You've got grow lights, your plants, all of your liquid nutrients, and everything you need to grow. You set up your grow tent, set your plants in place, strap your rope hangers to your lights and hang 'em. But before you switch those lights on, are you sure you know how high you need to hang them above your plants?

Hanging your lights too low can burn your plants, but hanging them too high can leave them stretched and weak. Some grow lights need to be hung far away from plants so they don't get bleached, but some lights are too weak to hang more than 10 inches away- so what do you do? The trick is to hang your lights right in that sweet spot of effectiveness and safety. The distance of your lights to your plants is all about getting your plants the power they need to turn nutrients into food for growth.

How To Measure Light for Plants

Let's quickly define PAR and PPFD. PAR (Photosynthetically Active Radiation) describes the portion of the visible spectrum that plants "see" and use for photosynthesis (400nm-700nm). PPFD (Photosynthetic Photon Flux Density) measures the amount of light (PAR) a plant receives over time. PPFD represents the light density a plant receives over time and is measured in micromoles per square meter per second.

A way to visualize PPFD is to imagine the sun is "pouring" light onto the plant leaves. As the sun pours light onto the plants, their leaves are collecting the energy. PPFD is the measurement of the amount of light (photons) the sun "pours" onto the plants over time. PPFD is an important metric because it helps growers
accurately measure the light intensity for photosynthesis at the canopy level. It is also important because lights placed too close to the canopy can cause burning, bleaching, stunted growth, or discoloration.

LED Grow Light Placement

If you do not have reflective walls, the inverse square law is very important, and you should never raise the lights higher than the manufacturer's recommendations. Moving the lights too high in this case greatly reduces light intensity and yield.

Reflective Walls
While the inverse square law applies to any light - these changes drastically when you use reflective walls.
Reflective walls bounce the light back to your plants and greatly reduce the light loss from moving the lights higher. So, when using reflective walls in a grow room or grow tent, it is possible to place the lights much higher without losing a large percentage of the light.

If you have efficient reflective walls - going higher is normally better than lower regardless of which LEDs you are using. With reflective walls, the light bounces from many different angles into the plant and you will get move even yields and deeper penetration into the canopy.
The distance between your LED lights and your plants is, in fact, an exact science. Seemingly minor adjustments make a huge difference when it comes to giving your plants the kind of light intensity that makes them flourish, without giving them so much that they burn and wither.

Oveerlapping Lights
In larger grows where there are many lights in the same room along with reflective walls, it is often more effective to puts the lights higher so that the is overlap between many lights. This means that each plant will get light from many angles and this will promote much better penetration into the canopy and better overall yield.

What is the distance of the plant in each growth stage of the LED light?

In general, LED grow lights should be 12–30 inches from the top of the plant. As the plant grows, you will adjust the light to maintain a consistent distance from the plant. However, the distance will vary depending on the wattage of the lamp, the number of plants, and the amount of ambient light.

Again, there is no general rule for the distance between LED lights and plants at each growth stage. It is important to understand this. Different manufacturers manufacture their LED lights in different ways. There are many details to consider, such as the power and angle of the individual LED diodes in the luminaire, and the use of lenses to reflect light downward. The best way to obtain the perfect distance between the lamp and the canopy is to consult the manufacturer.

The second thing to remember is that the correct LED light placement changes during each growth stage. If your LED is dimmable, you can use dimming to adjust the light intensity on the surface of the plant. If your LED is not dimmable, your only option is to change the suspension height. These examples are not hard and fast rules, so don't follow them without the support of the lamp manufacturer. But just to give you a general idea of ​​the position of the LED light in the entire growth cycle:

The placement of LED lights in the seedling stage

Seedlings need a minimum intensity and maximum distance between the canopy and the light. You may instinctively illuminate these little guys with high-intensity light to promote their rapid growth. The fact is that they are young, fragile, and more likely to thrive in a gentler way.

Vegetative stage
During the plant growth stage, you want to bring the LED light closer to the plant. Generally speaking, the plants in vegetables only need about half of the light they need to bloom. Therefore, you can have up to twice the hanging height used during flowering.

Flowering
During the flowering stage, you really need the highest intensity. When you transition from vegetable to flowering, it is important to establish in the "hardening" stage. Slowly increase the strength or decrease the suspension height until the flower height recommended by the manufacturer is reached.

Again, these are general guidelines. Discuss the ideal distance with your manufacturer and pay close attention to whether your plants show signs of overload. If your plants have too much light, they will notify you.
How to Recognize Light Burn On Your Plants

Because LED lights give off so little heat, you don't run much risk of causing heat damage by placing your fixture too close to the canopy. What you do risk is light burn. Unfortunately, the light burn is easily mistaken for nitrogen deficiency, and many growers wind up misdiagnosing their grow and addressing the wrong problem.

If your plants have light damage, you'll see symptoms such as:

Leaves point upwards.
Bleaching, which takes the form of white or yellow discoloration, typically on the leaves closest to the light.
Veins remain green even as the rest of the leaves turn yellow.
A nitrogen deficiency has similar characteristics, but there are a few notable differences.

A plant with a nitrogen deficiency also develops yellow patches, but the damage starts from the bottom of the plant and works its way up, while light burn begins at the top, closest to the lights.

In the case of a nitrogen deficiency, leaves wilt. In the case of a light burn, leaves turn upward and sometimes even become brittle.

The leaves of a nitrogen deficient plant fall off on their own. This does not happen to plants suffering from a light burn.

If it sounds like your plant has a light burn, it's time to reevaluate the placement of your LED lights.

Conclusion
The intensity or distance between your plants and LED lights is crucial to the health of your grow. Run the lights too high power or place the lights too close, and you can damage the plant. Underpower or hang them too high, and you miss out on the highest possible yields and potency.