How Many Grow Lights Do Your Plants Need?

How much light is enough to promote excellent growth in my plants?

That’s a question a lot of growers ask themselves at some point when trying to decide on the best grow light for their plants. Whether you're new to growing or you've been growing for a long time, figuring out how much light you need can be tricky.

If you’re feeling overwhelmed, don’t worry. We’ve created this guide to help you determine exactly how much light your plants will need in a grow tent or grow room environment.

Table of Contents

1. Light Coverage
   1. Research Your Plants Specific Lumen/PAR Needs
   2. Determining Coverage for Your Canopy
2. How to Figure Out Many Grow Lights You Need
   1. How to Calculate the Number of Lights You Need
   2. Determining the Necessary Wattage for Your Plants
3. The Best Grow Lights for Your Grow Room
   1. Editor’s Choice
   2. Perfect for 5ft x 5ft Grow Spaces
   3. For Commercial Growers
   4. Best for Beginners
4. Finding the Right Lights and Intensity for Your Grow Room

If you’re ready to grow your own plants and want to know how much light they’ll need, read on. We’ll give you tips on what to look for and what factors to keep in mind when considering your plants’ lighting needs.

Light Coverage

Knowing how much light your plants need isn’t as simple as choosing any light and then hoping for the best. You’ll have to take into account two important factors:

1. Plant Coverage — This is pretty self explanatory and means the amount of light each individual plant will receive.
2. Canopy Coverage — This type of coverage refers to the light that will be spread over the entire group of plants in your grow tent, otherwise known as the canopy.

Whether you’re going with plant coverage or canopy coverage, it’s essential to know how much PAR or lumens you’re going to need for optimal growth. Remember:

• PAR, or photosynthetically active radiation, is used to measure the intensity of a grow light. This is the primary measurement of HID and LED grow lights because it tells you the power a light will have on your plants.
• Lumens measure the brightness of a grow light, like the brightness of HPS grow lights, MH grow lights, and CMH grow lights.

You might be asking yourself, “Why do I need to factor lumens/PAR? Can’t I just use a certain number of watts per square foot in my grow room?” While you could just base the amount of necessary light for your plants on the size of your grow room or grow tent, doing so won’t give you enough light intensity for all of your plants.

• Plants require a certain amount of lumens or PAR to achieve optimal growth.
• As such, it’s best to figure out the exact amount of lumens/PAR the specific plants you’re growing will need.
  • Notice that we're ditching the whole "x-watts/per sq.ft." method. Now, the coverage area of any given light will have varying PAR/lumens the further out from the center you go. That's why if you have a large growing area and need to use multiple lights, PAR and lumens is going to be especially handy.
• This will allow you to figure out how much light they need more accurately than just trying to get a certain amount of wattage per square foot.

Research Your Plants Specific Lumen/PAR Needs

It’s important that you research the lighting needs of the plants you plan on growing. While you could potentially get decent results from simply estimating, you’ll see the best yields and the healthiest plants with proper lighting.

• Weak Light Intensity: If you don’t provide enough light for your plants, they’ll struggle to grow. This can lead to stunted plants and weak results. Not to mention, you simply won’t get the flavors and aromas you want out of your plants.
• Too Much Light Intensity: Too strong of a light will also cause problems for your grow. If you give your plants more light intensity than what they need, you could end up burning them.

If you want to determine how much PAR or how many lumens you’ll need for each of your plants, it’s pretty simple. All you have to do is a bit of research to find the right values for the plants you want to grow.

• Leafy greens like lettuce and spinach, for example, would need a PAR value of around 200.
• Meanwhile tomatoes, which flower and produce fruit, would need a PAR value of 400 to 500.

Once you’ve figured out the lumens/PAR your plants will need, finding the right light or lights for your grow tent will be a lot easier.

Determining the Coverage for Your Canopy

Once you know what your plants need to grow, it's time to figure out how much coverage you need.

But to figure that out, you've got to first measure the size of the overall canopy you need to light.

1. Measure your canopy's length and width: Your canopy is the part of your plants that absorb light. This is what you want to measure when considering grow lights. Measuring the entire grow space will give you an inaccurate estimate of the necessary light for your plants.
   • Note: the size of your growing area is important because it's the area you can fit a certain amount of plants in.
2. Measure your canopy's depth: Don’t overlook the depth of your canopy. You’ll need lights that are strong enough to penetrate deep into your plants. Simply covering the tops of your plants won’t be enough.
3. Find the Best Grow Light(s) for Your Grow Tent or Grow Room: Now that you’ve determined the size and depth of your canopy, as well as the lumens/PAR you need, you can make the most informed decision and purchase the right grow light.
   • By knowing the PAR/Lumens your plants need, you'll be able to determine how strong of a light you need.
   • By knowing how much area you need to cover, you can figure out how strong of a light you'll need so that it can spread the light it gives off the higher its raised.

How to Figure Out Many Grow Lights You Actually Need

As you can probably tell by now, asking yourself how many grow lights you need upfront is a bit too tricky and broad. Ultimately, a better question to ask yourself is:

How much strength and light intensity do my plants need, and how many lights will I need to achieve this goal?

Thankfully, you now know how to get the answer to that question. You’ve determined the amount of lumens or PAR that each of the plants you’re growing needs. You’ve also figured out the light intensity required for the size and depth of your canopy.

How to Calculate the Number of Lights You Need

Now that you have a general idea of how grow lights, lumens, and PAR work in your grow room, the question remains:

How exactly do you calculate the number of grow lights necessary for a successful harvest?

Here are some step-by-step instructions to help you figure out how many grow lights you'll need.

1. Research the lumen and PAR requirements of the plants you plan on growing. Are you growing tomatoes? Cucumbers? Peppers? Herbs? Whatever it is you plan on growing is going to have unique lumen/PAR requirements, so start there when trying to decide how many plants you'll need in your grow room.
   1. Research the minimum and maximum requirements for your plants. Tomatoes, for example, have a minimum PAR requirement of 400 and a maximum PAR requirement of 500.
   2. Remember: The outer edges of your grow light's coverage are weaker. If you're only using a single light, keep in mind that the outer parts of your canopy will have weaker light intensity. Depending on the plant, you may need to invest in more than one light to ensure your entire canopy is receiving strong doses of light.
2. Measure your canopy's potential size. Your plants will take up a certain amount of space in your grow room or grow tent. Measure the ideal space your plants will take up. By figuring out an accurate length, width, and height, you'll be able to decide on the proper grow light coverage and strength.
   
   1. Keep in mind the size of the plants you're going to grow. Ideally, you want to avoid crowding your plants together. For best results, be sure your plants have adequate space so they can grow comfortably.
      • If you have a small grow space but you want to grow large plants, you may only be able to fit 1 to 2.
      • If you're growing small plants, you'll realistically be able to fit more in that same small grow space.
      • Whatever the case may be, make sure you account for their growth so you have enough space for fully-formed plants.
3. Choose a grow light based on the coverage and intensity your canopy will need. With all of the factors mentioned above taken into account, you should now be able to more accurately and easily select the perfect grow light or grow lights for your plants.
   1. Consider your grow light placement if you're using multiple lights. If you need even coverage across your canopy, aligning multiple lights will essentially create a third intense point by overlapping the light coverage on the edge of each grow light. This will give you 2 or more strong points of light coming from the center of the light footprint, as well as intense light in the places where intensity would normally drop off.

Determining the Necessary Wattage for Your Plants

Figuring out the necessary lumens/PAR for your plants will determine the wattage you need from your grow lights.

When growing with HIDs (HPS and MH lights, for example), consider these options:

• Yield Lab 400W HPS Air Cool Hood Reflector Grow Light Kit (Price: $184.95) — A 400w HPS light provides 55,000 lumens. This light intensity is enough to provide coverage for 1 to 2 plants.
• Yield Lab 600W HPS Air Cool Hood Reflector Grow Light Kit (Price: $209.95) — This grow light kit can deliver 90,000 lumens with its HPS bulb. As such, it could provide solid coverage for up to 4 plants.
• Yield Lab 1000W HPS Cool Hood Reflector Grow Light Kit (Price: $229.95) — With a 1000w grow light, you’ll get 130,000 lumens from an HPS bulb and 110,000 lumens from an MH bulb. That’s sufficient to provide coverage for up to 6 plants.

If you’re using LED lights in your grow tent, you should consider these LEDs and their values:

• Advance Spectrum 240W Sun Series Model E 4-Bar Full Spectrum LED Grow Light (Price: $399.95) — An LED grow light like this 240w has an ideal footprint of 6sqft. Its max PAR rating is 436 PPFD at a 24-inch height. This makes it recommended for 1 to 2 plants.
• Advance Spectrum 400W Sun Series 4-Bar Full Spectrum LED Grow Light (Price: $595.95) — This LED grow light has a footprint of 9sqft at a 2ft height. It also has a PAR rating of 897 PPFD at a 12-inch height, so you can grow 4-5 plants through the vegging and flowering stages with no issues.
• Advance Spectrum 900W Sun Series Model E 8-Bar Full Spectrum LED Grow Light (Price: $995.95) — You’ll get a lot of power from an LED like this. The PAR rating for this light is 1618 PPFD at a 12-inch height. That means its got a minimal footprint of 16 sqft and will provide coverage for 7 to 10 plants.

As you can see, grow lights with different wattages provide coverage for different numbers of plants. The key is to find a grow light that has the right amount of lumens or PAR for the plants you’re growing — and from there you’ll decide on wattage.

Need Stronger Coverage? Here's What to Do

Now, lumens and PAR begin to weaken on the outer edges of that footprint. You’ll want to keep your plants well within that footprint. If you’re growing a lot of plants in a larger space, your best bet is to install multiple grow lights to overlap those weak points.

• For more intense coverage, one option is to lower your light closer to your plants. The lower a light is, the more intensely your plants will feel it. However, doing that will decrease your coverage area.
• For increase intensity, you can add more light(s) to your grow. This has the potential for increase PAR significantly, so it' important to raise or dim your lights if it's starting to get overwhelming for your plants.

Need to Cover More of Your Canopy with Light? Here's the Solution

If you need to cover a large area (like in a commercial grow, for example), you’ll want to use multiple high lumen output grow lights that you can hang higher than usual. Multiple lights also help give your plants much more intensity- as we've already demonstrated. But there are a couple of key things to look out for when you need a larger footprint across your canopy:

• Hanging Lights Higher Increases Coverage Area, Lowers Intensity- To get more coverage, the first thing to do would be to raise your lights. Keep in mind the higher you position the lights in your grow tent, the weaker their intensity will be on your plants. So be aware of how much light you're sacrificing if you need to raise your light.
• Re-evaluate your needs- If your plants need more intensity and/or coverage than you're giving them and you're running multiple lights already, evaluate the lighting you have. You may need to upgrade your lighting to stronger lights.

The Best Grow Lights for Your Grow Room

Now that you know the importance of lumens and PAR in relation to your grow room, it’s time to pick out the best light or lights for your grow room. Below are some of our top recommendations. Whether you’re going with HIDs or LEDs, we’re sure you’ll find the perfect light grow light for your plants.

Editor’s Choice: Yield Lab 600W HPS Air Cool Hood Reflector Grow Light Kit

If you want an all-around excellent HID grow light, we recommend the Yield Lab 600W HPS Air Cool Hood Reflector Grow Light Kit.

This HID kit will give you great coverage and light intensity for up to 6 plants. And because it includes an HPS bulb, your plants will be good to go for their vegetative and flowering stages.

Though HIDs are known for higher temperatures than LEDs, you can keep the light’s heat controlled with the digital dimming ballast. On top of that, this kit also features an air cool hood, so your grow room’s temperatures will never reach extreme levels.

HPS Grow Light Specs

• Wattage: 600w
• Lumen Output: 90,000 lumens (HPS bulb)
• Coverage: 4 plants (2x4 grow tent or 4x4 grow tent)

Our Price: $161.71 | Click Here to Buy This HID Grow Light

 

Perfect for 5ft x 5ft Grow Spaces: Yield Lab Pro Series 1000W HPS+MH Air Cool Hood Double Ended Complete Grow Light Kit

The Yield Lab Pro Series 1000W HPS+MH Air Cool Hood Double Ended Complete Grow Light Kit is all about big coverage and intensity.

This HPS and MH grow light kit will provide the necessary lumens to up to 6 plants in a 5x5 grow tent through the vegging and flowering stages. The power of this 1000w light is perfect for providing the coverage and depth to reach all of your plants.

If you’re growing plants in a 5x5 space and you’re looking for an HID grow light, it doesn’t get much better than this HPS and MH light kit.

HPS and MH Grow Light Specs

• Wattage: 1000w
• Lumen Output: 159,200 lumens (HPS bulb); 85,200 lumens (MH bulb)
• Coverage: 6 plants (5x5 grow tent)

Our Price: $315.95 | Click Here to Buy This HID Grow Light

 

For Commercial Growers: Yield Lab Pro Series 120/220V 1000W Double Ended Complete Grow Light Kit

For growers working with a lot of plants, the Yield Lab Pro Series 120/220V 1000W Double Ended Complete Grow Light Kit will get you huge results.

With a peak PAR output of 1776 PPFD, this grow light packs a huge punch for 6-8 plants. Plus it hood design allows for a wide but focused spread of intense light that will penetrate deep into your plants.

So whether you're in a grow room or a warehouse, this grow light is what your plants need for huge growth and big flavors.

HPS and MH Grow Light Specs

• Wattage: 630w (2 bulbs x 315w)
• Lumen Output: 33,000 lumens per bulb
• Coverage: 6 to 9 plants (5x5 grow tent or grow area, 6.5 x 6.5 grow tent or grow area)

Our Price: $289.95 | Click Here to Buy This HID Grow Light

 

Best for Beginners: Yield Lab 400W HPS Wing Reflector Grow Light Kit

Looking to start small? The Yield Lab 400W HPS Wing Reflector Grow Light Kit is the best HID grow light kit for beginners.

This kit includes an HPS bulb to help your plants get through all stages of growth. It’s perfect for 1 to 2 plants, though you could potentially squeeze in a third small plant.

At 400w, you’ll get enough light intensity for a small grow. In addition, priced at under $150, this grow light kit is a great way to get introduced to indoor growing!

HPS Grow Light Specs

• Wattage: 400w
• Lumen Output: 55,000 lumens (HPS bulb); 36,000 lumens (MH bulb)
• Coverage: 1 to 2 plants (2x2 grow tent)

Our Price: $144.95 | Click Here to Buy This HID Grow Light

Finding the Right Lights and Intensity for Your Grow Room

You can’t overlook the importance of light in your grow tent or grow room. Your grow lights make up one of the most important parts of your indoor grow.

Whether you’re using HPS grow lights or LED grow lights, you’ll want to make sure you provide the right amount of intensity and coverage for your plants.

With the information and tips in this article, you’ll know how to effectively calculate the necessary lighting for your canopy. That way you’ll see rich harvests and have peace of mind knowing that your plants aren’t lacking exactly what they need.

Editor's Note: This article was originally published May 7, 2015, and has been updated for accuracy and detail as of April 2023