Podcast

Do you grow plants indoors? Have you tried starting plants from seed or are you thinking of trying this year? In either case, light may be on your mind lately. It’s been on mine, so I invited Leslie Halleck, author of the new book Gardening Under Lights, to join me for this week’s podcast to discuss grow lights and some of the plant science of the world of light.

Leslie is a certified professional horticulturist, earning her M.S. at Michigan State. During her graduate research, she focused on greenhouse plant production using environmental controls including lighting, temperature and photo period.

Leslie put that research to work in a number of positions in the horticultural world, but these days, she works as a writer and consultant for homeowners and commercial members of the green industry alike. When it comes to the relationship between plants and light, Leslie knows her stuff.

She has long had a love of plants. So much so that, not only does she garden outdoors, she has quite the collection of houseplants and has converted her garage – and even a few closets in her home – into indoor growing rooms.


Leslie Halleck

Leslie is a lover of plants, both indoors and out, and she certainly knows her stuff when it comes to light science and how it relates to plant health. (photo: Stacey Jeminson)


The Importance of Light

As we each developed our own love of growing things outdoors and in, we learned that light plays a big role in the health of our plants. Hosta do best in shade. Tomatoes and other edibles need at least six hours of full sun. Our prized Christmas cactus will do best near a sunny window, while our African violets prefer to retreat to a darker corner.

Yet, it’s easy to lose sight of the science – the complicated biological processes which are going on as light interacts with plants of all types. While you may not like to geek out on science like Leslie and I do, a better understanding of some basic principles can be game-changing for your success with growing indoors.

Light at the Cellular Level

Leslie describes the cells within leaves as the harvesters of energy from particles of light, known as photons. Providing sufficient light for plants really boils down to providing sufficient photons for photosynthesis – in other words to provide the energy each plant requires for various functions such as foliage growth, flowering, fruiting, setting seed, etc.

Imagine a tomato plant in a rainstorm. If there is only a soft, brief rain; the tomato plant won’t become very wet. In a heavy downpour; the tomato foliage become doused in the dense delivery of water. Why? There was a greater volume of water to reach the plant.

The same principle is true of photons. In soft, brief exposure; the tomato won’t receive many photons – while in a dense, heavy exposure; the plant will become doused in photons. Plants don’t process energy from light based on brightness – but by volume.

A heavier volume of photons will drive more photosynthesis at a faster rate. The cells of the plant photosynthesize and become light energy collectors – like nature’s own solar panels. They become charged with energy which is then converted and delivered to the parts of the plant which need that energy for various processes.

Just like humans must consume a certain number of calories each day in order to function, plants require a certain volume of photons each day. How much volume varies from species to species, growth stage, whether or not the plant is in bloom or fruit, etc.


Growing indoors

Leslie has turned her garage into an indoor growing space, including grow tents like those seen here, which allow her more control over photo period and color spectrum range. (photo: Leslie F. Halleck)


Light Spectrums

When the human eye sees a rainbow, we see that ROYGBIV spectrum – Red, Orange, Yellow, Green, Blue, Indigo, and Violet. However, there are additional spectrums of light which the human eye can’t see.

The PAR – photosynthetically active radiation – range is the spectrum of light which is useful to plants for various processes. Chlorophyll, which gives most plants their green color, is a pigment which absorbs light within the PAR spectrum. The light absorbed by chlorophyll is then converted into energy through photosynthesis.

There are many colors of light within the PAR spectrum, but not all light frequencies are visible to the human eye. Yet, each color triggers different functions in plants. Some light range colors are more useful or efficient for plant health than others.

Humans tend to perceive green light as the brightest among the various visible color ranges. Although it looks bright to us, green light is not as efficient for photosynthesis as red and blue light waves. Blue light has been found to promote foliage growth (leaves and stems), while red light promotes flowering, fruiting and root growth.

Although we don’t perceive red light or blue light as being very bright, those two ranges provide the most efficient light volume that the plant cells require to produce energy. Some grow lights are designed to make the most of those efficient wave colors and use only blue or red. “Dual band” glow lights take advantage of both colors, blending them for a purple or pink cast to generate optimal plant growth.

Plants can thrive under a combination of only red and blue light. That said, who wants those brightly colored grow lights in their living spaces? Not me, and Leslie reserves those for the enclosed environments of her garage or grow space closets too.

So, what about white grow lights, which are more common? All white light is considered “full spectrum” light. In other words, white light includes the full range of light colors within the visible spectrum and PAR spectrum – but we perceive it to be white.

White light can vary too. The colors within white light are blended in varying ratios. If you’ve ever seen a “cool” light, it tends to have a slightly blue tint. That’s because it has a higher percentage of blue light. A “warm” light has a blend with a higher percentage of red light.

Because a full spectrum light includes some ratio of the PAR spectrum, it is providing all the different ranges which will influence different things on the plant. However, the blending of all the ranges is a less-efficient system to deliver what the plant needs most – red and blue.

Think of red and blue light as a vitamin pill. For the most efficient delivery of nutrients in your body, you just take the pill, right? What if the vitamin didn’t taste good, so you decided to grind it up and add the powder to cookie dough. You would have to eat the whole batch of cookies to get the full nutrient benefit of the vitamin pill. It works, it’s just not the most efficient way to take a vitamin.

Same is true of white light. It provides the plant with the light it requires to photosynthesize, but – just like those cookies – it’s delivering so many other ranges that it’s not the most efficient light delivery. It takes more full spectrum light to provide the volume of photons which a plant requires. This is the reason many commercial growers use only red or blue or dual-range bulbs – they want maximum growth and a lower power bill.

Most of us prefer to use a full spectrum grow light in our living spaces. They aren’t as efficient for the plant and will draw more energy, but they are more palatable for our eye.


Red and blue grow lights

Red light and blue light are the two within the PAR spectrum which provide the most efficient delivery of photons to plants. (photo: Garden Supply Guys)


Measuring Light

When it comes to our outdoor plants, understanding light requirements is a relatively simple process. Plant tags or seed packets provide information on light requirements for ornamental as well as edible plants. Oftentimes, indoor plants come without any light requirement information or vague details like “low light” or “bright light.”

Adding to the challenge – what we perceive to be “bright” doesn’t necessarily provide the volume of light the plant needs. Ultimately, many indoor plants require some supplemental light to make up for whatever energy they aren’t receiving from the natural light source of our windows.

So, how do you know how much they need? First, learn a little about your plant. Identify its species and do some online research to learn more about the plant’s requirements. What are its native conditions? For example, many orchids evolved in the canopy of the rainforest. Picture that environment. Does that tell you those orchids prefer lots of direct sunlight or bright, indirect light?

Next, learn more about the environment you are providing naturally. Every person’s indoor environment is different, and since we know our eye perceives light differently than a plant, it’s a good idea to measure the light in your spaces. Just bear in mind that the amount of light in each area will change throughout the day.

Many people believe that the stronger a grow light is, the less time it needs to be on to provide enough light. This isn’t necessarily true. Also when a grow light is labelled as “efficient” – that doesn’t mean it’s providing light efficiently to the plant. That simply means it’s using less energy while it’s on.

A bulb converts energy into two different things: light and heat. The less energy-efficient a bulb is, the more heat and the less light it will produce. As energy efficiency is increased, less energy is required to produce light since less of the energy is going toward generating heat. However, that doesn’t necessarily mean the energy efficient bulb is providing more photons for your plant.

Two different types of bulbs may appear equally bright to you, but they likely aren’t producing the same amount of PAR.

Leslie uses the example of a 54 watt LED strip versus a 250 watt fluorescent bulb. The LED is more efficient than the fluorescent, however the 54 watts isn’t capable of putting out as much light as the fluorescent, so the LED would need to remain on longer to provide the same amount of light photons.

Commercial growers often use very powerful 1,000 watt grow lights. Leslie sticks with bulbs which are 300-600 watts. That said, it’s not all about the wattage. What matters is the PAR photon volume.

How do you measure PAR in your indoor spaces? Well, with a quantum flux meter, of course. We all have one of those, right? No. In fact, very few home gardeners have one of these devices. They are expensive, used only for the very specific purpose of measuring PAR and, frankly, far beyond what most of us need.

There are more commonly-available (and affordable) means of measuring your light to get a better understanding of volume in any given area or from any given lamp. These options aren’t as accurate as the quantum flux meter, but they can provide decent insight without breaking the bank.


Houseplants under grow lights

Grow lights come in many shapes and sizes. Explore the options that fit your situation, then look for good quality products. Your local nursery or hydroponics store may be able to offer some guidance too. (photo: Leslie F. Halleck)


You can purchase inexpensive light meters and can even download light meter apps on your smart device. These options measure light in footcandles or lumens, which are measurements of visual brightness – not volume of light your plant will receive.

However using those measurements, you will be able to get a general sense of the brightness of your space. In her book, Leslie has provided some conversions which can help you turn those lumen and footcandle measurements into information which can help you determine if your light is in the range of what your plants need.

Some Darkness Required

You may not know this, but many plants are just as dependent on darkness as they are on light – but only when it comes to flowering. Poinsettia is a great example. Their bracts won’t change to their characteristic red color unless the plant is provided very specific time periods of uninterrupted, complete darkness. This is the reason it can be so difficult to get your poinsettia to turn red when you’ve grown it as a houseplant all year.

Plant species require different photoperiods – hours of light – because they need the corresponding time of darkness in order to bloom. Plants referred to as “short day” plants require many hours of darkness. Plants referred to as “neutral day” plants are more tolerant of varying light and dark periods. Leslie recommends day neutral plants as a less fussy option when you are growing plants indoors. Short day plants, like poinsettias, can be challenging unless you are willing to use a grow tent to shield the plant from all light for its preferred period.

While we’re on the subject, let’s geek out a little more on terms. Have you ever heard of qualitative or quantitative photoperiod plants? What the heck is the difference?

  • Qualitative photoperiod plants – Require a specific number of light hours in order to flower at all. There are short day and long day qualitative plants. These species must receive a specific period of uninterrupted darkness to produce a flower.
  • Quantitative photo period plants – Flowering can be quickened or slowed down by changing the number of hours the plant is exposed to light. The light period requirements for the blooms of these plants are less demanding, but you can impact the timing of the flower by the amount of either light hours or dark hours you provide.

Deep stuff, right? That’s more than the average houseplant lover or seed-starting fanatic needs to know, but it can’t hurt to be familiar with the terms.

Let me point out one more time that each of these terms – short day plant, qualitative photo period, quantitative photo period, day neutral – all relate only to plant flowering. When it comes to plant growth, it’s not the number of hours of light your plant receives – it’s the volume of light.


Seedling trays

Growing seeds indoors doesn’t have to be difficult, but it will take a little trial and error to find the right balance of light volume for your situation. (photo: Leslie F. Halleck)


Seedlings and Light

If you’ve ever grown seedling indoors or considered doing so, you probably know that getting the light right plays a big role in your success. You’ve probably also seen a lot of conflicting messages regarding how many hours of light your seedlings require. Now that you understand that the hours of light (or darkness) matters only in relation to flowering – you recognize that it isn’t how long your seedlings are under light. It’s the volume of light they are receiving overall.

Seedlings require more light volume than most mature plants. As they germinate, develop true leaves and a root system, and get off to a robust start – seedlings require a lot of energy in the form of light. They are light hungry. So again, it’s not that they require a specific number of hours. They require a high volume of light energy.

If you’re growing seedlings next to a sunny window, they are receiving more natural volume of light than seedlings grown in a basement room. So, it stands to reason that seedlings grown in those very different light environments will need different volumes of supplemental light.

Like many, I used to grow seedlings under 40 watt shop lights. I switched because, the low wattage fluorescent bulbs are inefficient – meaning they convert less energy into light and more into heat than more efficient bulbs. Since they weren’t putting out a lot of volume of light, I had to keep the bulbs very close to the seedlings, so they would be exposed to as much of the light output as possible. My seedlings grew well, but the bulbs were so close to the seedlings, they were always in my way.

Last year, I decided to experiment with LED bulbs. I switched to a 300 watt full-spectrum LED and let me tell you, that was a big adjustment for me. The higher wattage meant that there was more volume of light being put out by those LEDs. Since LED bulbs are more energy efficient, they were putting out more light than the fluorescents too. The intensity of output from the LEDs was more than I realized at first.

Initially, I put the 300 watt LED bulbs much too close to the seedlings. An excessively high volume of light can cause a sunburn effect. I was watching for this and caught the problem early, increasing the distance between the bulb and the seedlings several times before I finally found the sweet spot. Those bulbs needed to be 54” above the seedlings to provide enough – but not too much – light volume.

You could use the exact same bulbs and place them at the exact same distance from your seedlings, but the length of time your bulbs will need to be on to provide sufficient light will be different than mine. Why? Because it’s not just the wattage, and it’s not just the distance. Remember that it is about overall volume. The volume of natural light my seedlings receive is unique to my space.

How do you know if your seedlings are receiving the proper volume of light? Observation, trial and error. If the seedlings begin to look bleached out or burned, the light is too close and/or they are receiving light for too long – receiving too much volume. If the seedlings become leggy – they begin to stretch for the light – move the bulbs closer and/or leave the bulbs on for a longer period of time. There is no one-size-fits-all solution.

Even once you find the right power, distance and time period; remember that the light volume requirements will change as the seedlings mature. As they grow and reach the stage when it’s time to transplant them or move them outdoors, they will reach a point when less light volume is needed. When it comes to light, more does not always mean better.

Plants reach saturation points. For example, tomato seedlings are light hungry, but a more mature tomato plant will suffer under the same volume of light as the seedling. On the other hand, a mature pepper plant will produce well when receiving the same volume of light it required as a seedling. You could continue to provide that light to the pepper, but it’s a waste of energy.

The things of nature are ever-changing. That’s what keeps gardening – and growing indoors – so interesting. It continues to challenge us.


Seedlings growing indoors

When I switched to 300-watt, full spectrum LED lights, I had to raise them to 54″ above my seedlings in order to prevent them from receiving too much volume of light.


Purchasing Grow Lights

At this point, you have probably figured out that there isn’t a right or wrong answer when it comes to purchasing grow lights for your houseplants or seedlings. There is a sea of options, and your needs are unique.

Leslie uses many different types of grow lights – LEDs, fluorescents, HID (high intensity discharge) lights, CMH (ceramic metal halide), etc. That said, a common option is a high-output grow light often referred to as HOT5. This is a good, affordable fluorescent choice.

For seed starting, you should have, at minimum, four 54-watt fluorescent tubes. Whether you purchase all-spectrum bulbs or specific color waves, like red or blue, is up to you. Don’t be afraid to experiment. Leslie recommends that, if you opt for full spectrum bulbs, look for those marked 6500 kelvin (k). Those bulbs provide more blue light, which is more efficient for foliage growth.

If you have been using fluorescent bulbs and would like to switch to more energy-efficient LED bulbs, Leslie suggests that you look for HOT5 LED retrofit bars. These LED bulbs use the same connector as the fluorescent bulbs, so they can pop into the same fixture.

Whatever type, wattage and color you opt to buy – you get what you pay for, and quality makes a difference when it comes to the grow light market. There are lots of poor quality options, particularly in the world of LED lighting. Leslie and I have both had more success purchasing directly from a supplier, rather than an online commerce site such as Amazon. Ask your local garden center of hydroponics store for advice. You can also check Leslie’s book, in which she has listed some of her suggested resources.

As these products become more sophisticated and popular, manufacturers are beginning to incorporate PAR measurements on the packaging. So, you may only need to look at the grow light label to have a better sense of the volume of supplemental light they will provide your plants and seedlings. Look for acronyms such as PPF and PPFD.

Grow lights, like all products, will degrade over time. If you replace or switch to a new grow lamp, you may need to experiment to find the proper distance to keep the new bulb in relation to your plant foliage. Leslie’s book includes a helpful table to help you determine distance based on wattage.


Tomato vines

These tomato vines benefit from the natural light volume provided by the greenhouse windows as well as supplemental volume from grow lights. (photo: Leslie F. Halleck)


When you listen to my discussion with Leslie (which you can do by scrolling to the top of the page and clicking the Play icon in the green bar under the page title), you’ll hear more on the types of edibles Leslie grows throughout the year in her Dallas-area home, along with her latest plant passion and the plans I have to experiment with light this year. I hope you’ll join us. I encourage you to check out Leslie’s book as well. It’s not big, but it’s full of so much information to help you grow more successfully indoors.

There is a lot to know about light and the nuances of indoor gardening. We’ve only just scratched the surface here, but I hope it will help get you well on your way to being a better, smarter indoor gardener. What will you be growing from seed this year?

Links & Resources

Episode 037: Starting Seeds Indoors: The Non-Negotiables for Success, Pt. 1

Episode 038: How to Start Seeds Indoors: The Non-Negotiables for Success, Pt. 2

Episode 039: How-to Start Seeds Indoors: Digging Deeper, Pt. 3

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Gardening Under Lights: The Complete Guide for Indoor Growers, by Leslie Halleck

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About Joe Lamp'l

Joe Lamp’l is the creator and “joe” behind joe gardener®. His lifetime passion and devotion to all things horticulture has led him to a long-time career as one of the country’s most recognized and trusted personalities in organic gardening and sustainability. That is most evident in his role as host and creator of Emmy Award-winning Growing a Greener World®, a national green-living lifestyle series on PBS currently broadcasting in its tenth season. When he’s not working in his large, raised bed vegetable garden, he’s likely planting or digging something up, or spending time with his family on their organic farm just north of Atlanta, GA.

0 Responses to “083-Gardening Indoors: The Science of Light, with Leslie Halleck”

  • Nancy Wu says:

    Hi Joe,I live in Northern California and will be starting vegetable seedlings indoors in February/March. Our weather has been very erratic in the past couple of years so I’m going to set up a grow light in my dining room. As noted in your podcast, since it’s our living space, I would prefer a white full spectrum LED light to maximize my limited space. The number of seedlings that I start each year seems to be about 40% to 50% of what’s shown in your photo above. You used a white LED light last season to start your seedlings and had to really experiment with the height. Can you share how many 300-watt LED lights did you use for that area in your photo? Was there a particular brand that you used that you were happy with? Did you adjust the height based on the size and age of the seedlings? Thank you in advance for sharing your experience.

  • Shelley Warner says:

    Hi Joe,I immediately reserved this book from the library, was very interesting! As you know it can be difficult to maintain the proper distance from grow lights when plants grow at different rates. This morning I had the bright idea of using my sons’ old Legos to make little platforms to raise plants up. The Legos have been in my laundry room for years. Why I didn’t think of this many moons ago, I have no idea! They are waterproof, can be customized for pot and tray size then adjusted as plants grow. I did post this info on the Facebook group but thought you might want to share this other places, like your new project or a podcast. Thanks again for all your helpful information!

  • Joe Lamp'l says:

    Great idea Shelley. That is yet another brilliant idea for Lego uses! Well done and thanks for sharing.And so glad you found the book at the library. It’s the best one I’ve seen for packing so much important information into a format that is easy to follow. Thanks for writing!

  • Joe Lamp'l says:

    Hi Nancy. I had a total of 4 300-W, full spectrum LED light setups. I ordered them on Amazon. Here’s the link:…But note the link is now showing this light as 600W. I bought 300W but don’t see that available now. So even at 300W, 54″ above the plants was plenty! Those 4 lights were super bright and even kept the plants on the ground and to the sides fully lit to the point they never showed any legginess.I kept adjusting the light distance from the plants until I arrived at the sweet spot of about 54″. Anything close and the plants were showing signs of light burn.Good luck!

  • Nancy Wu says:

    Thank you, Joe! I will get a 600-W LED to try this season with growing the seedlings and will keep it higher than 54″. Will report back in the spring on how the seedlings responded to the light and my learnings. This was a really informative episode, as with all other episodes!

  • 吕美娟 says:

    Thanks for your article, it helped me learn a lot about how to care for plants and likewise helped me choose the grow lights. I got the Bestva W200 bat light, it works great, my plants are still vibrant indoors.

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