Last week we discussed how to choose the color and intensity of your grow light. This week we are going to dig in to a few more important topics when choosing a grow light.

We will take a look at:

• How many plants or how much space the grow light can support
• What the distance between the grow light and the plants should be
• How long the grow lights need to be on each day
• How to know whether the plants are getting enough light
• How to know whether your plants need more light or less light

Looking for Part 1? Click here to go to that post

Table of Contents

• How to interpret how much space or how many plants your grow light can support
• How to determine how far your grow light should be from your plants
• How to choose the length of time your grow light will be on
• Troubleshooting: How do I know if my plants are getting enough light?
• Troubleshooting: What are some signs I need to adjust my grow light settings?
• Related Posts

How to interpret how much space or how many plants your grow light can support

We are going to look at simple clues that anyone can use to estimate the physical growing space a grow light can support.

What do I mean by the physical growing space a grow light can support?

I mean how large of an area you can cover with plants (one shelf, one large table, one square foot, etc) and know that they will receive an adequate amount of light from your grow light.

Clues to understanding the area your lamp can support

Clue #1 How big the grow light is

The larger that a lamp or LED panel is, the larger the coverage area is likely to be.

For example, a single light bulb is likely to have a smaller coverage area than a large, rectangular panel of LEDs.

BUT there is one important rule when using the physical size of the grow light to compare them:

The intensity of the lights must be about the same.

What is light intensity? The intensity of the light refers to how much usable light your plants are actually receiving.

Click here to read Part 1 of the guide to grow lights for more information about light intensity.

For the example below, we will use PPF to compare.

PPF (Photosynthetic Photon Flux Density) is a measurement of usable light being produced by a grow light each second. The measurement used for PPF is micromoles per second.

To understand more about PPF, how it is calculated, and what it means, check out Part 1 of the Grow Light series linked here.

So now that we know we can only compare lights if the intensity (PPF) is the same, let’s look at an example:

Light A is a single LED grow bulb that delivers a PPF of 50 micromoles per second.

Light B is an LED panel that is about 18 inches by 12 inches in size and also delivers a PPF of 50 micromoles per second.

Light A is a small bulb that is approximately 3 inches in diameter so its footprint is small. Light B is a larger panel (18 in x12 in) providing a larger area of light.

Couldn’t you mount the single LED grow bulb higher to spill light into a larger area?

Yes, but the higher the light is mounted from the plants, the less usable light they are going to receive.

Think of the sun and your potential for getting a sun burn. If you are 5 feet from the sun, you will probably catch on fire and burn up. If you are standing on Earth, 91.98 million miles away from the sun, you might get burned, but you’ll live.

The same principle applies to grow lights.

Want another example? Let’s think of the light bulb as a shower head instead:

A shower head a few inches above your head is going to deliver a lot more water pressure than one mounted 10 feet above your head. The one mounted at 10 feet will still get you wet, but you’ll receive less water per second at a much lower pressure.

If you are just wanting to feel refreshed, a shower head 10 feet above your head might still do the job.

But if you have mud caked on your skin after doing work outdoors, using a shower head 10 feet above your head is going to make removing the dirt more difficult because the water will be trickling instead of blasting on you.

It’s a strange example, but it works the same way with plants and usable light!!

Clue #2 What the wattage of the lamp is

The higher the wattage, the more powerful the light is likely to be.

The more powerful the light is, the larger of an area it can potentially support.

A lot of sources make estimated recommendations and they are all different. I find this to be true of most specifications for grow lights.

HOWEVER, I’m going to give you the recommended wattages I would personally shoot for based on all of the different sources I have consulted.

Feel free to use these as well or research a bunch of different recommendations to see what feels right for you!

For Low Light Plants (75 to 200 footcandles) – I aim for at least 15 watts per square foot of grow area

For Medium Light Plants (200-500 footcandles) – I aim for at least 30 watts per square foot of grow area

For High Light Plants (500-1000 footcandles) – I aim for at least 50 watts per square foot of grow area

The recommendations above are using the assumption that the grow lights will provide all the light a plant needs to thrive.

If you are just supplementing the natural light you have, the wattage of the grow lights could potentially be less than the recommendations above and still help your plants to thrive.

Specific calculations you can use to determine the area your lamp can support

Are there calculations that someone can use to determine the exact growing area a light can support?

Yes, there is a way to use the actual amount of usable light (PPF) from a specific distance (PPFD) with the size of your growing area to determine how many lights the growing area needs to thrive or to determine the growing area that a light can support.

However, these calculations are beyond what I can reliably explain, so I’ll provide a couple ways for you to get the information needed to calculate the exact footprint your light can support.

There are online calculators that can help to provide these calculations, but I don’t know how reliable they are.

One that I have used is linked here: https://www.cocoforcannabis.com/grow-light-calculator/

The calculator tells you the fixture coverage based on the requirements of growing cannabis. Cannabis is a very high light plant, so these calculations will be more than sufficient for most houseplants (unless you are growing something that requires full sun, like citrus trees).

I’ll show how it works using the specifications from the GE Grow Light Bulb, PAR38 Grow Light Bulb for Indoor Plants, Full Spectrum, 32-Watt (linked to Amazon)

If you are interested in doing the calculations yourself, check out the book Gardening Under Lights by Leslie Halleck (linked to Amazon).

The exact calculations are covered on pages 79-80.

How to determine how far your grow light should be from your plants

The recommendation for how far away a light should be from your plants is generally 6 to 12 inches to start.

• 6 inches for full sun loving plants
• 12 inches for plants that do not want full sun

These are starting recommendations because the plants may tell us they need more or less light.

We will cover how we judge whether a plant is saying it needs more light or less light below!

How to choose the length of time your grow light will be on

If you are using grow lights to completely support your plants (meaning that your plants are receiving no natural light), it is recommended to start with the lights on for 12 to 14 hours.

If you are using grow lights to supplement the natural light your plants are receiving, you could begin by giving the plants as little as 4 to 6 hours of supplemental light or as much as 12 to 14 hours of supplemental lighting.

If supplementing, I personally would start with the lights on for 6 to 8 hours a day and then adjust based on how my plants are doing.

Troubleshooting: How do I know if my plants are getting enough light?

1. My plants are growing in a healthy way (they aren’t stretching or etiolating toward the light)
2. The color of my plants’ foliage is rich and healthy
3. My plants are using the water I provide in a reasonable amount of time (within a week)

Troubleshooting: What are some signs I need to adjust my grow light settings?

Signs your plants may need more light:

• the plant isn’t growing
• the plant is stretching (or etiolating) toward the light
• the plant is staying wet for long periods of time
• the plant is turning yellow and mushy
• the plant’s roots are discolored and/or mushy

Ways to remedy the issue:

The plants could be placed closer to the light to receive more usable light.

An additional grow light could be added.

The amount of time the grow light is on each day can be increased.

Other issues that can cause plants to exhibit similar signs to needing more light:

• a plant that isn’t growing could also be caused by:
  • nutrient deficiency
  • it is using its energy to grow its roots instead of the foliage above the soil
  • the plant is recovering from something (sunburn, root disease, a pest attack, shipping stress, etc)
• a plant that is staying wet for long periods of time could also be caused by:
  • the soil is too dense and retaining too much moisture
  • the plant is potted in too large of a container
  • the plant is dormant
• a plant that is turning yellow and mushy or has discolored, mushy, or unhealthy roots:
  • the plant is being watered too frequently
  • the soil is too dense and retaining too much moisture
  • the plant is potted in too large of a container

Signs your plant may need less light:

• Brown, crispy leaf edges (the plant is burning)
• Vines that are drying up and dying back
• Wilting

Ways to remedy these issues:

Increase the distance between your plants and the grow lights

Decrease the number of grow lights used

Decrease the amount of time the grow lights are on each day

Other issues that can cause plants to exhibit similar signs to needing less light:

• a plant that has brown, crispy leaf edges could also be caused by:
  • lack of humidity
  • fertilizer burn
  • underwatering
• a plant with vine tips drying up could also be caused by:
  • underwatering
• a plant that is wilting could also be caused by:
  • underwatering
  • root rot / over watering
  • severe fertilizer burn

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