In addition to sufficient water and nutrients, careful monitoring and the use of light are the most critical factors in growing fantastic cannabis plants indoors. If you want the best plant yields possible, you must absolutely make the right lighting system choices. Cannabis plant grow lighting can be more complex than most beginner cannabis growers imagine because the amount and kind of light affect the fragrance, flavor, appearance, bud size, and potency of cannabis buds.
This process is called photomorphogenesis, which is the development and form of plant structures using light. In this article, we will review the light spectrum, how it influences the cannabis growing cycle (such as flowering stage or vegetative stage), different lighting systems (just as LED and HPS), and recommended daily light cycles.
Light Spectrum & How It Affects the Flowering Stage
Ever hold up a prism and marvel at how that rainbow is created? White light contains all the wavelengths invisible to the naked eye. When white light hits the prism and gets refracted, the result is distinct color bands. Each color represents a different wavelength on the full spectrum. These colors represent different wavelengths that are measured in nanometers.
Each wavelength plays a key role in the growth process for plants. A plant’s chemical composition influences how it will respond to lighting. For example, the chemical makeup of a strain, intensity of light, length of light exposure, etc. all play a role in the growth process. Different colors help plants sense what is happening around them and consequently adapt during the growing process. Let’s begin by reviewing the different ranges and how they impact each growth stage.
Each wavelength happens within a specific nanometer (nm) range, and each one has a distinct role in the life cycle of a cannabis plant. Ultraviolet wavelengths are the shortest ones measuring between 280-400 nanometers. Many of us are familiar with UV rays because of the potentially harmful effects of UV-A and UV-B on human skin. These two wavelengths can also prompt plants to make antioxidants, enzymes, and chemicals.
UV-C light at 100 nm to 280 nm can damage plants, but UV-A (315 to 400 nm) does not damage plants. Ultraviolet wavelengths also offer some positive effects. Some growers think UV-B between 280 to 315 nanometers may increase THC counts, so the horticultural equipment market offers some newer models of LED (Light Emitting Diodes) grow lights that have UV-B.
Next in the spectrum comes blue wavelengths (420-490nm). Indigo ranges from 420-440nm. Blue is considered cool light and helps plants find the light source. The ideal nanometer point is 460. Blue light encourages root growth, strengthens the stems and produces shorter plants with larger leaves. Increasing the number of leaves means better light absorption.
Many indoor cannabis growers use compact fluorescent grow lights (CFLs) and T5 /T8 bulbs that emit blue light for the first few weeks to control stem length and avoid stretch too early in the growth cycle. “Stretch” is when the plant reaches to get more light. Stretching at early points can result in an uneven canopy, and lower buds will not get the optimal amount of light. Blue light also sets up the plant for growing healthy leaves.
Green wavelengths (490-570nm) are critical to the seed and vegetative stages because stem growth and water absorption are boosted. Indoor cannabis growers who use green light in addition to red and blue light for their cannabis plants report a remarkable difference in production over just using red and blue lights.
Many growers strongly suggest using green lights to check on plants in the dark so you do not disrupt the plants’ sleep period. Companies sell equipment that is made just for this purpose. CFLs also emit a significant amount of green light, and you can still produce a good crop even if that is your only lighting option.
Yellow wavelengths (570-585nm) encourage plants to stretch and bud. Plants will stretch to get more light. This growth also promotes leave growth. Plants use them as protective shields at night and as photoreceptors by day to absorb more light and thus increase the final yield. Orange wavelengths (585-620nm) are also suitable for the flowering stage. Orange light bounces off the leaves. Halogen bulbs are effective at emitting orange light.
Red wavelengths (520-720nm) signal to the plant to get ready for final flowering steps. Red wavelengths mimic the sun’s lower angle in autumn. Cannabis plants will grow longer stems in their final reach for light in this growing stage, and indoor growers believe 5660nm is ideal for increasing bud size.
Some growers will give small amounts of far-red light at night to plants that are slow or stubborn to bud or to enhance resin production. Red light activates the molecule phytochrome which helps the plant sense temperature and regulates activities such as leave production and flowering. Plants think that the far-red light means they are in the shade and respond accordingly.
Overall, knowledge of wavelengths is important to consider when buying and using a cannabis indoor lighting system. Regardless of the system you choose, plants use red and blue rays the most. The blues and reds are the most useful for photosynthesis making LED lights an efficient choice.
Experimenting with altering colors and nanometers is suggested, but make changes slowly. With each growth cycle, you’ll see differences in weight, density, CBD/THC potency, appearance and fragrance depending on your lighting methods.
How Does Each Color Affect Growth?
Now that we have reviewed the different wavelengths, let’s take a closer look at each color affects each of the four main stages: Germination, seedling, vegetative and flowering.
Germination lasts from 1-7 days. The germinating seeds do not need light while under the soil, so you can use this time for setting up any last-minute grow lighting needs.
The seedling stage can last up 2 weeks and start when you see your seeds sprouting. Then it’s time to turn on the lights and warm up the soil which prompts further growth. At this point, light is more important than heat for these sensitive shoots. LED grow lights are recommended during this stage for full-spectrum lighting. Once the plant has 5-7 leaves, you can consider the seedling stage over.
The vegetative stage lasts 3-16 weeks depending on the strain. During this stage foliage becomes fuller, leaves form the odd-numbered blades distinctive to cannabis, the root system becomes more robust, and the stem becomes more solid. Blue and white light are effective for the vegetative stage. If you have a lighting system with different light settings, switch to 30% blue/70% red in the pre-flower stage as plants start to focus their energy on resin production.
The flowering stage lasts 8-11 weeks. You will notice the buds becoming larger and the pistils darkening. Sometimes the leaves will turn a gold or yellow hue, indicating that the buds are sucking away nutrients from the rest of the plant. High-pressure sodium lights are good during the flowering stage and you can also use them with fluorescent lights.
Red, orange and yellow lights are conducive to the flowering phase, making High-Pressure Sodium (HPS) bulbs also an effective choice for this stage.
Choosing the right indoor grow lighting can be overwhelming for a beginner or even an experienced indoor cannabis grower who wants to try new lighting methods, so it’s best to research all your options and decide on your overall indoor growing goal. Is it to increase your yield? Is it to produce larger buds? Do you want to keep it simple and get one round of growing under your belt before conquering more advanced equipment and techniques?
If you want to take the most economical route and streamline equipment, look for versatile lighting that offers a range of colors/wavelengths to support all stages. A system that gives high amounts of red and blue light, moderate amounts of other colors (green, yellow, and orange), and low amounts of far-red and UV light is ideal.
This way you have a balance of different kinds of light, and you aren’t risking too much far-red which would potentially damage chlorophyll production. Light Emitting Diode (LED) lighting fits that description best. They’re relatively affordable and LED lighting systems also have advanced settings for the flowering and vegetative stages so that you can control which wavelengths the plants receive.
Indoor Grow Lighting Systems: PAR, PPFD, and Lumens
Some of the terminologies seem similar and can lead to confusion, so this section explains some key light measurement terms.
Photosynthetically Active Radiation (PAR)
Photosynthetically Active Radiation defines the kind of light needed for photosynthesis. PAR is measured by watts per square meter. An optical filter senses levels between 400-700nm which is the best range to support photosynthesis. Biologists have found that the quantity of light is more influential on a plant’s health than light quality. During germination, PAR should be between 150-200nm.
Levels should gradually be increased in the vegetative phase from 200-550nm. Raise the levels from 450-800nm as a transitional stage to the flowering phase for less than a week. Keep PAR between 700-800nm during the flowering stage. Growers can measure PAR at different levels of the canopy to gauge how much light plants are receiving.
Photosynthetic Photon Flux Density (PPFD)
Photosynthetic Photon Flux Density (PPFD) gauges the amount of PAR falling on a square meter of a canopy per second. Growers measure in micromoles per square meter per second (μmol/m2/s).
When picking out an indoor grow lighting system, make sure you select a setup that will adequately supply all plants with proper lighting. Make sure you match your needs with the PPFD values advertised on product packaging. First, mark where you plan to set your plants and measure the distance from the light source (both vertical and horizontal measures).
Indoor grow lighting manufacturers typically provide consumers with the recommended distance from the light source (vertical and horizontal), the number of measurements included in the average, and the minimum and a maximum ratio of plants in the area. Also, check the recommended mounting height for lighting based on the cannabis plant’s growth stage. PPFD will help determine how your equipment setup can maximize PAR.
Lumens means luminous flux light seen by the naked eye. Lumens are helpful to measure the amount of light emitted by fluorescent lights, LED, and metal halide grow lights. Each kind of indoor lighting system emits energy differently and Lumens is calculated so the number is weighted, so it is important to keep this in mind when taking a reading.
For example, LED grow lights function differently from other indoor grow lights because they project light in narrow wavebands. Therefore, the weighted numbers from lumens are not a suitable way to gauge how much usable light the plants are receiving.
Devices Used to Measure Cannabis Grow Lighting
Along with selecting a lighting system, it’s critical to monitor wavelengths and intensity with a light meter so you can increase your crop yield. Here are some optional, but recommended light meters:
A spectrometer measures wavelength, frequency, and energy by taking a photographic image of the wavelengths produced by grow lights. The data can be used to create charts of output from lamps to evaluate efficiency.
A Lux meter is a lumen meter, and it measures the amount of lumen per square meter. A semiconductor called a photodiode converts light intensity to electrical current. Growers use a lux meter to position their lights in the most ideal spot, test bulbs, and promote healthy plant growth. Many growers use lux meters to decide where to reposition plants if they are in shadier spots blocked by other taller plants or shadows. A minor drawback is that a lux meter does not give a full picture of how the plants are using light, only how much is light is falling on them.
PAR meters measure the amount of light that facilitates photosynthesis, but you must consider the usual factors which impact cultivation such as climate, temperature, absorption and nutrient quality. The ideal light to promote photosynthesis is usually between a wavelength of 400-700 nanometers, which is the blue and green colors of the light spectrum. Using a PAR will help you work towards ideal conditions. The meter measures an area’s PAR in millimoles.
Light Duration and Intensity
In the seedling stage, provide roughly 18 hours of light and 6 hours of darkness. Many recommend 12 hours of light and 12 hours of darkness for the vegetative stage, although some growers recommend more light hours to trick the plants into flowering sooner. Some suggest only 10 hours of light for indica-dominant strains.
During the flowering stage, provide 12 hours of light followed by 12 of darkness. It’s important to make sure that no light enters the growing space during the plants’ dark hours, so only check on them during their light times.
Other General Considerations Regardless of Lighting System
Consider the physical traits of each strain and how you need to adjust your grow space. Sativa plants grow tall and thin while indica plants grow short and squat, so place lights the recommended distance away from plants. Adjust the distance between lighting and plants as needed throughout the growing stages. Also, research the tolerance of a strain. For example, landrace strains like Afghani, which tend to come from higher altitudes, which are more tolerant of higher UV-B intensity.
Make notes during each grow cycle about lighting and effects on appearance, taste, aroma, potency, and yield. Be sure to document the nutrient applications, humidity percentages, and soil readings of each stage. Also, document color changes and oxygen levels if you have equipment designed to do so.
These careful steps will inform your decisions about adjustments during the current growth cycle as well as future indoor growing efforts. One fun and informative experiment is to take a light camera and track the wavelength and color changes in a 24-hour cycle. You might notice some color changes that correlate with plant responses to these factors.
Make sure you consult plenty of lighting and growing guides before you start out, and keep reading as you progress along with your indoor growing efforts. Most growing guides either in a book, video, or online form will discuss lighting options. Beginners can purchase a ready-to-assemble grow lighting kit or piece together their own by shopping online and at local gardening and hardware stores.
Whether you choose to grow in soil or hydroponically, you will still need to consider lighting that will work best for your needs.
Most growers agree that blue and red colors (40% and 60% respectively) are most effective if you must rely on the same lighting system throughout growing stages because these colors maximize THC count and create bigger buds. There is a big trend to use LED grow lights with settings for vegetative and flowering stages, and such systems should benefit the novice and experienced cultivators alike.
Decide which light measurement device might work best for your budget and needs. You can get by without a light measurement device and many home cultivators live by the motto “listen to your plants”; however, investing in a spectrometer, lux meter or PAR meter may make the difference between satisfactory weed and savory, aromatic bud.