How far should the LED grow light be from the plant? 

Analysis of the hanging height of LED grow lights for indoor cultivation of cannabis and other plants.

Nowadays, more and more growers no longer use traditional HID and T5 fluorescent lamps, but install LED lighting. Compared with HID light source, LED emits less heat, which means that the proximity of LED grow light to tree crown is different from that of traditional planting lamp. It is also important to understand how the best growth distance affects different stages of plant growth.

This paper summarizes the different lighting requirements of all kinds of plants (including hemp) according to the growth stage of plants. It also discusses how to determine the appropriate distance of LED lighting from plant canopy to nourishing plant growth, as well as the importance of PPFD (PAR) and other popular growth lamp terms.

How to measure the light used for plant growth.
Let’s quickly define PAR and PPFD. PAR (photosynthetically active radiation) describes the part of the visible spectrum that plants “see” and use in photosynthesis (400nm-700nm). PPFD (photon flux density of photosynthesis) measures the amount of light received by plants over time (PAR). PPFD represents the light density received by plants over time, expressed in micromoles per second per square meter [photons].

One way to visualize PPFD is to imagine the sun “pouring” light onto the leaves of plants. When the sun shines on the plants, their leaves are collecting energy. PPFD is a measurement of the amount of light (photons) that the sun “pours” onto plants over time. PPFD is an important indicator because it helps growers accurately measure the light intensity of photosynthesis at the canopy level. This is also important because lights too close to the crown can cause burning, discoloration, growth hindrance or discoloration.

The distance map of LED grow light.

Distance from Plant Canopy

 (m / inch)

Coverage or “Light Footprint”(m² / ft2)*
2m / 79 inches 955lx 670 7.6m2 / 81.8ft2
1.5m / 59 inches 1692lx 1170 5.0 m2 / 53.8ft2
1m / 39 inches 3663lx 1670 3.0 m2 / 32.3ft2
0.5m / 20 inches 12,500lx 2170 1.5 m2 / 16.1ft2
0.2m / 8 inches 50,300lx 2670 0.5 m2 / 5.4ft2

Table the distance between 1:LED grow light and plant canopy (600W LED grow light).

* the coverage will vary depending on thegrow light used. The LED grow light shown here does not use a lens to guide the distribution of light.

Table 1 shows the performance of 600W LED grow light at different distances from the plant canopy. It outlines the light intensity (lux), provides PPFD (μ mol/ mmur2 / smur1 or micromoles per square meter per second), and also shows a “light footprint” or canopy coverage. Intensity, PPFD / PAR and “light footprint” all change with the increase or decrease of the distance between light and plant canopy.

Table 1 also highlights how changing the distance of the same 600W LED lamp changes the light intensity and “light footprint” or canopy coverage received by plants. When the light source is located closer to the crown, the light intensity increases. In general, for the vegetative stage of plant growth, the growth lamp should be installed closer to the plant canopy, while it should be higher (away from the plant crown) during flowering.

Where should I put the grow light?
For seedlings, LED grow light should usually be installed between 24 and 36 inches above the plant canopy-however, this depends on the power of the light source (wattage). Place the LED grow light farthest from the seedlings (~ 36 inches)-this keeps the heat and light intensity low and helps prevent the seedlings from drying. Once the root system is established and budding, the lamp can be moved closer (usually within the first 2-3 weeks).

During the plant growth stage, the LED grow light should be 12-24 inches from the top of the crown. At this stage, photosynthesis needs more light, so the light source should be located near the plant.

As plants enter the flowering stage, the demand for bright light decreases. The top leaf of the canopy should be 18-24 inches from the light source to produce flowers. It is at this stage that the plant grows taller and produces fruit. Depending on the light and the way you want the crops to grow, you don’t have to change the height of the light during flowering, especially if you don’t want tall plants.

How far should the LED grow light be from the seedlings?
In the early stages of growth, the seedlings are fragile and require less light intensity. This means that you do not want to increase the intensity prematurely, because the seedlings will reproduce in a milder way. Depending on the size of the light, the growing light can be safely kept somewhere between 24 and 36 inches from the top of the soil.

How far should the LED grow light be from the plant clone tissue?
Cannabis cloning is a process in which cuttings or cuttings from mature plants are used to grow another plant of the same kind. The growth light distance of LED above the clone is different from the height needed by the seedlings. For clones, they will need strong light to start. Depending on the light intensity and maturity of the plant, there may be a big difference between 14 and 36 inches from the top of the plant canopy.

How far should the LED grow light be used?
As plants grow, their needs change. After the completion of the nutritional stage, the plant enters the flowering or “flowering” stage. For mature factories, they have become a necessary place to flourish. During the flowering stage, the LED grow light should be located between 16 and 36 inches from the plant canopy. Moving the growth light closer increases the light intensity, thus maximizing photosynthesis. However, if the growth lights are too close to the top of the plant, they may cause the growth range to expand, spread or even damage the plant.

What about the other stages of growth?
According to each stage of growth, plants need different levels of light. In order to know exactly how far the light a plant should grow, it is important to consider the different stages of growth. It is also important to consider the power output of the grow light-this is usually related to the power of the light source, which is the main factor in determining the optimal distance of the LED at the beginning of the growth process.

Plant growth can be divided into three stages-seedling raising, nutrition and flowering.

This means that once a healthy root is firmly established, it is time to increase the intensity and reduce the light.

Seedling stage.
In the initial stage as a seedling, the LED grow light should be placed above the plant to prevent the soil from drying.

Some growers may try to explosive the seedlings with high-intensity light to promote faster growth, but this will not help until the plants are established. At this early stage, the seedlings are too fragile and a milder approach is needed.

Vegetative period.
In the vegetative stage, plants respond well to strong light-during this period, plants gradually mature and use photosynthesis to grow rapidly. In order to increase the light intensity, the LED grow light should be located closer to the plant canopy. Strong, healthy stems and roots are the key to successful repeatable yields.Increasing light intensity can promote growth during the vegetative period, but it is important to closely monitor plants to ensure that attention is paid to adverse symptoms caused by too much or not enough light.Flowering stage.
Flowering is the final stage of the plant growth cycle-during which fruit yield and stem growth accelerate. With the transition of plant growth from vegetative stage to flowering or flowering stage, the good practice is the transition process of “phasing”. To do this, gradually raise the height of the LED grow light away from the roof of the plant (using the height recommendation outlined above). Closely monitor the distance and keep in mind the crop height and flowering requirements for a particular plant / crop.The adverse effects of excessive light.
When plants such as marijuana are exposed to too much light at any stage, they usually show disturbing symptoms. Since LED does not emit too much heat, the main problem that needs to be closely monitored is any sign of “light burning”. Other side effects associated with the growth lamp being too close to the plant canopy may be discoloration or growth obstruction / irregularity. Both must be quickly identified and the height of the growth lamp adjusted accordingly.

There are signs of mild burning in marijuana plants, including upward leaves and so-called “bleaching”. Bleaching is the discoloration of white or yellow on the leaves closest to light. Mild burns can also be identified when the veins of the plant turn green and the rest of the leaves turn yellow.

Increase the distance of light-marijuana and other plants.
As growers move crop production indoors, the use of LED planting lights for marijuana production is becoming more and more common. LED also brings benefits to growers because they can use specific spectra to provide more targeted lighting for different crops. Other benefits when considering using LED grow light compared to traditional light sources include lower power consumption, lower forward heat, and increased production in a shorter period of time.

The increased distance of light should reflect the height and plant uniformity desired by the grower, and be dotted with leaves and healthy flowers. If we look at the crop requirements of salad vegetables or lettuce compared to marijuana, we will find that salad vegetables and lettuce are suitable for short-term, wider growth, while marijuana is more suitable for taller and narrower growth.

No matter which crop you use, farmers and marijuana growers want to achieve the highest quality per unit yield in a shorter growth cycle.

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