Determining the Ideal LED Grow Light Setup

Black Dog LED examines layouts and designs for using LED lights in a grow room.

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This article has been republished with permission from Black Dog LED. The original article may be found here.

Determining the Ideal LED Grow Light Setup

Determining the ideal LED grow light setup for your grow depends on a lot of variables:

  1. Your goals:
    1. Vegetative, flowering or both?
    2. Economize vs. maximizing yield
    3. Active growth vs. maintenance
  2. Plant / Crop:
    1. Light demands
    2. Photoperiod (day / night hours)
  3. Growing area:
    1. Size
    2. Shape

By selecting the appropriate footprint coverage using our lights, you can achieve the ideal grow!

Your Goals

The ideal LED grow light setup for your situation depends on what you want out of your grow, for example:

  1. Growth phase use: Vegetative, flowering or both?
    1. If you want to use the growing area for only vegetative or flowering, you only need to consider the lighting intensity for that growth phase. When using the area for both phases of growth, it is best to design for the most light-demanding phase (usually flowering).

  1. Economize vs. maximizing yield.
    1. Once the plants’ minimum lighting requirements have been met, increasing the total light output will increase yield. Up to a point, increasing the total light output by a certain percentage will increase yield by a “linear” equivalent percentage; for example increasing the total light output by 10% may increase yield by 8%, and increasing total light output by 20% would increase yield by 16%. When you start giving the plants as much light as they can handle, this yield return on increased intensity starts to fall off and is no longer linear; for example if you’ve got a very high light output to begin with, increasing it by 10% may only give a 4% increase in yield. Past a certain point increasing the total light output will not increase yield at all, and may even hurt it if the plants are being harmed by too much light.

Plant Lighting Intensity and Duration

Different species of plants (and sometimes even different varieties or cultivars within species) require different amounts of light for ideal growth, and flowering plants often require more light while flowering than during vegetative growth.

The total amount of light received by a plant each day is determined by both the intensity of the light and the duration of the illumination. For example, if you have a light running for 2 hours per day, it only takes 1 hour for a light that is twice as intense to create the same total amount of light.

To measure the total amount of light per day, the Daily Light Integral (DLI) counts the total number of photons per day in the PAR (Photosynthetic Active Radiation) range of 400-700nm. DLI is measured in units of mol·m-2·d-1 [moles of light (mol) per square meter (m-2) per day (d-1), where 1 mole of light is roughly 6×1023 photons].

For every plant there is a DLI range which will maximize growth and production; less than this range will result in poor growth or no growth, and more DLI will be wasteful, costing more than the additional yield justifies, or even harmful to the plant. Determining this ideal DLI range is tricky as it not only depends on the plant type and growth stage, but can also vary with other factors in your growing environment such as temperature, humidity, fertilization / nutrient availability, CO2 supplementation, the exact lighting spectrum (including outside of the PAR range measured by DLI), and more. This deserves an entirely separate in-depth discussion, coming soon.

Complicating matters more, plants can be divided into 3 categories:

  1. Short-day plants flower when the night length is greater than their critical period; until then, they stay in a vegetative growth mode. Cannabis sativa, Cannabis indica, cotton and rice are examples of short-day plants.
  2. Long-day plants flower when the night length is less than their critical period. Peas, lettuce and carnations are examples of long-day plants.
  3. Day-neutral plants do not initiate flowering based on the night length, but instead on some other stimulus such as reaching a particular size or age, or period of cold or drought. Cannabis ruderalis (“auto-flowering” cannabis), peppers, tomatoes and cucumbers are examples of day-neutral plants.

Sometimes very similar plants fall into 2 or 3 of these categories, as is the case with Cannabis, strawberries and mulberries.

Note that some plants still require a dark period each day, even if they are day-neutral, short-day (vegetative stage), or long-day (flowering stage). For example, tomatoes grown under 24/7 light will get chlorotic leaves and grow poorly. Giving plants a 4-6 hour resting period each night is generally beneficial both to your crop and your wallet.

To the extent allowed by plants’ night-length constraints, you can increase the DLI by keeping the light on longer each day, without having to increase light intensity. For short-day plants like Cannabis indica and C. sativa, it is particularly necessary to have high light intensity during the flowering stage, as the DLI cannot be increased by extending the daylight hours.

LED Grow Light Intensity and Coverage

Each model of our PhytoMAX lights uses 5W LEDs to create light. The average light output (intensity) per LED is the same; the larger models simply have more of them to create more light. All Black Dog LED lights are designed to spread the light as evenly as possible over the entire lighting footprint; the intensity of the light within the footprint and the size of the footprint are controlled by how high you hang the light above the plants.

Moving the light closer to the plants will decrease the size of the footprint but increase the intensity because you’re spreading the same amount of light over a smaller area. Moving the light further from the plants increases the size of the footprint but decreases the intensity.

No matter what size light you’re using, you can get an equivalent percentage increase in DLI by either shrinking the footprint area or increasing lights-on hours. For example, you can get a 25% increase in DLI by shrinking the covered footprint by 25% (e.g. from 16 square feet to 12 square feet), or by increasing the hours the light is on by 25% (e.g. from 12 hours to 15 hours).

Growing Area Shape

Our lights have a square footprint area, but that doesn’t mean you can’t use them in rectangular growing areas. There are three options for covering non-square areas:

  1. Use a light big enough to cover the longest dimension:
    1. For slightly-rectangular LED grow light coverage this can be the least-expensive option. Using reflective materials on the longer sides of your growing area will help to make use of all of the light in this setup. If your growing area’s longest side is more than 1.5 times the length of the shorter side, you probably want to use option number 2 or 3 for better, more-economical results.

  1. Use multiple lights:
    1. By hanging two or more lights side-by-side you can create a rectangular footprint. For example, two PhytoMAX 200’s hung 2.5 feetapart (on center) will cover a 2.5 x 5 foot rectangular area, using our maximum recommended flowering footprint for photoperiod-sensitive Cannabis. In a partial combination with option #1 above, these same two lights could also cover a 2.5 x 4 foot area, a 2 x 5 foot area, or even a 2 x 4 foot area.
  2. Use a light mover to “stretch” the footprint shape:
    1. Hanging the light from a light mover will change the shape of the light’s effective footprint. The actual footprint is still square, but by moving this square footprint back and forth in a line, it is effectively transformed into a rectangle. Often the cost of an appropriate larger light with a light mover is less than the cost of two smaller lights without a light mover, making it the most economical choice for many rectangular growing areas. Light movers have a lot of benefits and considerations, detailed below:

LED Grow Lights on Light Movers

Light movers are beneficial for two reasons:

  1. Changing the effective shape of a light’s footprint:
    1. By moving a light back and forth in a line, light movers “stretch” the effective footprint of a light into a different shape, as long as you keep the effectively-covered footprint area the same. For example, a PhytoMAX 800 can normally cover a 5 x 5 foot area (25 square feet) when it isn’t on a light mover, but moving it back and forth can make it cover a 4 x 6.25 foot area or a 3 x 8.3 foot area (both 25 square feet), although each setup requires a different light hanging height above the plants. When on a light mover, the light should ideally be hung at the appropriate height to get a square footprint as wide as the shortest side of the effective rectangular footprint area. For example, to cover a 4 x 6.25 foot area, the PhytoMAX 800 hung from the light mover should be hung at the appropriate height for a 4 x 4 foot footprint (24 inches above the average height of the canopy). This square footprint is moved by the light mover to create the rectangular effective footprint.
  2. Increasing light distribution to raise yield
    1. When a light is hung overhead in a static (un-moving) manner, plants will naturally orient their leaves to catch as much light as possible. This often results in upper leaves shading out the lower leaves and flowers of the same plant. To some extent this issue can be overcome by increasing the intensity of the light, but by simply moving the light back and forth and mimicking the sun’s natural daily traversal of the sky, light can be more evenly and efficiently delivered to all leaves on the plant. We’ve seen 10-20% increases in yield when a light is on a light mover covering the same size area, even when the light isn’t being moved very far.
    2. To some extent, hanging multiple un-moving lights provide many of the same benefits; having the light come from many angles helps to nourish more of the lower leaves. However, placing multiple-light setups on light movers will still increase light distribution and yield.

Considering how inexpensive light movers are to purchase initially, and how little electricity they take to run (typically less than 5W!), light movers are often the most economical way to squeeze extra yield out of a grow light.

Large Areas

When setting up larger growing areas, it is important to consider plant access. You’ll need to be able to reach the plants to take care of them and harvest them; we generally recommend setups with no more than a 4 foot reach to access any plant. This generally requires aisles in larger grows, unless the plants are on movable carts to allow on-demand creation of aisleways.

In general, the ideal setup for larger growing areas minimizes aisle space while still allowing easy access to all plants. This requires adjustment for the exact size and shape of the area, as well as the placement of doorways. The staff at Black Dog LED have decades of experience setting up larger growing operations and we’re always happy to help with lighting setup design; simply contact us or call us at 720-420-1209!

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  1. Want to learn more about subjects touched upon in this article? Check out our articles on subjects such as:
    1. Using Light to Repel and Attract Insects
    2. Optimize Your Grow for an LED Lighting Strategy
  2. Want to get in touch with person? They can be reached via the following methods:
    1. Website:
    2. Email: [email protected]
    3. Phone: (800) 380-2291

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About the Author

Our philosophy is simple: we want to create the best possible plant grow lights available. This means we don’t always worry about providing the cheapest light or even the most profitable light, but it does mean that we don’t cut any corners.