All You Need To Know About Grow Lights

Gone are the days when the only way to grow crops, greeneries, and other veggies was to kneel in wet dirt outside.

Nowadays, we can grow virtually everything indoors and even do so more effectively if we know what we’re doing. Growing veggies and greens indoors is a great way to maximize the use of your indoor space without being subject to weather conditions.

And one of the keys to indoor gardens? Grow lights.

In this article, we cover what you need to know about grow lights, the types of grow lights, how they affect your garden’s yield and how to set them up.  We’ll attempt to answer all of those and more below.

What are grow lights and how do they work?

Grow lights are artificial lights designed to help optimize the growth of your indoor garden. They are different from normal light bulbs in that they prioritize different parts of the light spectrum’s output. That’s because their goal isn’t to produce efficient visible light but to help your plants grow.

So, how do grow lights work?

There are two main things you need to familiarize yourself with here – the light spectrum output and the light intensity output.

What Is Light Spectrum Output?

We’ve all learned about the visible light spectrum in school. It includes the different wavelengths of light (measured in nanometers or nm) which dictate the color of visible light.

  • 400 to 500 nm is purple to blue
  • 500 to 600 nm is blue through green and to yellow
  • 600 to 700 nm is yellow to red.

That 400 to 700 nm spectral range is the visible light’s wavelength. It’s called PAR (photosynthetically active radiation)

Why does this matter for grow lights?

Because it’s the blue and red spectrums of PAR that are beneficial to plants’ growth cycles. Specifically, the 400-500 nm blue light causes vegetation and the 600-700 nm red light causes flowering. The “middle” 500-600 nm green light is largely unnecessary for plant growth because plants tend to reflect it (hence why they look green to us).

It’s actually the spectrum of blue light that attracts the plant foliage – that’s why plants tend to bend toward the sun when possible (it’s called positive tropism).

So, for optimal grow lights, you’d need bulbs that prioritize blue and red light. Some users are tempted to buy separate red and blue lights but that’s usually too expensive. Instead, simply opting for well-made and specialized red-blue grow lights is usually your best bet.

What Is Light Intensity Output?

In other words – brightness. The grow lights’ intensity output or brightness is even more important than the brightness of ordinary lights. It’s measured in lumens and it should be carefully calibrated based on the type of plants your growing and the grow lights’ distance from them.

This sounds simple enough at first – just research how many lumens your plants need and get the proper grow lights, right? Not quite. The problem here is that light is dispersed very quickly after being emitted from the bulb. This means that the lumens of the light as it comes out of the bulb are not the same as its brightness after traveling a foot or two. Put even simpler – light gets dimmer with the distance it travels.

So, what can we do about that?

Here’s a simple formula for calculating the light intensity’s dispersion over distance:

  • I = L / D^2  (Intensity = Light output divided by the distance squared)

So, for example, a 1,000 lumens grow light that’s placed two feet away from your plants means your plants will only receive ~250 lumens of light. The other 750 will get dispersed in the air.

And this gets us to our next point:

How Should You Place Your Grow Lights And Why?

With what we said above about light intensity getting lost with the distance traveled, the intuitive response of most people is to put the lights as close to the plants as possible. And while it’s true that we should try to keep the lights as close to the plants as possible, there are a couple of complicating factors here:

  • The closer you put the grow lights to your plants, the fewer plants they’ll be able to cover. This makes choosing the right position and distance of your grow lights a tricky balance to strike. The room size and interior design also matter. Grow lights manufacturers will detail the correct usage of their particular lights on/in the lights’ packaging so choose according to your own setup.
  • Grow lights, like all other light sources emit heat. This means that if you put them too close to your plants, you’ll risk burning the latter. Different types of grow lights emit different amounts of heat and we’ll cover those below.

For now, let’s take a look at the general rule of thumb for the optimal distance of grow lights. We’ll use the standard wattage that most lights use and not their lumens but again – adhere to each specific lights’ own guidelines.

5 by 5 feet room or 1.5 by 1,5 m1000W31 to 37 inches or 80 to 95 cm
4 by 4 feet room or 1.2 by 1.2 m600W24 to 31 inches or 60 to 80 cm
3.5 by 3.5 feet room or 1 by 1 m400W16 to 24 inches or 40 to 60 cm

What Impact Does The Grow Lights Choice Have On Your Garden’s Yield Rate?

We’ll cover some more of the grow lights’ specifics as well as the 3 main types of grow lights below. First, however, let’s briefly explain why it’s vital to pick the perfect grow lights for your indoor garden.

Light, together with water and nutrients from the soil, is one of the three most important factors for successful plant growth. Furthermore, different plants have different plant requirements or different “Photoperiodism”  – the amount of light and darkness they require to grow well.

In fact, while we’re used to saying that plants “need light” it can just as well be said that they require darkness as well. Striking the right photoperiodism cycle for each plant is crucial for helping it grow well.

Some plants require a lot of light, others – just a bit. The simplest way to differentiate between them is to categorize them in two groups – short-day plants and long-day plants:

  • Short-day plants require less than 12 hours of light per day to bloom as well as possible. These include plants such as the salvia, soybeans, green onions, and others.
  • Long-day plants need 12+ hours of light each day. Popular examples include the petunia, spinach, lettuce, and others.

If you’ve ever taken care of an outdoor garden you’ve likely noticed that as well. The difference between these plants is that the former (short-day plants) typically grow in the shadow of the latter (long-day plants). That’s why even for a simple outdoor garden the setup is important – which plants will be in the shade by the fence, while will be in the middle of the garden, which will be under the trees, etc.

For indoor gardens with grow lights, the difference between short-day and long-day plants is in how you’ll have to set up the grow lights. The lights can be timed to only work for a limited time or more complex lighting setups can have the lights rotating above the different sections of your indoor garden. Either way, if you make sure to get the right type of grow lights for your indoor plants and to set them up correctly, you’ll maximize your yield quite substantially.

The 3 Main Types Of Grow Lights

There are different types and sub-types of grow lights but they can be summarized most simply into three categories – fluorescent, HID (High-intensity discharge lights), and LED (Light-emitting diodes) lights.

1. Fluorescent Grow Lights

Fluorescent lights have been the standard for plant grow lights for a long time. They aren’t too expensive, are quite efficient and effective, and are very reliable. Fluorescent lights work by ionizing mercury vapor inside their glass tubes.

You can get fluorescent lights for your indoor garden either in the form of standard CFL bulbs or as large T5 tube bulbs. The former are great for smaller spaces and emit mostly red lights, making them great for the flowering stages of the plants’ growth. The latter are better for bigger spaces and for vegetation rather than flowering. They also have a better light distribution.

The pros and cons of both types of fluorescent lights can be summed up like this.


  • Very affordable to both purchase and use.
  • Simple to install and use.
  • Emit very low levels of heat, allowing you to position the lights better over the plants.


  • Not as efficient as LED lights.
  • CFLs are better at flowering and T5s – for vegetation. Using both is advisable but that can pump up their price tag.

2. HID Grow Lights

HID lights are made to offer dependable and bright light. They emit light by igniting the gas inside the bulbs. They can also be divided into two subtypes based on the type of gas they’re using.

HPS (high-pressure sodium) bulbs emit mostly red light for flowering and MH (metal halide) bulbs which are better for blue light and vegetation. Both types also require a highly reflective hood above them so don’t make the mistake of only buying the bulbs. Aside from the type of gas they’re using and the type of light they give off, most of their other characteristics are pretty similar. As such, their pros and cons read like this:


  • Both types of HID lights produce an abundance of usable light, allowing them to cover large areas.
  • HPS HID lights, in particular, are the best red light grow lights on the market and are perfect for the flowering stages of most plants.


  • Both HID types produce A LOT of heat. This means two things: 1) It’s very common for novice indoor gardeners to accidentally burn their produce with HID lights, and 2) they need to be set up higher than LED or fluorescent lights which counteracts their brightness.
  • Because of the excess heat, you’ll also likely need to get exhaust fans.
  • HID lights don’t last as long as Led or fluorescent lights so they need to be changed more regularly.
  • Because of how bright and hot they are, HID lights are really mostly meant for larger indoor gardens and not for smaller spaces.

3. LED Grow Lights

The “new kid on the block”, LED grow lights are quickly changing the old balance of power between HID and fluorescent lights. LED grow lights aren’t as popular as residential LED lights but they’re getting there.

LED lights offer multiple positives to your indoor garden and only a few negatives:


  • LED lights are great at producing both red and blue light at the same time which makes them much easier to use for novice gardeners. This means that you won’t have to use two different types of bulbs, you won’t need to switch them depending on the plants’ growth cycles, etc.
  • LED lights give off the lowest amount of heat out of all grow lights. This means that they are extra safe for your plants as well as that you can position them even closer to your plants for maximum brightness.
  • LED lights are much more energy-efficient than HID or even fluorescent lights. In other words, they’ll have the lowest impact on your electricity bill out of all options. This is great for both large indoor gardens which tend to rack up enormous power bills and for smaller gardens with more modest means.
  • As with residential LED lights, grow LED lights tend to last the longest out of all types. This means both that you won’t have to bother changing them too often and that you’ll safe money from subsequent purchases.


  • LED lights are more expensive than the other options. While that initial expense us usually compensated for overtime through the lower power bills, it’s still an investment that can be unpleasant at first.
  • Quality can vary a lot with LED lights – some sub-par models can offer insufficient light so always make sure you’re buying quality.
  • LED lights are often advertised as “Full-spectrum”. This is a marketing term that doesn’t offer much value to the gardner, to the contrary – it means that these lights also emit white and green light which are largely useless for your gardening needs.

Which Type Should You Choose?

Which lights you’ll go with depends on several different factors – your setup, the plants you’re growing, the space you’re growing them in, as well as your personal preferences. For most situations, we’d usually recommend LED grow lights because of their simplicity, effectiveness, and efficiency. However, HID and fluorescent lights also have their benefits so don’t ignore them – they may very well be even better for your particular needs.

The Bottom Line

The right grow lights can help your indoor crops grow healthy and strong, without the sun. For this, understanding the science behind grow lights will assist in choosing the right products and installing them in a way as to enhance the optimal growth of the plants.  

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