Nowadays, more and more growers use LED grow lights instead of traditional T5 fluorescent light and HID lights.Compared with HID light, LEDs emit less heat,So, the distance of LED grow lights to the canopy is different from that of traditional grow lights. It is also important to understand the optimal growth light distance varies with the different stages of plant growth.
In this post,we will explore the different lighting requirement of various plants according to their growth stage, how to determine the proper distance for grow light to the plant canopy,the importance of PPFD (PAR) and other grow light terms.
How to measure the light used for plant growth
There are two terms of PAR and PPFD. PAR (Photosynthetically Active Radiation) describes the range of the visible spectrum (400nm-700nm) that plants need for photosynthesis. PPFD (Photon Flux Density of Photosynthesis) measures the amount of light (PAR) that plants get in a certain period of time,the unit is the micromoles per square meter per second.
PPFD is an important indicator because it can help growers accurately measure the light intensity of photosynthesis at the canopy level.
How Far Should Grow Lights Be From Plants
Whether it is growing fruit or vegetable plant, the plant needs a certain amount of light,while the light intensity plant got will vary with the distance between the plant growth light and the plant.So, how far should the grow lights be from plants. The short answer is “it depends”, but you also view the recommendations below:
The distance between the LED grow light and the canopy:
In general, the height of grow lights to canopy at 24-26 inches is better for the seedling stage of growth (the first 3 weeks), and at 18-22 inches is better for the nutrition and flowering period. The a little higher or lower height depending on the light is powerful or weak.
The more troublesome thing is that almost every LED grow light is different because they are from different manufacturer and different power.
Of course,you could get the recommend hanging heights from manufacturers, but these are only estimates and are not always suitable for all situations.
When the distance between the plant and the growth light is too far, It will cause the plant stem stretch because the plant stretches its stem to get closer to the light source and get more light intensity.If you encounter this happens, it means that the light intensity is not enough,you should should hang the growth light closer to the plant.
If you can not get a recommended distance, you can hang it according to the chart below：
| LED Wattage | VEG | Bloom |
| 30-40W | 12-20 inches | 8-16 inches |
| 80-100W | 10-27 inches | 13-21 inches |
| 120-150W | 30-38 inches | 18-30 inches |
| 160-250W | 32-42 inches | 19-34 inches |
| 300W-500W | 36-46 inches | 21-36 inches |
How to determine the distance and lighting time
It is not hard to purchase an LED grow light for indoor plants.However,How to use it better is the another problem. The key to this is the distance and lighting time control of the plant light.
LED grow light lighting distance
There are two factors to be considered including the distance of the grow light to plant. Other is the distance between the grow light and the adjacent light when planting in a large area.
The distance between the grow light and the plant
First of all, It is reasonable to install LED plant growth to ensure the illumination efficiency of plant lights, when a single grow light or a small area that can be illuminated by a grow light,It should ensure the installation of the light is directly above the plants, and the layout of the plants should be arranged according to the effective irradiation range of the LED plant growth light.
Determine the light distance according to the needs of the plants. First determine the amount of light intensity that the plants need. When determining the light area, LEDs starts with the light distance is about 0.5 meters, depending on the different power and the needs of the plants, then adjust the distance appropriately.
The LED grow light is a cold light source and emits very little heat. It can be closer to plant in special applications such as tissue culture and seedling cultivation.
The distance between LED plant lights and adjacent lights
When the area is relatively large and not within the one range, we need to use multiple LED grow,and refer to the uniformity of the color on the ground then then adjust the distance appropriately.
LED grow light lighting time
The growth of plants is more complicated, and the light required for each growth stage is different. For plants in the growth period, In general, it will be used for 5 hours under the condition of a certain amount of sunlight, and the time is controlled at about 10 hours on cloudy and rainy days, and about 12 hours when there is no sunlight at all.
Since plants growth also need dark environment, it should not provide the light at night for a long time.
There are many types of plants and different factors, so the actual situation are extremely complicated,we need to constantly explore and get the best results.
Inverse square law of light to determine the distance
The inverse square law of light is a physical law that can help determine at a specified distance intensity of the grow light to the plants,which will help you determine the right distance.
The formula is intensity = light output/(distance*distance)
It can be applied to plant lights. When the distance between the plant and grow light is doubled, the area covered by the light will increase, but the intensity of the light will be reduced to 25% intensity of the original distance.
The Distance to HID grow light (metal halide and HPS)
Both Metal halide light and HPS light wil produce more heat than LEDs, So,we can test HID heat with the back of the hand as a general rule.
Just place your hand on the top of plant and the back of your hand facing the light source for 30 seconds. If you feel too hot and can’t bear it,the same as your plant,So,go ahead to increase the distance.
How to get the start distance to test?You can view the chart below:
| Bulb Wattage | Metal halide | HPS |
| 150W | 4-10 inches | 4-12 inches |
| 250W | 5-13 inches | 5-15 inches |
| 400W | 7-18 inches | 7-21 inches |
| 600W | 10-27 inches | 10-28 inches |
| 1000W | 12-31 inches | 12-35 inches |
For HPS light, it can improve efficiency and reduce investment when choosing high-power light as much as possible. For example, many commercial growers generally choose 600W or 1000W power.Click here to know more what size of grow light do i need.
The distance to fluorescent grow lights
There are four types of fluorescent growth light-T5, T8 and T12 and compact fluorescent light.
The fluorescent light do not product more heat than other lighting fixture,So,you can be closer to the plant only they are not touching the plant.
To obtain the ideal distance,you can also use hand test method like HPS light.
We can start with 4 to 6 inches on the canopy of plant for flowering or vegging.
For seeding, It is a reasonable height about 6-12 inches to start because they need higher light intensity.It is a reasonable height about 12-16 inches to plant from nutrition to flowering,
In general, when using T5, it is best to keep them as close as possible, but monitor for overheating or drying out.
Wrapping up
The distance between the grow light and the plant is closely related to the power and type of the light fixture, So,when using the grow light light, we can first inquiry the seller to obtain a relatively accurate distance. If not, we can follow the above the rule of thumb, calculate and find a reasonable distance. In addition, the distance between the grow light and the plant is also related to the growth state of the plant and the size of the plant,We should adjust the height according to actual situation.