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Growing Under Lights

by ZippyGrow on July 04, 2019

Harvest Your Own Food All Seasons

Author: Danny Donello / Category: Article / Published: Jul-04-2019

Why Plants Need Light

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. If you understand what is Photosynthesis and Chlorophyll, you know 90% of all you need.


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.


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.

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.

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 com-bine to form O2, or oxygen gas. This is why we characterize plants as breathing in car-bon 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.

How Plants See Light

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. The paramount here is that not all light is equal. Different types of light both drive and limit photosynthesis, change plant morphology, and influence flowering. Maybe, you have already heard the so called "PAR" (nothing in common with "golf").

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).

Therefore, lamps that emit only blue light, with its shorter high-energy wave-length, 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.

The spectrum of light an outdoor garden receives will vary according to geo-graphical 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.

Ultraviolet and Infrared Light

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.

Radiation wavelengths that fall just above the red light spectrum are called infra-red (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 chlo-rophyll, and they are involved in important biological processes beyond photo-synthesis. 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.

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.

Red vs. Blue Light

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 occur 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.

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 photo-synthesis and other functions.

When you grow plants under only red light, they can put on additional leafy growth, or bio-mass. 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.

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.

Grow Lights

Now that you are armed with an in-depth understanding of how your plants respond to light and how much light they need, you can choose the right lights for your home-growing needs. Many indoor gardeners wonder if they can use shop lights. It’s possible, but you will not get the same results, efficiency, or aesthetics from standard shop lights or other common household lamps. You will likely end up with a lot of unwanted heat, wasted electricity, and less useful light. As a general rule, the less expensive the purchase price of the light setup, the more expensive and less effective it is to run. You will get much better results if you provide the right intensity of selective or full-spectrum light that is specific to plant growth.

Here are a few key factors you should consider when choosing the type of grow light, or combination of lights, you will use to grow your plants: the spectrum of light provided; that is, the balance of cool and warm colors and how much of each is provided; the usable quantity of light the lamp provides; the amount of heat output from the lamp; and the amount of light, or the number of hours, you intend to light your plants given their daylength requirements and the lamp’s energy efficiency.

Don’t base your lighting decision solely on the number of electrical watts the lamp needs. Watts are an input requirement of the lamp, not an output of resources for your plant.

When comparing grow lighs, don’t rely solely on the listed lumens of the lamp. Lumens are a general measure of brightness for the human eye, not a measure of usable light for photosynthesis.

Ultimately, the type of lamp you will need depends on the types of plants you are growing (leafy, flowering, or fruiting), the number of plants you will be growing, the size of the plants, and the dimensions of your growing space.

If you simply want to keep a single houseplant or a blooming orchid in your office or living room, you need only a single smaller spotlight fixture. A small crop of leafy greens, microgreens, or herbs is also simple to light with a small-footprint fluorescent or LED setup. Many self-contained home-growing units are already complete with full-spectrum LED lamps for growing seedlings and microgreens. If, on the other hand, you intend to grow groups of fruiting crops such as tomatoes, citrus, or cannabis, you will have to upgrade the type and quantity of your grow lights. Grow lamps will emit decreasing amounts of light over their life span. This is known as depreciation. The packaging or product description usually includes an approximate number of hours you can expect to receive full PAR. If the life of the lamp is 10,000 hours, the PAR may begin to degrade at about 8000 hours. The number of hours a day and number of days per year you’re running the lamp can help you determine when to replace it.

With so many variations of grow lights, it wll take us more than a day to consider all Pros and Cons. And here in ZippyGrow, we are 101% sure that LED Grow lights should be your first and only choice, ignoring the higher price (eventually, LED Grow lights will save you more money, time and troubles).

LED Grow Lamps

The world of LED technology for growing plants has exploded over the past few years. Their growing popularity is based on improving cost and energy efficiency. Most of the self- contained growing units on the market, such as those intended for growing greens or herbs on your kitchen counter or even integrated into your kitchen fixtures, use LEDs.

While LEDs did not start out as the best lamps for growing plants, and there is still a lot of variability among the options available, they are earning a good reputation as low-energy cost and low-heat grow lights. While some indoor growers still use LEDs only as a supplement to other forms of HID lighting, newer advances are making LEDs a good consideration for primary plant lighting, especially in intensive indoor food-production operations that use vertical farming. You can also use LEDs to lengthen photoperiod during the shorter and darker days of winter, or shift easily between vegetative and flowering stages of plant growth using spectrum-specific LEDs.

Small LED lamps and fixtures are typically very affordable. But the larger multiband rigs that emit enough light for multiple plants in a grow tent can cost just as much as, if not more than, HID lighting, even if they do not emit as much PAR. An LED lamp is a semiconductor that produces light when electrical cur-rent passes through it. This is solid-state lighting, as opposed to lamps that use electrical filaments and gases to produce light. This solid state can make LEDs a longer-lasting, more efficient choice. LEDs are lightweight and don’t emit a lot of heat, which is very important in a controlled growing environment.

LED lamps produce only about a quarter of the heat that other high-intensity lamps emit because they can convert more watts into light. Less heat means less transpiration in your plants, which cuts down on watering. Low heat output also means you can place LEDs close to your plants without the risk of scorching foliage or damaging young seedlings. When you are growing plants that need a more concentrated delivery of light (such as seedlings), the closer you can place your lights without burning them, the better. But keep in mind that larger LED rigs can still generate heat, especially in enclosed growing spaces.

While the low energy usage and heat output of LEDs are great, the ongoing concern for growing plants relates to both the amount and kind of light (PAR) they can deliver to your plants based on their electrical draw. To date, LEDs are not as efficient for growing plants as other types of lamps. Newer LED lights, depending on the wattage, claim to be just as intense as traditional HPS lights in terms of the amount of PAR they can produce. Real outputs will vary among different manufacturers, and quality construction matters. LEDs may draw less wattage than they claim on the label, so their output of light may be lower than you expect.

Dual-band LEDs that mix primarily red and blue light produce light that looks pink to purple in color. You can use this mixture for continuous growth through vegetative and flowering stages. You can also use single-band nanometer-specific blue LEDs for vegetative growth, then add red if your plants need to flower. Or bulk up young vegetative plants using single-band red LEDs, then add in blue light to encourage flowering.

As LEDs have evolved, they came to include deep red and royal blue light, and were referred to as three- or four-band lamps. LEDs can include multiple bands of light within the PAR spectrum, including green, yellow, orange, violet, and others, as well as UV and IR. These types of multiband LED fixtures are often offered with specific spectrum mixes that encourage different states of growth and development.

Some of these dual-band or multiband LED fixtures are zoned so you can turn on only the color or mix of colors you want, depending on your plant’s stage of growth. This is handy when you have a small growing space and cannot move plants to different lamps between the vegetative and flowering stages.

White full-spectrum grow LEDs are not really white; they are blue diodes coated with a yellow phosphor, which creates the appearance of white light. Over time this phosphor coating can degrade, and the color of light it produces will change. You can check the spectral output chart on the lamp, if provided. Full-spectrum LEDs will usually lean to the cool or the warm side, and you can use them accordingly for either foliage or flowering plants.

Multicolor white LEDs, also known as RGB LEDs, mix separate red, blue, and green diodes together in a balance to create white-colored light that is both visually pleasing and efficient for plant growth. RGB LEDs are a bit more complicated to manufacture than standard white full-spectrum LEDs, so they may cost more. Many small two-band LED lamps can be used in standard home light fixtures, floor lamps, or spotlights. These small lamps typically fall in the 3- to 13-watt range and will be labeled as E6 or E27 base. LEDs with a GU10 base that snap in are often used in recessed lighting fixtures. These individual low-wattage LEDs are useful for only individual houseplants. Do not expect them to light up a large area or to be suitable for multiple plants.

LEDs lamps are available in other configurations: large multiband fixtures for inside grow tents and grow rooms, light bars you can mount or hang from shelves or cabinets, and even flexible strips you can wind around fixtures you build or the inside of grow tents or closets. You can also use LED strips to build your own LED hanging fixtures. These options give you the ability to interlight plants, or to run lighting between plants to improve growth on the lower areas.

No matter what LED lamp you choose, be at least a bit wary of claims of proprietary spectrums or unique combinations of light colors. If you understand the basics of light science, you know your plants can use all the light within the PAR spectrum, and blue and red light used individually are most efficient for photosynthesis and best employed to shift developmental stages of plant growth.

As LED technology advances, these types of lamps will likely outpace other types of traditional grow lamps in light delivery and energy efficiency. As with any evolving technology, part of the bigger price you pay now (with perhaps less efficacy) is an investment in future advancement and efficiency.


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