How Many Plants Can You Grow Per Light?

As a merchant, one of the most frustrating questions that I get asked all the time is, “How much area will this light cover?” Even worse is this question: “How many plants can I grow under this light?”

The reason both of these questions are difficult to answer is that there’s a whole lot more to it than just a simple cookie-cutter answer.

I have seen so many different garden lighting configurations over the years that it makes my head spin. Some growers want to see how few lights can be used in their garden space, while others try to cram in as many as they can. Extreme examples range from two 1,000-W lamps on movers covering four 4- by 8-ft. trays to four 1,000-W lamps over a single 4- by 8-ft. tray.

I decided to write this article after receiving the following email from a grower:

“I need to know about how much square footage your 1,000-W fixtures cover. For example, if I had a 3,000-sq.-ft. warehouse with a 14-ft. ceiling, how many fixtures would I need?”

There are several problems with this question. The total square footage of a building tells me nothing about how it’s designed. Many configurations can occupy 3,000 sq. ft.

Even when I'm provided with the dimensions of a grow space, it's still a challenge to recommend a number of lights to cover the space because it can’t all be dedicated to growing plants. Aisles are needed as well as access to entrances and exits, plus space for fans, filters and supplies.

Additionally, your arms are only so long. If you plan to access each plant individually, you will need to take this into consideration. Three- to 4-ft. trays are as wide as you can reach, assuming you have access to both sides. Any larger and some serious acrobatics might be necessary to maintain the garden.

Light Levels

Even when you know exactly how much garden space will be illuminated, how much any particular light will cover also depends on what you’re growing as well as the stage of growth.

Plants like wheatgrass require low lumination, whereas tomatoes require much higher levels. Light requirements also vary for each stage of plant growth, from the seedling stage to vegetative growth to the blooming stage.

Seedlings and cuttings require the least amount of illumination while plants in bloom generally require the highest amount of light. Plant light requirements are listed in micromole (µmol) values, not in lumens.

Lumens describe the amount of visible light rather than photosynthetically active light, and are used to determine lighting requirements for humans, not plants. Crop requirements in micromoles can be looked up online for various plant crops.

Once you have an understanding of how much light your garden needs, you can begin to plan the number and placement of light fixtures. Most manufacturers can provide some basic guidance on the output of their lights.

Ultimately, you will need a quantum meter tuned for the type of light used (HID, induction, LED, etc.) to accurately measure delivered garden illumination when growing in a large, multi-light garden.

A quantum meter measures in µmol, the actual number of photons contained within the light striking the surface. Knowing the photon density will allow the gardener to move lights closer together, further apart or up or down, creating better light uniformity and density.

Too Much Light?

It's often said bigger is better, but this is not the case with garden lighting. For many commercially grown plants, scientists have worked out the maximum amount of light a plant can receive on a daily basis. This measurement is called the Daily Light Integral (DLI).

Exceeding this amount will cause the plants to defend themselves from the light instead of using it to create valuable sugars for growth. Never exceed DLI values—too much light does not necessarily mean larger harvests and can harm your plants!

Do Your Own Homework on Grow Lights

When it comes to how many lights your garden needs, kind of like everything in life, you need to do your own homework. Lighting is the single largest investment in an indoor garden, so you need to make informed decisions.

Solely relying on opinions of others, especially anonymous strangers on Internet forums, is not the best method for making this critical decision.

Just because someone is having fantastic results in one situation does not automatically mean it will work for you in your situation. Experimentation is required to find the perfect set-up in many cases.

So ultimately, how much does “this light” cover? The answer is like that old accounting joke: What do YOU want it to be?

For more information on lighting up your grow room, check out Maximum Yield's lighting article archives.