What Color Light Spectrum Do Cannabis Plants Grow Best In?

What light do plants really absorb?

In the 1970s a scientist called McCree determined the wavelengths of light a plant can absorb. He assessed the relative quantum efficiency of different wavelengths of light on photosynthesis. In other words he recorded the rate of growth of plants exposed to each wavelength or colour of light.
McCree discovered that plants use all parts of the PAR spectrum but at different rates. The quantum efficiency for red is close to 100% efficient whereas blue and green were less efficient, ranging from 65% to 75%. McCrees action spectrum identifies what colors of the visible spectrum are best for photosynthesis. McCrees action spectrum is the correct reference for assessing the suitability and effectiveness of a grow lights spectrum output.
Blue Light
Blue lights with a Kelvin color temperature from 5000K to 6500K or wavelength in the 400-500 nanometer range are best for the vegetative stage of the grow cycle and for starting seeds indoors. This color range light is responsible for leaf growth and stimulates the production of secondary pigments in the leaves that can enhance the plant’s color and fragrance. Be careful when using blue light LEDs though because too much blue light may stunt the growth of certain species. These lights are particularly good for lettuce and other crops where a short, dense growth is desired. The chart below shows the results of a study done by Michigan State to show the effect of different combinations of light on a salvia splendens ‘Vista Red’ plant.
Green Light
Green lights are commonly used during a plant’s dark cycle because it doesn’t trigger photosynthesis or photoperiod hormones. This means it does not disrupt the plant’s “night” cycle while allowing you to navigate the grow room to water, check timers, and spray nutrients or bug repellant. These are particularly useful if your crop requires short periods of daylight with long nights.
Red Light
Red lights from 2000K to 3000K and 640-680 nanometers are most important in the flowering stage. This range of light promotes stem growth and overall plant height because red light signals that there are no other plants above it so it can grow unimpeded. They also encourage fruit and chlorophyll production, so they can be used to extend the light cycle of long-day plants and vice-versa for short-day plants.
Light Intensity
Sunlight provides far greater intensity than artificial lighting. Not all plants need the same light intensities, though. Some plants prefer the high intensity of full sun, while others prefer moderate sun or shade. In artificial lighting, the plant needs to be close to the light for highest light intensity.
Light Duration
Light duration is important, especially during the blooming period. For indoor plant growth under artificial lights, a minimum 12 to 14 hours of light per day is usually needed. Darkness is important to allow the plant to rest and trigger the flowering response. Some flowering plants, known as short-day plants, need long periods of darkness to bloom. Short-day plants such as poinsettias, chrysanthemums and June-bearing strawberries bloom in the spring when days are shorter and nights are longer. Long-day plants, such as onions and spinach, flower at the end of summer when days are long and nights are short. Day-neutral plants bloom without regard to the length of darkness but usually do best with longer light durations.

What Color Light Do Plants Like Best?
If you’re thinking sunlight, you’re right. Natural light is best for plants for any part of the plant’s life cycle. But what can you offer indoor plants?Plants make their food through photosynthesis: the chlorophyll transforms carbon dioxide into food. Three things influence the growth of the plant: the intensity of the light, the time the plant is exposed to light, and the color of the light.When plants grow in sunlight, they get full intensity and the full spectrum of all wavelengths. However, plants only really use the red and blue wavelengths. Blue light helps the leaves and stems grow (which means more area for photosynthesis) and seedlings start, so fluorescent lights are a good choice, since they are high in blue wavelengths.
What Color LED For Grow Light?
Many manufacturers of LED grow lights still subscribe to the outdated notion that plants need only red and blue light. As a result, they make “purple” lights with only red and blue diodes.
Stay away from those lights.
Instead, these two types of lights do the best job:
lights with mostly red and blue diodes, but also a decent amount of all-white diodes mixed in
full-spectrum white LED grow lights
There are plenty of good options when it comes to the first, but far fewer for the second.
The best lights that feature red, blue and white diodes are the ECO FARM LED grow lights.
Is HID Light A Good Color For Plant Growth?
Traditionally, HID grow light meant either high-pressure sodium (HPS) or metal-halide (MH) light. The former contains a lot of red light, but virtually no blue, while the latter contains a lot of blue light, but virtually no red.
As a result, best results are achieved when both types of bulbs are combined to give plants all the colors they need. But that results in a lot of bulbs, which really drives up energy costs and heat. It also means you need a lot of reflectors and ballasts.
Using MH bulbs for vegging and HPS bulbs for flowering is the most common solution, but it is not ideal. Even though plants want more blue during vegging, they still benefit greatly from also having red light. And the reverse is true during flowering. They need tons of red, but still want some blue.
It is for this reason (among others, like operating cost and simplicity), that I much prefer LED and generally recommend LED grow lights over HID, since they don’t even really cost more these days.