Many flowering plants, Cannabis among them, are called "diurnal" plants; they use the day-night cycle to trigger patterns of growth. This is important to understand when growing plants indoors so that your crop management takes this cycle into account. There are two phases to the growth of most flowering plants: vegetative and flowering. The vegetative cycle focuses on establishing a solid root system, a strong main stem, and ample foliage to absorb light for photosynthesis.

Seedlings can get by with less light intensity until vigorous growth starts; heat is more of an issue. Keep seed beds warm, but not hot, and give enough light to nourish the seedlings without scorching them. If you're starting with seedlings, full-spectrum LED grow lights are a good choice because they give all the light needed without any overheating concerns. Don't use a weak light that causes seedlings to “stretch” in order to get sufficient light.

Once the plants’ roots are established and it’s in the vegetative phase, it will require plenty of light in the right frequencies to stimulate growth. Outdoors, the sun can provide more than enough light in all frequencies, but indoors, it’s up to the grower to give plants the quality of light they need. Leafy plants like Cannabis need a good amount of blue and red light in the proper wavelengths for optimal growth and bud production. Mixing various light sources can approximate cannabis’ needs, but the simplest and ultimately most economical way is through the use of properly designed, full-spectrum LED grow lights.

Editor’s Note: Plasma lights also have a very interesting spectrum, although they produce a lot of heat. There’s always a tradeoff!

A lesser-known and often overlooked process in plants is what is known as phototropism. Plants locate the most powerful light source and turn towards it in order to gather the most energy. An insufficient amount of blue light will of cause the plant to stretch toward the light source and become “leggy” and weak. High pressure sodium (HPS) lamps don't emit enough blue light for proper phototropism. You can solve this with metal halide lamps during the vegetative stage. In our opinion, however, full-spectrum LED lights eliminate the need to change light sources during the grow cycle, which has other benefits as well.

While cannabis plants don't have a "sleep cycle" per se, many growers feel that at least some time in the dark reduces stress and lets them relax and catch up on some other processes that improve plant quality such as root development. At a minimum, Cannabis plants require less than 12 hours of dark to stay in the vegetative cycle, so a good approach is 18 hours on and 6 hours off during the vegetative state. This method saves considerably on energy used for lighting and ventilation with very few negative effects on plant growth.

The flowering phase in Cannabis is triggered by an increase in the amount of dark time. Most growers start by changing their light cycle to 12 hours on and 12 hours off. During this phase, the plants will continue to grow vigorously and require even more light because of its size. Some growers switch to HPS at this time because the plants need more red light than they did previously, and HPS lights provide a high intensity of light. Full-spectrum LED grow lights eliminate the need to switch lights, saving time and money, as well as reducing the need for ventilation to control the large amount of heat produced by HPS lamps.

While it's technically possible to give plants too much light to the point of oversaturation, it's unlikely that you’ll do this with an indoor grow room because you’d probably start a fire in the building first or blow out a breaker. Practically speaking for LEDs, a minimum of 37 watts per square foot is a minimum, and 65-75 watts is your upper limit. Keep in mind that other lighting technology increases your power demands greatly per square foot, so use it wisely.

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