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How Distance Plays a Role in Overall Light Quality, Efficiency

Sometimes, the smallest details of your grow’s design can make the biggest impact. One of these seemingly minor details prove to play a significant role in the success of a harvest: the height at which lights are hung above the crop. Here’s why the distance between LED lights and the plant canopy should matter to every cultivator.

How are your lights working for you?

The ultimate goal of your light fixture is to ensure plants receive enough photosynthetically active radiation (PAR). Plants use this type of radiation to convert light energy into chemical energy for plant growth. If there isn’t enough PAR uniformly distributed, a crop is at risk of being stunted, never able to reach full maturity.

Another important measurement to keep in mind when considering light effectivity is PPFD. This number indicates how many photons of light are actually reaching the plants themselves and being absorbed into the photosynthetic process. This is measured in micromoles per second per meter squared. For mathematicians out there, the formula looks like this: μmol/s/m2.

This measurement can be used to understand the Daily Light Integral (DLI), or total moles of light delivered to a plant during a 24-hour period. A common misconception with lighting is that more light equates to more product. Rather, understanding the DLI will help grows understand how much PPFD is needed to maximize their yields.

Check out our recent post, What Lights do Plants Prefer?,  to learn more about these measurements.

Discovering a happy medium  

Now that we have established the metrics for determining if a LED light delivers the appropriate PAR and PPFD, it is important to also consider how far away these lights are from the plant canopy.  Light follows the inverse square law, which states the intensity of the light is inversely proportional to the square of distance from the source. According to this law, the farther away the light is from the plant will drastically reduce the PPFD to the crop.

However, it is important to keep in mind that even though LEDs run at a much cooler temperature than most older lighting technologies, it generally is not recommended to hang the lights as close to the plants as possible. With lights that do not deliver the full spectrum, this may lead to bleaching or interveinal burns. In outdoor agriculture, this is often described as a sunburn. To ensure rich, healthy growth, gradually increase intensity over time and avoid putting plants too close to their light source. Think of it this way: in a plant’s natural environment, the sun’s light gradually increases as we approach the summer solstice. At the solstice, the light is at its most intense — but it naturally builds up to this intensity, rather than being present through the entire plant life cycle.

The trick is determining a happy medium. Too far away from the plants and you might see stretching as the plant reaches toward its only source of energy. When lights are too close, we see bleaching and other stressors.

Measuring PPFD and PAR throughout the growth cycle

It is important to check in on the crop’s condition throughout the growth cycle. This might lead an experienced cultivator to move the lights closer to the plants, or take it back a few inches. Many lighting technologies have a dimming capability, which can be used to help reduce or increase PPFD levels. Generally, cannabis growers tend to keep the light farther away or dimmer during the vegetative stage, while gradually increasing intensity when flowering.

  • The vegetative stage represents the adolescence of the plant’s life. It is during this stage that the plant establishes it roots system, starts to produce an abundance of leaves and develops a thicker stem for its vascular network. In the natural world, plants are in their vegetative stage usually during the spring and early summer, prior to the summer solstice. When using a 600-watt light, the industry standard calls for 30-38 inches away from the canopy. For an 800-watt light, 32-42 inches away is more appropriate. During this phase, PAR is ideally at 150-450, gradually increasing over time.
  • The bloom stage is when plants reveal if they are male or female, producing buds and seeds, hoping to carry on their genetics. With short-day plants, the bloom stage is often realized in the natural world as the days begin to shorten, in late summer and early autumn, following the summer solstice. A 600-watt light calls for a cozier distance of 18-30 inches. In turn, an 800-watt light should generally be placed 19-34 inches away. In turn, PAR is at 450-900, gradually increasing.

Of course, these numbers are estimates. Each individual grower understands their crop the best, so adjust as needed. Sometimes finding the best solution takes trial and error.

Any questions? Feel free to reach out to our team of experienced lighting and cultivation professionals for advice when installing LED lights.

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