How far should my grow light be from my plants?
It is a simple question to ask, right? But there are various things you have to consider to get the best illumination, growth, and yield. You have to base everything on your chosen species’ light requirements for fast-growing. You can get away with 500 to 700 micromoles, but 800 to 1,000 should be your goal for maximum levels of production.
In this post, we will explore a detailed LED grow light distance chart and all the factors that help determine the ideal distance from the plants’ canopy.
LED Grow Light Distance Chart
| Wattage | Vegetative Stage Distance | Bloom Stage Distance |
| 1000W | 36 to 46 Inches | 21 to 36 Inches |
| 800W | 32 to 42 Inches | 19 to 34 Inches |
| 600W | 30 to 38 Inches | 18 to 30 Inches |
| 400W | 20 to 27 Inches | 13 to 21 Inches |
| 200W | 12 to 20 Inches | 8 to 16 Inches |
What Factors Affect LED Grow Light Distance?
The most important factors that affect LED grow light distance are:
PAR (Photosynthetically Active Radiation)
This simply refers to a measure of light intensity for plants. PAR is measured using a PAR meter, which is a digital measuring device that counts photons that are traveling at specific wavelengths. Usually, it counts photons in the range of 400 and 700 nanometers, which is the band of radiation that people can see.
Is More PAR Better For Your Plants?
Any LED grow light you buy needs to be efficient at turning electricity into a healthy plant usable light. At about four hundred to five hundred nanometers photons usually carry more energy. At around six hundred to seven hundred nanometers the red and orange part of the spectrum have photons with half the energy. In other words, the blue part carries more energy compared to the red part. But here is the thing folks, PAR meters count all the photons as equal, whether it is red or blue.
How Distance Affects PAR
| 600-Watt Lamp | Total PAR / PPFD | Percentage Increase |
| 24 inches | 661 μmol/m2/s | -18% |
| 20 inches | 734 μmol/m2/s | -8% |
| 16 inches | 806 μmol/m2/s | Ref |
| 12 inches | 863 μmol/m2/s | +7% |
According to the chart, raising or lowering the 600-watt LED grow light will have a very significant impact on the growth of your plants. By dropping the light by about four inches, you get a significantly more PAR, but you get into the zone where the light intensity directly under the fixture is way too high and may end up damaging your plants. When you raise the fixture; however, you get an eight percent reduction in the amount of light reaching your plants.
Where do you start in optimizing your lighting setup? You want to hang your lamp as low and close to your plants as possible. Why? The reason for this is that the higher you hang your lamp the more light is spread out on the sides. In an indoor environment, this light will be reflected down onto your plants. But there will be reflectance losses.
There is a trade-off when you lower your light closer to your plants, which is the possibility of damaging your plants.
How Do You Prevent Damaging Your Plants
Look at the species of plant that you are growing and determine what is the maximum level of light that the plants can use. Let’s look at medicinal herbs for example. In the flowering stage, the maximum power level is around 1,200 micromoles per meter squared per second. PAR intensity reduces as you move away from the center point underneath the grow light.
Grow Light Distance vs Coverage
A 100-watt LED grow light works well in a 28-inch grow room. If you double that and work with a 200-watt lamp, you should expect to cover around 56-inch by 28-inch area at a height of 24 inches. But the lighting may not be completely uniform.
The PPFD or PAR numbers may range from 306 to 614. If this is for seedlings, some plants would grow much better than others, especially if the grow chamber is too elongated. If you shorten the area to 44-inch by 28-inch, the PPFD range will only fluctuate by 150. Dropping the light to 18 inches will change those numbers.
What does this tell us? The height or grow light distance makes a big difference. Over the coverage area, we have discussed, 24 inches is the sweet spot. The average PPFD is 584. For an 18-hour light cycle, you get a daily light integral of 37.8. That is really great for normal grow room production. The light level will work for all types of nursery plants, including basic high-light fruiting plants, such as tomatoes.
If you have plants with special needs like Mari tomatoes, which need a 12-hour light cycle. The aforementioned PPFD levels would yield a daily light integral of 25.2. But most indoor growers would shoot for a daily light integral of 41 to 51. That means PPFD values need to be over 950. To get such values, you need to reduce the coverage area, squeezing all the available photons into a smaller space.
So, if you try a 3-foot by 2-foot area at 18 inches. The average PPFD would be 892. If you settle for coverage of 36-inch by 20-inch, you would get a PPFD of about 1054. At 12 hours of light, the daily light integral would be 45.5. At that light intensity, you can cover two eighteen-inch wide plants. This would be fine for a mini grow room.
For a larger coverage area with more plants, the aforementioned dimensions can help you calculate the density requirement for your grow light. You will either need more lights or even stronger fixtures. You can always use two light fixtures at the same distance for greater flexibility over various growing areas.
Based on our figures, we estimate that low-light indoor plants would do well in a 48-inch by 34-inch coverage area with a light height of 27 inches when side reflection is provided.