Grow Light Guide to Indoor Gardening

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The must know info for growing indoors

Grow light is an increasingly popular way to grow plants and vegetables inside. Especially if you live in an area that has a long winter, with extended days of low light.

You may have heard about grow lights being used in greenhouses, or from keen gardeners who want to get a head start on the growing season.

There are lots of vegetables that require a longer growing season, which can be helped by starting them inside first.

Whether you are starting seeds inside, trying to grow salad greens during the depths of winter or simply want to give your plants a bit of extra pep, this guide will help you select the lights that are best for your needs.

Here are some key things you’ll need to understand when trying to pick out a grow light:

  • Colour temperature
  • Understanding daylight
  • Light spectrums
  • Photosynthesis
  • Common Types of Grow Lights

Colour Temperature

Let’s talk colour temperature for a minute. Colour temperature is measured in Kelvins, and represented by the measurement K. You might see charts on light boxes that say 5600K. (that’s the rough colour temperature of the sun during peak daylight hours).

But before we dive into what all that means, let’s talk about fire.

Have you ever been around a campfire, or watched flames burning from a candle? Usually if you watch long enough, you’ll see that different parts of the flame are different colours.

If you stare into a fire you can see blue and white flames flickering below the orange, yellow and red flames above. That’s because the white and blue are physically hotter than the orange, red, and yellow flames on the outside, near cooler air temperatures. 

Ever hear the expression white hot? That’s referencing the white hot of the fire. The closer to the base of the flame, the cooler the actual colour of the flame will be. 

Its the same thing when talking about temperature in terms of colours. The hotter the temperature, the cooler the colour, and the cooler the temperature, the warmer the colour.


When we use this to talk about colour, the colours we typically call warm colours are actually colder, or lower on the Kelvin scale. And cold temperatures are higher on the Kelvin scale

Color temperature black body 800-12200K.svg

By Bhutajata - Own work, CC BY-SA 4.0, Link
By Bhutajata – Own work, CC BY-SA 4.0, Link

Kelvins are how we measure the temperature of colour, but it doesn’t mean that the lights we are using are that hot or cold to touch.

The Kelvin scale is talking about if a black body radiator (something that absorbs electromagnetic radiation) reached those temperatures, it would emit those colours or wavelengths of colour.

All of this basically to say that the way we measure colour temperature is based on the science of colour waves, and we use Kelvins to do it.

Daylight is about 5778K, but you will often see it represented on packaging around 5600K

Now that we have that sorted, you’re ready to read the packaging. But first you should have an understanding on what colour temperature means for your plants’ growth.

Understanding Daylight 

Its important to understand the colour temperature of daylight, but its also important to understand how plants use daylight.

Making the right choice about a grow lamp will really depend on how much light your plants need to grow, and what wavelength of light will best support their growth. This is especially true when choosing an LED light. But more on that later.

Light Spectrums

What does your plant need?

Knowing what your plants need is a really important aspect of choosing the right lights for you.

If you’re looking to grow indoors all year round, and have very little window light, your choice will likely be different from someone who is trying to start seeds indoors.

Knowing how you want to grow, and what you are growing is key in making the right choice for you.

If you are growing inside all year, and lets say you want to grow tomatoes, but don’t have any windows, lets break down what to look for.

Tomatoes requires full sun to grow well outdoors. In our indoor garden, we need to mimic full sunlight (as well as nutrients using fertilizers and soil).

So we know we want a broad spectrum light that can mimic the colour temperature of the sun, and something that will have to be on the majority of the time, which can be expensive if using a traditional bulb.

LED grow lights are a great alternative option to making a selection that uses less power, is cool to the touch, and can mimic the right colour spectrums and temperatures.

A major drawback with today’s technology is that we don’t have enough studies to tell us long term outcomes of growing with LEDs yet. They haven’t been available long enough to study growth outcomes.

How Photosynthesis works

Photosynthesis is the process of turning light energy into chemical energy, which plants use as fuel to grow. Carbon dioxide combined with water and light produces sugars, and oxygen. This is where the type of light a plant gets becomes important.

Photosynthesis en.svg
By At09kg : original Wattcle : vector graphics – This file was derived from: Photosynthesis.gif, CC BY-SA 4.0, Link

In order to mimic sunlight, we want to use a light that gives off the same wavelength as sunlight. That means choosing a light that can produce something in the 4600 – 6500K range.

Anything above 6500K will have a bluish hue, and anything lower than 4600K will have a yellow or orange/reddish hue.

For most indoor plant growing, you will want to look for something that has a range of 5000-6000K.

Importance of Light

Some types of lights will offer a higher red hue, or a higher blue hue. This means those wavelengths are stronger within the light source. Depending on what nutrients your plant needs, it might be impacted by the type of wavelength it is receiving. Growing more or less depending on how helpful it is to that plant.

In terms of figuring out how much light your plant will need there are actual equations if we want to get really into the science. But for most of us, it will mean knowing whether or not something is a low light, or a high light type plant.

Spinach, kale, lettuce, and other leafy greens typically require lower levels of light (fewer hours). Whereas things like tomatoes, cucumbers, and peppers require more hours of light. Plants that are fruit bearing typically require higher quantities of light also.

Common Types of Grow Lights

  • LED (Light Emitting Diode)
  • HID (High Intensity Discharge)
  • MH (Metal Halide)
  • CMH (Ceramic Metal Halide)
  • HPS (High Pressure Sodium)
  • Fluorescent
  • Combination styles

LED Lights

Light Emitting Diode (LED) lights don’t offer the same type of colour spectrums as a traditional incandescent bulb, so its important to know what light spectrums your plants need to grow.

When it comes to growing plants, we don’t know a huge amount about how the light spectrums of the LED will contribute to growing patterns.

One of the main benefits of using an LED is the very low power consumption. These lights can have a very high light output (wattage), with very low amperage, and very little heat being created by the light itself.

LED lights also have a wider range of colour spectrum options, making them more viable for highly focused colour temperature needs. If you are just starting out though, this probably won’t be a concern for you.

I recommend LEDs as a great way to get started with grow lights. They are very easy to set up. Because they don’t produce a lot of heat, they are also a safer option, especially if you have young kids or animals in your house or apartment.

You may need to experiment a bit with these lights, because the way they emit light is not as even. But overall, they are a great advancement to indoor growing.

HID (High Intensity Discharge) Lights

High Intensity Discharge lights(HID) are a really common, and popular type of grow light, because they have high light output with reasonable power consumption.

They work by keeping a high pressure gas chamber (inside the glass envelope of the bulb) wrapped around two tungsten electrodes held apart by a space. This gap between the electrodes causes an electrical arc which burns at a colour temperature close to that of the sun.

This makes them a popular choice for indoor gardening. However they can be quite finicky, and need an electrical ballast to work properly. Even if you have the right bulb for your needs, you have to have the right ballast to match.

These are still a great option for grow lights, but be mindful to check the labels before you buy your supplies to make sure everything is compatible.

MH (Metal Halide)

Metal Halide lights are another form of HID bulbs. They require a ballast, though new advancements have made this type of bulb easier to use than a traditional HID.

Metal Halide tends to emit blue and violet colour temperatures that closely resemble sunlight. They don’t have noticeable colour distortion, so they are very popular in stores, and displays where growers want their plants to look like they are under natural sunlight.

Because of the wavelengths produced by this type of bulb, they tend to be a good option for starting vegetables. They encourage strong root development early in the growth phase.

Metal halide bulbs can come in a range of colour temperatures, but are commonly found in 3000K to 7000K, though ultraviolet versions reaching 10000k can also be found.

MH bulbs have a fairly decent energy efficiency in terms of light output per watt. But they will still consume a fair amount of electricity.

MH bulbs also have a shorter lifespan than HPS which last on average 2X longer, and LEDs which last years. Which could factor into cost decision making.

CMH (Ceramic Metal Halide)

Ceramic Metal Halide bulbs are very similar to Metal Halide, except that they use ceramic to contain the chemical reactions under pressure in the light bulb’s envelope.

CMH bulbs still require a ballast, as they are another form of HID. They can burn at quite high temperatures, so caution should be used, as the amount of heat they throw off a lot of heat.

This could be a concern if there are any pets or small children who may want to touch them. Place them in an area where they can safely be out of reach, and not in contact with anything that may potentially catch fire.

The colour temperature will vary depending on the chemical combinations inside the bulb. But they tend to emit a bluish hue, close in colour to sunlight.

These bulbs are also popular for industrial and film and studio use as their lifespan is quite long. Somewhere around 24,000 hours.

HPS (High Pressure Sodium)

High Pressure Sodium bulbs are a bit different in that they tend to produce light on the lower end of the Kelvin scale, mimicking warmer colours like reds, and oranges. This makes them a good option to promote continued growth and flowering, but not as ideal for seed starting.

HPS bulbs are often used as supplemental light in greenhouses where plants are already getting blue light from the sun.

HPS bulbs are still a form of HID though, and also have a ballast that helps regulate the amount of electrical current reaching the bulb.

Due to the limitations of the colour spectrum of HPS bulbs, its not an ideal all purpose grow light. But it can be very helpful in providing supplemental light.

Combination styles

Combination style bulbs refer to a combination of Metal Halide and High Pressure Sodium bulbs combined to create a HID bulb that can cover both warm and cool light spectrums. Its sometimes referred to as a “dual arc”.

This type of light can be a good option in situations where you need to provide light throughout the entire growth cycle.

Like all the bulbs in this category though they do require a ballast, and generate a lot of physical heat.


I think the most common thought about fluorescent bulbs are that they can feel like a cold office space. While those flickering overhead lights at your office might be the same style bulb, they come in many different shapes, sizes, and colour temperatures to make them useful for growing.

Fluorescent bulbs are one of the cheapest options next to LEDs. They also require a ballast, unless using a compact bulb version which has a limiter set into the base of the bulb.

The tubes used most commonly for grow lights are called T5, T8, and T12. T8 and T12 are better suited for lower light needs, but T5 is useful for higher light needs. T5 is one of the most common bulbs used to help get plants started by seed.

CFL (Compact Florescent Light)

As mentioned above, there are some fluorescent bulbs that don’t require an additional ballast. These are the spiral type fluorescent bulbs you might use in your home to reduce power consumption.

They have a high light output, use less power, and are also produced with colour temperatures used for grow light.

These bulbs are great options for a concentrated spot of light. If you have a desk plant, use one of these bulbs in a lamp for concentrated light on one plant.

They are not ideal for larger trays of plants, unless you use multiple lamps to create lots of light coverage. Though this approach is not that practical if you could use a bank of T5 bulbs instead.

How to choose?

Making a decision around what type of grow light will be best for you will depend on a few factors. For me, the main things I always try and consider are:

  • How much will it cost to operate?
  • Can it safely operate without my being constantly present?
  • What is the upfront cost?
  • How often will it need to be maintained?
  • What do I need from it? (am I seed starting, or hoping to grow an entire cycle of plants or vegetables)
  • Do I have space to store it?

Find what’s important to you

There are so many other factors that could go into choosing the right product for you. But this is how I tend to approach things.

For me, I want to use grow lights to start seeds, and to grow vegetables through winter months. I also know I don’t want to spend a lot of money on electricity costs. I want to be as environmentally friendly as possible.

None of these bulbs are environmentally friendly when we consider how they are manufactured, however some last longer than others.

For my needs, I have chosen to use LED bulbs. I like that they have a long life, and that their light spectrums are broad, and can be supplemented. I also like that I can leave it on without worrying about how hot the fixture will get. And, I know that my power consumption will be lower than other options.

Choose based on your needs

There are a lot of unknowns in growing with LED bulbs though. There isn’t enough research to know how using them might impact flavours, growing patterns, and overall plant development.

Because I’m growing for myself, and do not need to rely on any of these products to run a business, I am okay in experimenting and trying things out.

If I needed to rely on a product with certainty, I would consider using a T5 fluorescent tube bank. Or a metal halide bulb. Both of these options are more costly than LEDs in the long run. But they have more reliable information at the moment to know that they are functional options.

Ultimately gardening is somewhat of a personal journey. One of the things I love most about it is the ability to experiment, discover, and then try again. We don’t have to get it perfect, because nature isn’t perfect.

Let me know in the comments below what type of light you’ve chosen, and how its working out for you.

Happy Growing!