For Beginners

The distance of the light is measured from the light bars to the top of the plants. It varies depending on the plants, the light, and the brightness. The ultimate goal is to get a certain amount of light to the top of the plants. Less powered light needs to be placed closer to the plant and vice versa. Because the light density gets lower and diffuses more as the light is further away from the light source. A reflective grow tent could be used to even out the light distribution and lessen the light wastage. As the plants grow higher, it is needed to adjust the height or the brightness of the light. Growers can adjust the light using a lux or par meter or using the PPFD chart provided by manufacturers as a reference. 

For plants at vegetation state, it would require 2,000 to 3,000 lumens of light per squire foot at least. The plants also need to be spaced properly to make sure each plant can receive about 2,000 to 2,500 lumens. The 2,000 to 2,500 lumens per plant are based on plants like lettuce and cannabis. The lumens of light required for each plant is depending on its size. The idea is that they do not get too clustered so each plant could receive enough light. 

For plants at the flowering or fruiting stage, it would require 5,000 to 10,000 lumens of light per squire foot. As the plants form fruits, they need more energy. It is crucial to provide them with additional light to achieve optimal results.

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For cannabis, there are mainly two stages after germination, the vegetation stage and the flowering stage. In each stage, the light cycle required is different. Recommendations several tips are as follows.

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Under the vegetation stage, the plant will mature and keep growing after reach maturity. If the grower does not stimulate the flowering stage, the plant will just keep growing into a larger plant. Such plants would also yield more in harvesting. Another extreme approach is to stimulate flowering right after maturity. Cannabis usually reaches maturity in around 5 weeks at least. It varies depending on what bread it is. Growers decide on how long to keep the cannabis in the vegetation stage depending on whether the extra time and resources are worth the extra yield. Sometimes, a largely grown plant could yield as much as 10 barely matured plants. 

Cannabis would require at least 14 hours of light in the vegetation stage. The daytime needs to be noticeably longer than the nighttime. It is recommended to give 18 hours of light and 6 hours of darkness in each daylight cycle. It is feasible to give more light. 20 hours of light is usually the maximum that growers give to cannabis. More light means more photosynthesis, which in turn makes the plants grow faster. It is not recommended to give 24 hours of light each day. Forcing the plants to grow non-stopping could cause stress for the plants, which would lead to a higher chance of defection.

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As the name sounds, cannabis will produce flowers in this stage. Growers can induce flowering by providing 12 hours of light and 12 hours of darkness. When the day and the night are the same lengths, cannabis will release hormone level which triggers the flowering process. The hormone would need a few days to a week to build up until flowers start to appear. 

In this stage, growers need to make sure all sources of light are blocked in the nighttime. Even as little as a small LED light on equipment could spoil the flowering stage. A light leak would risk the grower three things: 1) The plant would not flower at the growers' expected time. 2) The plant may revert back to a vegetation stage. 3) The plant will Hermie and produce seeds. 

There are a few types of cannabis that will enter the flowering stage upon maturity regardless of the light cycle, which is called auto-flowering. Growers can place these plants under any light cycle. But the recommended light cycle is still 18 hours of light and 6 hours of darkness. The advantage of this type of plant is that growers can place them with any other non-auto-flowering plants that are at a different stage or place multiple auto-flowering plants that are in different stages together to save space and management effort.

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Vegetables and fruit generally need 14 to 16 hours of light to achieve maximum growth. Vegetable and fruit growth requires full-spectrum grow light to mimic natural sunlight. Rushing the growth with too many hours of light or using non-full-spectrum light not only could cause defections, but also bitterness in taste. 

Leafy vegetables, such as lettuce, usually need consistent light throughout their lifespan. Fruiting plants, such as tomatoes, would require different brightness of light in different stages. More will be covered in the light distance section below. 

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In commercial flower growing, indoor growing can provide much more control over the plants for growers. The amount of light, hydration, and ventilation can all be controlled precisely. On the other hand, the cost associated with grow light and ventilation system also increases. Moreover, there are natural predators that eat insects that harm the plants. When growing indoor, those insects need to be eliminated by hand or other means. 

For flowers, the presentation matters more. It is crucial not to rush plant growth. Most indoor flower growers would give 12 to 14 hours of light per day. Forcing the plants to grow could lead to lower immunity to disease and insects which cause defection. 

Studies have suggested that the red spectrum (640nm – 680nm) could induce blooming. Growers could adjust the spectrum to make flowers bloom at the desired time, or a batch of flowers bloom together.

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UV light is a crucial part of plant growth. Just like to humans, UV light is essentially harmful to plants. Too much UV light could kill the plants. However, giving the plants the right amount of UV light exposure could boost up the plant’s immunity to disease and insects.

 
UV light is present in natural sunlight. It can be braked up into 3 sections of different wavelengths: UVA (400nm - 315nm), UVB (315nm - 280nm), and UVC (280nm - 100nm). UVC is not present in natural sunlight due to the earth's atmosphere. Plants will still grow without UV light. However, UVA and UVB are key factors to plants’ healthy growth. Growers can provide the correct amount of UVA and UVB by simply using grow light that has UV light emissions. 

Under UV light, plants will secrete oil or resin to protect themselves like sunscreen. This could increase harvest quality, such as the improved flavor of vegetables or fruits, the better fragrance of flowers and texture of leaves, a higher concentration of THC and CBD for cannabis. For most plants, UV light can also induce them to grow a larger root mass. UV light can be looked at as a way for plants to exercise.

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IR light is also present in natural sunlight. It will make plant stem grow faster. Because IR light is better at penetrating plant leaves than visible light. A greater share of IR light would essentially tell plants that there are higher plants blocking visible light around them. So that they will grow tall faster. 

IR light is not necessary for plant growth. Using it at the right amount could increase the growth for plant stems.