1-866-710-GROW Online Customer Service Hours M-F 9:00-5:00 pm

This article is part of a series written for HTGSupply by plant biologist Dr. E.R. Myers.

This is the fifth of a multipart series concerning plant growth and limiting factors (previous
articles can be found at the htgsupply wesite ). This month I will discuss light and plant
growth, next month carbon and ventilation.

As you know, the most important thing when considering plant growth is understanding limiting
factors. Plant growth is determined by limiting factors, if your plants do not receive enough of
any one (1) factor they won’t grow at the maximum rate no matter how much of any other factor
you give your plants. To be a better grower find your limiting factor. Increasing your limiting
factor, will increase growth without having to adjust or do anything else.

There are several major factors that can limit plant growth, listed in order of importance they are:
light, water, temperature, carbon and nutrients. Light is the energy source for photosynthesis, and
photosynthesis provides the energy for all the plants physiology. If you don’t give your plants
enough light it does not matter how much fertilizer, CO2 or any other factor you give your
plants, they won’t grow bigger or faster. If you are reading this, I assume you are one of the
smart people who have a high quality light source from HTGsupply.com . Understanding what
light is and how different types of light affect plant growth will make you a better indoor grower.
After reading this, you can tell your friends exactly why they too need a high quality light from


1. What is light
2. How light is involved with photosynthesis
3. Light as wavelengths
4. Light, plant physiology, and flowering
5. Basic lighting regime for most plants
6. Light fixtures
7. Light Measurement

What is light?

Visible light is just a small part of the massive amounts of energy released by the sun, all the energy the sun emits is collectively called radiant energy. Light is sometimes called white light, because it appears white to the human eye. Have you ever used a prism to make a rainbow from a light source? A prism separates white light into its various components. White light consists of all the colors of the rainbow, red, orange, yellow, green, blue, indigo, and violet (remember ROY G BIV from junior high school?). The energy of white light is what plants use for photosynthesis and interestingly is the same energy we interpret as vision. Color, is actually how our minds interpret the various energy waves that make up white light. (there are no colors, there is only your minds interpretation of energy of different wavelengths that you think are colors--- sit down and think about that sometime). As an indoor grower, it is important to keep in mind that different light fixtures emit different percentages of white light, not every light source is good for every plant.

How light is involved with photosynthesis

Photosynthesis is the process of using light (light energy) to make carbohydrates (chemical
energy) which plants use to grow and flower. Photosynthesis occurs in plant leaves, specifically
inside cells in something called a chloroplast. Photosynthesis is a metabolic process, meaning
there are many steps involved from the first reactant to the last product. Basically, photosynthesis
begins with pigments inside the chloroplasts (chlorophyll a, b and others) that absorb specific
types of light energy (colors). The captured light energy is used to make high energy electrons,
which then use their energy to make a more stable energy source which is used up in order to
combine carbon dioxide (CO2) into carbohydrates/sugars (C6H1206) and in the process give off
oxygen (02).

The balanced equation for photosynthesis may be written as:
Light Energy + 6CO2 + 6H20 ---> C6H1206 + 6 02

The chemical formula C6H1206 is the formula for the sugar glucose. Plants make many different
sugars, but glucose is usually the sugar used in textbook diagrams. Thus, we see the three biggest
inputs required for photosynthesis and good plant growth, are light, carbon dioxide and water.

Light as wavelengths

Light wavelengths (colors) are measured in units called a nanometer (nm). Different wavelengths
appear as different colors. Indoor growers need to have a light that emits the proper type of light
wavelengths for their plant and the plants life cycle. Some plants need high amounts of red to
flower for example. However, light intensity is also important (More below).

White light ranges in wavelength from roughly 400 nm to 700 nm. When looking at light and
photosynthesis, red and blue light are by far the two most important colors concerning plant
growth. Red and blue are absorbed by the chlorophyll pigments very efficiently. Other pigments,
caretenoids for example absorb more yellow. The diversity of pigments in plants helps them
absorb energy from various colors. More pigments help plants get more energy from white light.

With less pigments there is less energy available for plants to absorb. The same concept explains
why some people are color blind. Some color blind people can not tell green from red because they are lacking one of three pigments in the human eye used to differentiate (absorb) these colors.

Generalities due to different wavelengths of light energy

400-500 nm (blue) helps in keeping internode length (the space between branches along the main
stem of a plant) small and will increase the number of chloroplasts in a plant, making them turn a
dark green. A certain about of blue light is required for most higher plants to grow and flower.
500-560 nm (green) Not necessary for photosynthesis, but is a significant component of some
light sources.

560-590 nm (yellow) Not a major wavelength for photosynthesis, but this color is absorbed by
some plant pigments and can be linked with some plants and flowering. Excess yellow light can
lead to long internode lengths and lankyness.

590-630 nm (orange) Not a major wavelength for photosynthesis but is absorbed by some
pigments so it can contribute to photosynthesis in some plant species.

630-700 nm (red) Required by most plants for maximal photosynthesis and normal physiological
activities. Some red light is required for most higher plants to successfully grow and flower.

700-750 nm (far-red) Involved with flowering, stem elongation, and is used to keep track of day
length of certain plant species (as a function of the red/far-red ratio)

Red and yellow light are associated with flowering in many plants. I have worked in greenhouses
where plants that were grown under florescent bulbs (high blue) had an incandescent light (high
yellow) added to induce or promote flowering. This is also one reason that some growers use
high pressure sodium lights, their light output being more intense, but also emitting more energy
in the red/yellow end. (See light fixtures below)

Light, plant physiology, and flowering

Green plants use light for purposes other than photosynthesis. The duration of light is ultimate in
determining when and how many plants flower (See Basic lighting regime for most plants
below). An interesting phenomenon of light is phototropism, when plants bend toward the light.
You can see this in a plant placed in the window, the leaves will turn to face the incoming light
through the window. Predominantly blue light is responsible for this in plants. Light is also used
by plants to regulate plant life processes, this is called photoperiodism. Light can determine
when perennial plants stop growing, when leaves fall off deciduous trees, and light even affects
seed germination. Red and far-red (infrared) are often associated with photoperiodism. Since plants do not wear watches or even have eyes, how do they tell night from day? Plants have a pigment in them called phytochrome that is sensitive to light. To understand the basics of light detection you just need to know that phytochrome comes in two types, light and dark. When light is hitting the plant, dark phytochrome turns into light phytochrome, and you guessed it, in the absence of light, light phytochrome turns to dark phytochrome. It is the ratio of dark to light phytochrome that plants use to measure day length. When the amount of dark phytocropme gets to a certain point i.e. the nights are a certain length, the plant will begin to flower. To be honest, plants measure the length of the night (also called the “dark period" by indoor plant growers and it is when the lights are turned off). Translation turning your lights off in the middle of the day (light cycle) to fix something, install new equipment etc. will not affect growth and flowering. BUT, if you turn the light on in the middle of the night (dark cycle), you WILL affect flowering. If you do not have a light proof room you will have “light leaks” into your garden during the “dark period”, you may end up with some very odd or no flowering responses. If you can’t devote a whole room to growing, you could try using a grow tent. Grow tents are light proof and provided an excellent “microclimate" to grow your plants in.

Concerning plant physiology and growth, a common misconception I hear is that pruning big
shade leaves and older leaves will allow the smaller leaves to have more light. The big shade
leaves on broadleaf plants have more chlorophyll (sometimes seen as being darker green). In most cases, it is a bad idea to remove big shade leaves, they are contributing to the photosynthesis rate, and hence the growth rate of the plants. Move them so that younger leaves on that or other plants are not blocked from light. Older leaves on plants can actually have high rates of photosynthesis with lower light intensity, so don’t think the big shade leaves are being wasted if you tie them down, they are contributing to the growth rate. If they don’t get enough light, they will yellow and die, but at least this way, all the nutrients in that leaf will be used by the plant.

Basic lighting regime for most plants

If you are interested in flowering and fruit, which is why most of us grow, the changing day length determines when many plants flower. The change in the duration of night is why different plants flower in spring (nights longer than day) summer (nights shorter than days) or fall (nights getting longer). Temperature also plays a part in the plants life cycle and some plants just flower after a certain amount of time. Other plants need to build up specific nutrients over time to get big enough before they will flower. There are many plants that need a certain amount of darkness in order to trigger them to begin flowering. Indoor growers must know the light requirements for their plants in order to promote flowering. This is simply the ratio of how long the lights are on versus off. This can be determined with some basic knowledge: does you plant naturally (in the wild) flower in the spring, summer or fall? Spring plants will need warmth to start growing and light at least equal to dark. Summer plants will need a grow period and increasing light versus dark. Fall plants will need a growth period with long days followed by shorter days, light on for 12 hours off for 12 for example to simulate the shortening fall days.

For most plants, I start off giving seedlings 24 hrs of light for the first couple weeks. Most plants
do not need the dark cycle for the first two weeks so the theory isà the more light the faster the
growth. I always start seedlings under high output T5 fluorescent lights. Bulbs for these lights
that are high on the blue end of the light spectrum (6500K Grow Bulbs) are available and
recommended to use so that seedlings will have short internode lengths and be dark green. I have
noticed that some plants do start to yellow with 24 hr light after 2 weeks, switching to 18 hrs on
6 off tends to help them begin to grow better. Eventually, all plants need to have a dark period
for certain molecular cues to go off and normal plant growth to occur.

Lighting Fixtures

There are numerous plant lighting systems on the market today, each with their own good and
bad aspects. Choosing the right light for your situation is the most important part to growing
indoors effectively.

1. Incandescent lamps - These are the standard household “light bulbs” that Thomas Edison
improved upon in 1879 which was then a 50 year old idea. Incandescents are not good
for growing plants as the only light source; they are very inefficient, using a lot of
energy to produce little light output. They also emit predominantly red/yellow light
causing seedlings to grow lanky (over stretched) and fall over. They may be used to
supplement the flowering period to increase the amount of yellow light which can help
induce some plants to flower.

2. Fluorescent lamps -The fluorescent lamp is well suited for indoor plant growth, especially “High-Output” T5 fluorescent lamps. Fluorescent lamps are 3-7 times more efficient than incandescent lamps meaning you will use less electricity for the same amount of light. Fluorescent bulbs come in a wide spectrum of light, 6500K grow bulb shave more light on the blue end so they promote bushiness in plants whereas the 3000K bloom bulbs have more light in the red end of the spectrum so they promote flowering. However, these lights have a low light output, meaning you need to keep the lights within a few inches of the tops of plants. This means that fluorescent lights are not usually a good light source for big plants (over 3 feet). You can use fluorescents with big plants if you have lights above and along the side of the plant.

3. High Intensity Discharge (HID) lamps - These include metal halide (MH) and high pressure sodium (HPS). These lights are traditionally used for lighting large areas such as gymnasiums, warehouses and parking lots. Today, they are the primary choice for indoor plant growth. HID lamps are 4-8 times more efficient than incandescent lamps. These lamps have a high light output, (and produce a lot of heat) so they should be placed about two feet or more above the top of the plants. Metal halide lights have a wider range of wavelengths so they are a good all purpose light able to be used in all stages of plant growth, seedling, vegetative and flowering. I use high pressure sodium which have a higher intensity of light per watt compared to MH, but they also emit more red and yellow light. This is great for promoting flowering and fruit. In spite of this, they are not the best light source for seedlings. A MH conversion bulb is a good option for vegetative growing plants, but it may be more economical if you have the room to have one area with fluorescent lights for seedlings, and young plants, and when the plants get bigger put them in another area where you promote flowering.

4. LED or Light Emitting Diodes – These are a new lighting source for growing plants indoors. These lights are very efficient, using very low amounts of electricity and giving off minimal heat energy. However, they emit light in a very specific range. This could mean that plants do not get all the light wavelengths they need, or it could mean that plants get just what they need and nothing more. The idea is that you don’t go any further by over filling your gas tank vs. a full tank, so why use electricity to provide light that plants won’t use. There are numerous experiments testing to see what percentage of red and blue light an LED needs to promote good growth and flowering. One thing has been established by scientific studies: You will not be able to grow a plant with just one color LED! All LED for indoor growing should have red and blue, and possibly yellow, although testing is underway in many labs around the world looking at what amount of each color is best for different plant species. You could definitely use a single color LED to supplement your current lights. You could use blue LED with HPS during vegetative and early flowering to reduce internode length or use a red LED with MH to promote and improve flowering.

Light Measurement

I will end this letter discussing how to measure the amount of light your plants have available to them. The light used for plant growth, i.e. the energy absorbed by plant pigments, is different than the light absorbed by human pigments in the eye. This means that the foot-candle which is based upon the sensitivity of human eye pigments (light in the 550-560nm range) is a poor measure of light for your plants. Before you throw out your foot-candle light meter however, keep in mind it can still be used for measuring light intensity and how well light is spread evenly in your grow area.

Foot candles are a measure of light intensity. While a lamp (the source of the light energy) produces lumens. Lumens are a term for measuring the visible output of a lamp or “bulb". A standard 60 watt incandescent lamp is rated at an output of 900 lumens. A 400 watt metal halide lamp produces 36,000 lumens and a 400 watt high pressure sodium lamp produces 50,000 lumens. Generally, HPS lamps are more efficient light producers (more lumens per watt), these lamps produce a range of 100-140 lumens per watt, while MH lamps produce a range of 70-110 lumens per watt. As I mentioned above, metal halides have a broader spectrum of light having more blue light while HPS has more yellow – red light as a part of their total intensity. Scientists who study light and plant growth don’t use foot-candles or lumens. A light meter that counts the number of photons is more accurate in predicting photosynthesis than foot-candle meters. Smaller growers have continued to use foot-candle meters, which as I said, measure light for humans. Foot candle meters have errors of up to 45% when used to measure light for photosynthesis (ouch!). Measuring light in scientific experiments is done in something termed Photosynthetic Photon Flux (PPF) and its units of measure are micromole per meter squared per second (μmol m-2 s-1). This term is based on the number of photons in a certain waveband, per unit time (s) on a unit area (m2) divided by the Avogadro constant (6.022 x 1023 mol-1). It is used commonly to describe photosynthesis rates in the light wavelengths plants use i.e. the 400-700 nm wavebands. I am not recommending you spend thousands of dollars on a Quantum measurement device, I simply want you to understand how light should be measured in  scientific experiments. There are devices available, but most are 100’s to 1000’s of dollars, and maybe not in the budget of a hobbyist.

Considering photons of light energy you should ask how much do your plants need? Keep in mind that a daily average irradiance of 26 μmol m-2 day-1 will effectively grow most species of higher plants. The maximum solar irradiance around midday of 2000 μmol m-2 s-1 is transient and is not necessary for normal plant growth since the plants growth is a factor of the average daily irradiance of light. Light intensity is important over time. This is why you can sometimes greatly improve or expand you grow room with a light mover such as a high quality Light Rail or Solar Revolution light mover. If you are giving you plants more than enough light, you can spread the light out (move it) so long as the daily average is still at the maximum.

Keep average daily irradiance in mind when you are planting your outside garden if you have sun loving plants, make sure they get the sun of the midday, it might not matter that they miss a few hours of morning and evening sunlight if they are in full midday sun. This also explains why some plants only prefer morning or evening sun, they can not tolerate the full intensity of the midday sun, they prefer some shade. If you are growing a plant such as this, like an African violet and some types of terrestrial Orchids, you  may only need fluorescent lights, since the light of an HID may be too intense. What ever you are growing, start with a good light source and you will be well on your way to good growing.

Good Growing,
Dr. E.R.Myers