Gardening Under Lights

The Complete Guide for Indoor Growers


By Leslie F. Halleck

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“If you want to grow plants indoors, you need this book.” —Niki Jabbour, author and staff writer at

Gardening Under Lights is a highly-detailed, accessible guide for seed starters, plant collectors, houseplant fans, and anyone who wants to successfully garden indoors any time of the year. You’ll learn the basics of photosynthesis, the science of light, how to accurately measure how much light a plant needs, and details about the most up-to-date tools and gear available. Also included are tips and techniques for helping ornamental plants (like orchids, succulents, bonsai, and more) and edible plants (arugula, cannabis, oregano, tomatoes, and more) thrive indoors. Whether you are a vegetable gardener who wants to extend the growing season, a balcony gardener short on outdoor space, or a specialty plant collector, Gardening Under Lights is a must-have.



Being obsessed with plants by the time I was 18 years old didn’t exactly make me popular at parties. As my original plant friend, Carolyn, will tell you, the two of us could clear a room quickly once the surrounding college partygoers heard us talking about our cactus or cross-pollination. We just couldn’t help ourselves, and we’re still at it today. However, my status at The University of North Texas as one of only two students (at the time) concentrating in botany, my curious reputation as a gardener, and my college job as a garden-center employee made my phone ring a bit more frequently than my social status warranted. If only I had known I could have made some cash telling anonymous callers how to stop killing their closet so-called tomato plants. But I was never interested in what was actually growing in their closets. I was content to be knee-deep in my outdoor ornamental and vegetable garden, not to mention obsessed with an increasingly large collection of houseplants. But I’m glad those closet gardeners called, because it was the start of my horticultural consulting career—and, as it turns out, this book.

I spent my first two years of university life at UNT as an art major before I switched to biology and botany. As such, aesthetic considerations are infused into all my pursuits, even the scientific ones. The fusion of art and horticulture is natural. Growing plants and food indoors doesn’t have to be utilitarian; it can be a beautiful practice that blends into our living space and lifestyles. Within this book you’ll find examples and inspiration for your own attractive plant lighting.

As a graduate student at Michigan State University, my research focused on greenhouse production and flowering of perennial plants. Therefore, you will also encounter some science and math, which may seem a bit confounding at first. If you don’t need this information, feel free to skip it. If you have intensive indoor gardening goals, however, the more in-depth how-tos on measuring and calculating your indoor lighting needs will likely form the basis of your long-term success.

As it turns out, writing this book feels like I’m coming full circle to bring the closet garden to light. I hope this book encourages your interest in, and creates new possibilities for, growing plants where you once thought you could not. Perhaps you’re on a mission to grow more of your own food or medicinals, even if all you have is a kitchen counter, a guest room corner, or a small closet. You may want to extend your vegetable-gardening season by getting a jump-start on propagation or growing indoors off-season. Having control over your own food source is a powerful feeling. It’s good to eat fresh, hyperlocal, and clean. Or maybe, like me all those years ago, you have caught the plant-collecting bug and there just aren’t enough windowsills left in your home to feed your growing plant family. In any case, you’ve come to the right place.

Some plants, such as haworthias, can survive in a windowsill.


My window is bright enough, right? New indoor growers ask this hopeful question all too frequently. While many books and websites insist that you need only a bright window to get your tomato seeds going or your orchids reblooming, this approach can result in a lot of disappointment.

Ambient light levels inside your home are significantly lower in intensity and ultimately different in spectrum than natural outdoor light, especially during the winter months. Even a seemingly bright window may not provide enough light, or the right kind, for young seedlings or fruit-producing plants. Have you ever started seeds indoors in a sunny window, only to watch the tiny seedlings lean so far that they topple over? Perhaps you’ve tried some beautiful succulents and watched them do the same. Your plants are starving for, and stretching toward, the light.

There are plenty of low-light tropicals and blooming plants that you can grow successfully indoors with good ambient light, and you can maintain certain light-loving succulents for a while in a windowsill. But even a bright windowsill is typically not the right location for plants you intend to harvest for food. The same goes for heavy-blooming plants. Reproduction is an energy-intensive process. Producing flowers, fruit, and seed requires a lot of juice. Plants need enough light, and the right kind of light, to get the job done.

You may have learned that in your outdoor garden, plants that produce large fruit, such as tomatoes, require roughly double the duration and intensity of direct sunlight as plants that produce mostly foliage, such as leafy greens and lettuce. A plant’s heritage—that is, the geographical area where it naturally evolved—dictates its requirements for specific light intensity, duration, and spectrum to reproduce successfully. If you are going to invest time, effort, and money in indoor growing, quality supplemental lighting should be your number-one priority.

Before we draw up your shopping list of types of lamps and gear you should be using indoors, or determining how many lamps you will need for different types of plants, we must understand how plants use light; how to distinguish light quality, quantity, and duration; and how to measure each. Artificial lighting is complex, and there is a lot of misinformation in the marketplace, which can ultimately lead to wasted resources. To make good grow-lighting choices and produce better yields, it is necessary to grasp some basic botany and light science.

If you already garden outside, growing plants indoors under lights is a great way to supplement your efforts and extend your seasons and yields. If gardening indoors is your only option, grow lighting can transform your living and eating experiences and bring some much-needed nature into your home.

Many plants will need extra light indoors.



Understanding how plants use light is crucial to learning how to grow them successfully, especially indoors. A bit of Botany 101 is a good starting place for all plant enthusiasts.


Photosynthesis is a reaction to a transfer of energy. Think of a plant’s leaves and green stems as light-energy collectors; nature’s very own solar panels. Plant appendages (and several types of microorganisms) use photosynthesis to convert radiant light energy into chemical energy. Light hits the surface of a leaf or a green stem, and specific cells convert the light energy into sugars. These sugars move around the plant, driving various biological functions. A major by-product of photosynthesis is oxygen; hence, we breathe.

When growing plants indoors, your goal should be stimulating and enabling successful photosynthesis. The amount and type of light you provide your plants will ultimately determine their success or failure. Depending on where you live and the time of year, the amount of sunlight penetrating your living room window or filling up your glass greenhouse likely won’t be enough to grow many types of plants in an enclosed environment.

Photosynthesis occurs only in the green portions of plants, such as stems and leaves. More specifically, it takes place within the chloroplasts of those plant parts. Chloroplasts are tiny structures inside a plant’s stem and leaf cells, like a cell within a cell. Chloroplasts serve as the plant’s kitchen and pantry, as they create and store all the needed pigments and food.

Chloroplasts are most likely alien to original plant physiology. Much like our own cellular mitochondria, the organelles we refer to as our cells’ powerhouses, chloroplasts were once likely autonomous organisms or bacteria found in the environment—until another organism absorbed them. This organism responded positively to the energy created by its new captive, and the two coevolved. Evolutionary theorist Lynn Margulis designed this idea of a beautiful, codependent, mutually beneficial relationship that is commonly referred to as the endosymbiosis theory.

This philodendron leaf turns light into plant energy.

Chloroplasts are the structures in plant cells responsible for photosynthesis.


When light reaches a chloroplast, chlorophyll absorbs the pigment inside the chloroplast. What makes plants special is their ability to use this chlorophyll pigment to convert light into sugars to use as energy. On the light spectrum, chlorophyll absorbs and employs more red and blue light, leaving more of the green light to bounce back to the human eye. This phenomenon results in the green-colored appearance of most plants.

Biological pigments create the rich colors of this begonia foliage.

Two types of chlorophyll are involved in photosynthesis: Chlorophyll A and Chlorophyll B. Chlorophyll A absorbs most of the usable light. Chlorophyll B is a yellow pigment that plays a supporting role by absorbing mostly blue light and transferring it to Chlorophyll A.

Carotenoids, flavonoids, and betalins are additional support pigments—sporting shades of yellow, orange, red, pink, and purple—that also absorb small amounts of light. These pigments are responsible for different colors throughout plant structures. As chlorophyll pigments break down in the autumn months in response to temperature and daylight changes, these carotenoid pigments become visible, resulting in the much-anticipated fall foliage show.

Photosynthesis is the process by which plants use light to generate their own food.

While light is the key trigger and engine of photosynthesis, adequate amounts of water and carbon dioxide are also necessary to the process. For the plant’s powerhouses to operate properly, they need carbon dioxide from the air, water from the soil, and light energy from the sun—each in the right amount. If you restrict or eliminate any of these main ingredients, photosynthesis can be impaired or stop altogether.

During the first step of photosynthesis, when light is available, water molecules are split apart. When chlorophyll absorbs light it becomes charged, like a battery, which gives it the ability to split two water molecules (2H2O) into four electrons, four protons, and two oxygen atoms that combine to form O2, or oxygen gas. This is why we characterize plants as breathing in carbon dioxide and exhaling oxygen. There is a common misperception that plants convert carbon dioxide into breathable oxygen, but the water molecules supply the oxygen gas that plants release back into the atmosphere. The electrons and protons that remain are then stored in proteins within the cell and combined with carbon dioxide in the second step of photosynthesis to form the carbohydrates the plant burns for fuel.

This plant ran out of resources and now it’s toast.


While respiration in humans is primarily an exchange of gases in the form of breathing, it is not the same in plants. The primary action of plant respiration is burning sugars generated from photosynthesis to drive growth and development. Basically, respiration is how plants burn calories. While photosynthesis can take place only when there is light, respiration occurs continuously in a plant, just like humans burn calories all the time. Your plants will burn energy even if there is not enough light, water, or carbon dioxide available. If those inputs remain limited or cease, eventually your plant will burn more energy than it can generate. This results in dormancy or death.

While plants can generate usable energy within their own bodies, they still rely heavily on many environmental conditions and inputs for photosynthesis to work as intended. When you grow plants indoors in an artificial environment, they become completely dependent on you to create favorable growing conditions and provide them with the right ingredients they need to thrive. As any good baker will tell you, finding the right balance of ingredients and technique takes practice, and every oven functions a bit differently. You will probably flatten a few soufflés—and kill a bunch of plants—before you learn to get it just right.


Understanding how plants see light is the first step in making the right grow-lighting choices. While humans qualify light in terms of visual brightness, plants qualify it in terms of wavelengths, or spectrum.


Not all light is equal. Different types of light both drive and limit photosynthesis, change plant morphology, and influence flowering.

Photosynthetically Active Radiation

When a beam of white light hits a glass prism at an angle, it is then split into different wavelengths of color: violet, blue, green, yellow, orange, and red light. Each of these colors of light measures a different wavelength, falling between 400 nanometers (nm) (violet) to approximately 735 nm (red). This range of visible light is also the range used to fuel photosynthesis. This range of spectrum is known as Photosynthetically Active Radiation (PAR).

In the process of photosynthesis, the red and blue light spectrums most efficiently drive carbohydrate production in plant cells, but all PAR in the 400 to 735 nm range is useful for photosynthesis. PAR is not a measurement of quantity of light, but rather the quality of light. PAR tells you the color spectrum of light it delivers that your plants can use for photosynthesis.

PAR is made up of light particles, or photons, that will eventually strike a leaf’s surface. The leaf absorbs these photons in quanta. One photon equals one quantum of light.

You can think of light photons as having calories that plants use for energy, just as our bodies burn the energy delivered from calories in food. Different foods offer different amounts of calories, and so do different wavelengths of light. Light with a short wavelength and high energy (blue) delivers more calories to a plant than light with a longer, lower-energy wavelength (red).

A prism splits full-spectrum light into separate color spectrums.

Plants use light that falls into the 400 to 735 nm range for photosynthesis.

Therefore, lamps that emit only blue light, with its shorter high-energy wavelength, are the ticket to tons of giant tomatoes, right? Not so fast. Plants need only a small percentage of their light delivered in the blue spectrum to be able to grow and function well. Red light, despite its longer wavelength and lower energy value, is more efficient at driving photosynthesis than blue light. Cells, structures, and pigments (called cryptochromes) that are not involved in photosynthesis also absorb blue light and use it for other operations, such as opening and closing stomata, which are pores in plant leaves that allow for the exchange of water and gases. But only the plant’s chloroplasts that contain the chlorophyll, the pigment responsible for driving photosynthesis, absorb the red light photons. While red light may deliver less energy, the plant uses much more of it efficiently to fuel photosynthesis and produce sugars.

This light-emitting diode (LED) lamp mixes only blue and red light, resulting in a pink-colored light in the growing area.

The spectrum of light an outdoor garden receives will vary according to geographical location. Sunlight that reaches areas north of the Fortieth Parallel contains more blue light, while sunlight at the equator delivers more red. The atmosphere absorbs light at different levels in each area.

You may have learned that plants don’t use any green light, that it all bounces off the plant and makes the plant looks green to the human eye. That is partly true, but it is a myth that plants do not absorb any green light photons or do not use them for photosynthesis. In fact, scientists currently believe that most of the green light spectrum is useful to plants for photosynthesis. It may be that plants have simply figured out a more efficient way to use green light photons, and absorb smaller amounts of it to get the job done. Studies have shown that green light is involved with seedling and leaf development, flower initiation, and plants’ use of carbon dioxide and water, and even plays a role in stem growth and height. Green light is not as energy efficient to deliver as red or blue light, but it is easier on the human eye. Green light makes it easier to spot problems such as disease issues or nutrient deficiencies because plants appear their natural color. Green light can also penetrate the leaf canopy more easily, meaning it can reach the lower leaves better than other colors of light. The addition of some green light could help keep the lower leaves on your plants photosynthesizing instead of dying and dropping off.

A tomato plant growing exclusively under yellow-colored light emitted from a high-pressure sodium lamp.

A tomato plant growing under white-colored light emitted from a compact fluorescent lamp.


Light that plants can use and humans can see is only a part of all the electromagnetic radiation that surrounds us. Wavelengths that measure below violet light, 100 to 400 nm, are invisible to the human eye and referred to as ultraviolet (UV) radiation. UV radiation does not provide much heat, but it can damage living organisms at the cellular level. The oxygen and ozone in the atmosphere absorbs most UV radiation, which is why ozone is so important.

Plants do not use UV radiation for photosynthesis, and it can cause cellular damage and plant death, but there are some benefits to providing small amounts. Cryptochromes absorb several wavelengths of UV radiation for various beneficial functions. UV damage to plants stimulates them to produce protective antioxidants, resins, oils, and other chemicals that give them flavor. UV light can also toughen up young seedlings so they can more easily transition to higher-intensity lighting without experiencing shock. Some grow lamps include the UV spectrum to boost nutritional value and flavor in edible crops.

If you use grow lamps that emit UV light and you’ll be working under or around them, be sure to take the same precautions you would if you were spending time in the sun, such as donning protective clothing and UV-blocking sunglasses.

UV and infrared light fall outside the visible PAR spectrum.


Radiation wavelengths that fall just above the red light spectrum are called infrared (IR), and this radiation is also not visible to humans. Yet IR radiation makes up about half the solar energy that hits the earth’s surface. You can’t see IR light, but you feel it as heat. Plants do not use IR radiation for photosynthesis, but it regulates other crucial growth and developmental changes in plants, known as photomorphogenesis. Some lighting systems emit IR at the end of a growing cycle to speed up plant growth and improve blooming.

Plant Biologically Active Radiation

Green plants do not use UV or IR light for photosynthesis, nor do they use radio waves or X-rays, wavelengths even further away on the light spectrum. But these types of nonvisible light do interact with other plant photopigments besides chlorophyll, and they are involved in important biological processes beyond photosynthesis. This wider range of spectrum between 350 and 800 nm is known as Plant Biologically Active Radiation (PBAR), but there are likely biological plant responses within a much larger range of 100 to 1100 nm.


Overall plant growth and development depends not only on the spectrum of light received, but also on the color combination, color sequence, and duration of each. Plants have evolved to employ finely tuned sensors to use different spectrums of light for different stages of growth and reproduction. Plants use light to regulate developmental stages (such as germination, rooting, stem elongation, leaf unrolling, flowering, and dormant bud development), which is known as photomorphogenesis.

Telling Time

For a plant to grow and bloom on schedule and in the right season, it must be able to tell time. A series of pigments and hormones regulates the time-telling function of photomorphogenesis. Plants produce chemical pigments, called phytochromes, that act as triggers. A blue pigment phytochrome, PR, absorbs and responds to red light. When PR is exposed to red light during the day, it converts into a secondary form of pigment called PFR. When PFR is present in the plant, it tells the plant to produce short, thick stems and determines its overall shape. The presence of PFR is also required to trigger flowering signals. PFR absorbs and responds to IR light over the course of the night. Plants use IR light to tell when it is night, and to determine how much uninterrupted darkness has occurred. In darkness with IR radiation, over time PFR will naturally convert back to PR. This cycle is comparable to the circadian rhythms that help our bodies know when it is time to sleep and rise. The balance of the two forms of phytochrome helps your plant develop properly and on schedule.

Some plant seeds do not germinate until they are exposed to red light. If you have ever tried to grow lettuce from seed, only to be disappointed when the seeds did not sprout, you most likely covered the seeds with soil and blocked them from the light.

PR converts to PFR throughout the day as the plant is exposed to red light. In the dark, when plants can sense only infrared light, PFR converts back to PR.


When plants stretch or grow toward the sun, also known as tropism, they are reaching for more blue light. When sun-loving plants grow in too much shade, their growth slows down and their internode length elongates. The plants stretch to avoid the shade so they can compete with surrounding plants and reach more light. This internode elongation occurs when the quantity of available light is reduced and the spectrum of available light changes. When blue light is limited or blocked (shade trees block out more blue light than red), it triggers this shade-avoidance stretching.

This arrowhead plant, which sits about 5 feet away from a bright north-facing window, is stretching to reach more light.

Some seeds, such as lettuce, require exposure to light to germinate.

This LED lamp emits only blue and red nanometer specific light.

Red vs. Blue Light

Light-emitting diode (LED) grow-light technology allows you to narrow the spectrum of light you provide to just one color, referred to as nanometer specific light. You can purchase LED grow lamps that emit only a red spectrum of light or a blue spectrum of light (as well as other individual colors, such as orange and green). You can drive photosynthesis efficiently by using only blue and red light together.

While you can use red and blue light exclusively and successfully, only a few species of plants are able to grow well under only red or blue light indefinitely.

Blue light helps control excessive stem elongation, or stretching toward light. It influences how chloroplasts move around in plant cells and helps regulate stomatal opening. Blue light also increases antioxidant levels in crops such as lettuce. You can grow some crops (short-lived ones such as microgreens) using only blue light, which supports ongoing vegetative growth, but don’t expect any flowers in certain types of plants. Over the long term without any red light, however, plant leaves may eventually develop too small or become deformed, reducing photosynthesis and other functions.

Oedema has become an issue on these pepper plants grown exclusively under high-pressure sodium lamps.

When you grow plants under only red light, they can put on additional leafy growth, or biomass. This is good for certain crops, such as lettuce. But as plants develop more leaves and other structures, they may not transpire properly or they can stretch, get too tall, or develop oedema (leaf blisters—a common issue in tomatoes grown under only red light) or other problems. If you grow your plants under only red light for too long, chlorophyll production can stop altogether, causing photosynthesis to cease. Plants may even flower too early under only red light in a final effort to reproduce before inevitable death.

You can grow certain quick-turn crops for short periods of time using single-band red or blue LED lamps. For example, if you’re growing lettuce plants in an enclosed space with artificial lighting, you can start them off using only red light, which causes the leafy greens to grow larger leaves more quickly. However, once the young plants begin to put on more growth, you will need to add 10 to 20 percent blue light to keep them from stretching.

While a little stretching should not concern you, too much can result in weak seedlings that topple over or puny stalks that cannot support flowers or fruits. If your plants are stretching too much, they are not getting enough overall light or they need more blue light.

However, if you want to graft your plants, you may want to encourage them to grow overly elongated stems. Crops such as tomatoes and cucumbers are often grafted onto hardier rootstock, and red and IR light are used to elongate the seedlings to make the grafting process easier. Another scenario for elongated stems is when you are readying a plant, such as cannabis, to flower. If you’ve been growing it under mostly cool blue light to encourage dense plants, you can then expose it to red light and IR radiation to begin elongation to make room for large flower buds.

Seedlings growing under full-spectrum fluorescent lamps.

Mixing and Matching Light

Switching between color spectrums and types of lamps can help you influence plant-growth habit and trigger different stages of development. You can also mix and match different spectrums of light.


  • “This scientifically grounded yet accessible book is an essential guide to just about anything we might want to grow indoors.” —Kelly Norris, director of horticulture and education at Greater Des Moines Botanical Garden

    “This book nails it! Everything from breathtaking yet practical ways to grow your own food to basic starts for your indoor or outdoor garden.” —Bren Haas, founder of Creative Living with Bren Haas and host of GardenChat

    “If you want to grow plants indoors, you need this book.” —Niki Jabbour, author and host of “The Weekend Gardener”

    “Redefines how we approach growing plants in any indoor environment.” —Brie Arthur, author and horticulturist

    “If you are a plant enthusiast, this is a must-read and will become an indispensable resource.” —Shawna Coronado, author, gardener, and wellness lifestyle advocate

    “Halleck writes engagingly for those with more than a passing interest in indoor gardening.” —Booklist

    Gardening Under Lights is a wonderful introduction to growing indoors. . .  appropriate both for readers who want to know what lights to find to make a few more houseplants happy, and for readers who envision a more industrial production, Gardening Under Lights is a timely and very useful new book.” —NYBG’s Plant Talk 

    “Explains all you need to know to grow almost anything indoors under lights. . . . Halleck will tell you what you need to be successful at indoor gardening.” —Garden Design

    “Get ready for knowledge bomb after knowledge bomb in this one, plant friends.” —Bloom and Grow Radio 

    “The definitive primer on indoor gardening.” —The Designer

    “Whatever your skill level, Leslie’s bright ideas will light up your indoor garden—no sun required.” —Grower Talks

    “If you are at all interested in gardening indoors, then this book is for you.” —The Professional Gardener Magazine

    “Perhaps the hardest subject for a garden enthusiast to tackle is indoor lighting. . . . Into the darkness of my ignorance has come Gardening Under Lights. . . . This book is the answer to the prayers of gardeners everywhere.” —Southwest Gardening

    “A really welcome book for those of us who haven't fully dipped our toes into the confusing world of artificial light for our plants.” —On the Ledge

    “Halleck’s book walks readers through the basics and turns potentially confusing concepts into common knowledge. . . Gardening Under Lights doesn’t just tell you how to have an aesthetically pleasing and successful indoor garden—it shows you.” —Garden Center Magazine

    “A detailed, accessible guide for seed starters, plant collectors, houseplant fans, and anyone who wants to successfully garden indoors any time of the year.” Michigan Gardener

On Sale
Jul 10, 2018
Page Count
248 pages
Timber Press

Leslie F. Halleck

Leslie F. Halleck

About the Author

Leslie F. Halleck is a dedicated horticulturist with a masters in horticulture from Michigan State University. She is a Certified Professional Horticulturist (CPH) via The American Society for Horticulture Science, with more than 25 years of green industry experience in research, greenhouse production, public gardens, garden center retail, landscape and design services, and gardening communications. Her work has been published in many scientific, industry, and consumer horticulture publications such as Fine Gardening, Greenhouse Management Magazine, and more. She currently runs Halleck Horticultural, LLC, a company that provides consulting services to green industry businesses, as well as horticultural consulting.

Learn more about this author