How Many Plants Can Fit Under a Grow Light?

Figuring out how many plants will thrive under a grow light isn't as easy as "x-wattage can cover y-amount of plants". Those generalizations are outdated and don't work for all types of grow lights.

So in this article we’re putting all those old notions to rest. We'll cover the difference between footprints and strengths of HPS grow lights and LED grow light coverage, and how to figure out how many plants you can grow with a grow light. Then we'll show you how to find put all this info to work and get you the coverage your plants need.

As a rule of thumb, you should plan on growing no more than 8 untrained plants or 1 to 4 trained plants under a single grow light. However, there are three factors that truly determine how many plants can grow under a single grow light: the grow light's footprint, the intensity of the light, and whether you train your plants. Let's take a look at the importance of each aspect:

Light Footprint

A grow light's footprint- or coverage area- is the first thing that determines how many plants you can grow under a light.

The footprint of a grow light refers to the shape and size of the light that comes from it. Two factors determine your grow light's footprint:

1. Shape- The shape of a light will determine the spread of light and the arrangement of plants inside of the footprint, which are the starting points of how many plants can fit under a single grow light.
   • In HID's, the shape of a light is determined by the shape of its reflector.
   • In LED's, the shape of light is determined by the physical shape of the light.  
2. Height above plants- A grow light's height above plants will determine the strength and size of a light footprint.
   • The closer to plants a light is, the smaller and more intense the footprint is.
   • The further away a light is to plants, the wider and less intense the footprint is.

As you can see, the footprint is the space in which your plants will receive light, so it's important to have the right size footprint for a given number of plants.

But it's not just the size and shape of your grow light's footprint that'll determine how many plants it can grow. Size and shape don't necessarily tell you about intensity, which is the test of effectiveness on your plants.

Intensity

A grow light's intensity (measured in PPFD or PAR) will determine how well the plants inside a grow light's footprint will grow. Without optimal, intense light, the widest coverage area wouldn't grow even 1x plant, let along multiple plants.

The PAR of a grow light is determined by its strength. The strength of a light (i.e. the power it takes to generate light) is the first factor into the overall intensity of a light.

• The more power it takes to run a light, the more intense it will be.
• Too intense of light will bleach and/or burn plants (over 900 PPFD); and not enough light will lead to stretching and stunted growth (under 200 PPFD).

So when determining how many plants a grow light can cover, it's important to look at the footprint's intensity.

But how are you supposed to know what's best for plants? Well, plants thrive at the following intensity levels:

• Seedlings and Clones: 200 to 300 PPFD per second
• Vegging Plants: 300 to 600 PPFD per second
• Flowering Plants: 600 to 900 PPFD per second

So when determining how many plants can thrive in a footprint, it's important to focus on intensity as well as size. Moreover, the ideal light will give you optimal PAR across as much of the footprint as possible, as the edges of footprints tend to have less light than in the center.

Training Your Plants

Whether or not you plan on training your plants plays a huge role in how many of them you’re able to grow under each grow light.

Trained and untrained plants grow in different sizes and shapes. These shapes will take up more vertical or horizontal space. This ultimately affects the light footprint and its effectiveness on your plants.

• When you train plants, the canopy of each plant will expand outward. That means less plants will be able to fit comfortably inside the footprint of any given grow light.
  
  • Trained plants typically take up about twice as much space. So if you plan on training plants, make sure your light has the footprint and intensity for as many plants as you need.
• If you don’t plan on training your plants, pay attention to the overall size you want your plants to grow to. Untrained, wild plants can grow in directions you weren't planning. So remember to keep an eye on your plants and their growth under an given grow light to ensure they're getting optimal lighting.
  • You should also space every plant out with at least 1 square foot of room around each.

And while training your plants isn't necessary, it's important to know whether or not you plan on doing it. That way you can optimize your footprint to cover all of your plants optimally.

Put these three factors together, and you'll be able to determine how many plants can thrive under a single grow light:

1. Figure out the size footprint you'll need for your plants (remember: no more than 8 plants/light).
2. Determine the PPFD intensity you need for your plants, and find a light capable of giving you the intensity they need.
3. Determine whether you want to train your plants or not, giving them 1sqft. of room in between each plant.

From there, you'll be able to see how many plants you can grow in your grow room.

The Difference Between HID and LED Grow Light Coverage

The truth is the effectiveness on HID and LED grow lights on plants is roughly the same. Their coverage and light distribution, however, is another story.

You see, LED's can put out similar amounts of intensity as HID's, but use around half the energy. So when it comes to LED's, you have the potential to offer much more intense light- and even larger footprints- than HID's the stronger you go.

Confused? Don't be. We'll use a few Yield Lab HID's and Advance Spectrum LED's to explain the what we mean by comparing their PPFD ratings and footprints:

Remember when we said LED's can have the same (if not stronger) coverage as HID's for less energy? We grabbed a few Advance Spectrum LED's to show you what we mean, because it's pretty shocking:

Looking over these charts, we can come to a 3 important conclusions about how many plants you can grow under a grow light:

1. Generally, grow lights that use 200-400w's can grow around 1-3 plants per light; lights that use 500-700w's can grow around 4-5 plants per light; and lights that use over 800w's of power can grow 6-8 plants per light.
2. LED's can give you more intense coverage on the center of the canopy, but become weaker on the edges. That means LED growers may want to pack their plants a little closer together when growing.
3. HID's offer a stronger light on the edges of the footprint, but it's weaker in the center. That means HID growers can space their plants further out, but they'll need to lower their lights for optimal coverage.

And while these measurements are a good rule of thumb, remember that they’re general guidelines. There are a number of factors that go into figuring out how many plants can fit under a grow light, including...

• For HIDs with other reflectors - like wings and umbrellas- you’re going to get a slightly higher PPFD on the outer edges of the light footprint. However, overall PPFD will decrease, so while you’ll be able to fit some more plants, you’re going to be sacrificing some intensity.
• When it comes to double-ended grow light bulbs, those will emit at least 10 percent more PAR than single-ended ones, which means they’ll be able to grow at least 1 more plant. And that 1 plant can make all the difference in your overall yield!
• LED chip wattages and the amount of chips play a huge role in coverage.
  • The higher wattage the chips in each diode are, and the more of them there are, the more intense the light from that diode will be.
  • The lower wattage and amount, the softer the light will be.
  • Chips that have lenses on them will provide more intense light and a better spread, while those with no lenses provide a softer light.

Growing with HID and LED Lights

Ultimately, the number of plants you can grow under a single HID or LED grow light depends on your skill level and the strength of the light. So as long as you take everything we’ve mentioned above into consideration — from PAR levels to plant training — you’ll get a good idea of the number of plants you can grow under a grow light, whether you're using HIDs or LEDs.

By now you should have an understanding of grow lights and the coverage and light intensity they provide. As such, you’ll be able to determine how many of your plants will be able to thrive under any given grow light and, ultimately, make a well-informed decision.

And remember that when in doubt, you can always refer to one of the HID or LED lights we listed above!