Let’s face it:
You won’t be able to grow many of your favorite plants inside if you don’t have a proper source of light.
Any indoor space where the sun doesn’t shine for more than four hours a day is not suitable for most flowering and fruiting plants.
Many people give up on gardening when they realize that the sun doesn’t shine enough on their indoor plants.
Others, however, think of innovative solutions, such as growing plants with lights.
Lamps are also a source of light, so we should be able to grow plants under them, right?
The truth is, you can grow any plant with artificial light, and people have been doing it for decades.
And, with the newest light technology, growing plants with lights has become easier than ever!
But, if you want to grow plants with lights, you must be ready to learn all about it.
For that purpose, I wrote this definitive guide on how to grow plants with artificial light.
Although you’ll find a lot of new information in this article, I want to assure you that nothing mentioned below is difficult to understand.
Also, you don’t need to finish reading this guide in one sitting. Save this article in your bookmarks and come back to it whenever you want.
OK, without further delay, let’s start!
- Section 1: Introduction to Growing Plants with Lights
- Section 2: Choosing a Suitable Grow Light
- Section 3: Using Lights to Grow Plants
Section 1: Introduction to Growing Plants with Lights
Before you grow plants with lights, you should learn about the light requirements of plants, measurements of light and their importance, and the lights that are used to grow plants.
In this section, I will walk you step by step through the basics of growing plants with lights.
If you have enough information about the topics mentioned here, you can skip this section and jump to the next one.
Light and Plants
You have heard it before, plants need light for photosynthesis, the process by which they produce their own food.
But did you know that not all light can drive photosynthesis? Plants need specific colors (frequencies) of light to survive.
Sunlight has all frequencies of light, ranging from infrared (IR) to ultraviolet (UV).
However, plants rely almost entirely on red and blue light to make photosynthesis. Plants also grow better when they are exposed to slight amounts of UV and IR.
The unique light pattern, which ranges from blue to red and can make plants thrive, is called PAR, short for Photosynthetically Active Radiation.
Lamps that are used by gardeners to grow plants emit most of their light in the blue and red spectral range (PAR range). This is how their light spectrum might look like:
Besides the quality of light, the quantity of light also matters.
Light Requirements of Plants
Different species of plants need different amounts of light.
In gardening, we classify plants into three (sometimes four) categories according to their sunlight requirements:
- High-light or full-sun plants need between 6 to 8 hours of sunlight per day
- Medium-light or partial-sun plants need 4 to 6 hours of sunlight per day
- Low-light or partial and full shade plants thrive on less than 4 hours of sunlight per day
You can find the sunlight requirement of any plant by doing a simple search on Google. Here’s an example:
The above terms are mostly used in outdoor gardening, but they are important in indoor gardening as well.
To choose and use a grow light, you must first learn about the light requirements of the plants you want to grow.
Keep in mind that the light requirements of plants change from one growth phase to another. What are growth phases?
Plants have three major growth phases:
- Seedling phase
- Vegetative phase
- Flowering phase
The seedling phase is the stage during which seeds germinate and develop into adult plants.
After the seedling phase, plants enter their vegetative growth phase. In this stage, plants grow their leaves and stems.
The vegetative phase is followed by the flowering phase which is the last stage in the life cycle of plants.
In the flowering phase, plants focus all their energy on flowering and fruiting.
Studies and practices have shown that red light is mainly responsible for flower formation.
One study showed that the number of flowers on pepper plants was the highest when they received light that was 95% red and 5% blue.
So, in the flowering phase, plants need red light more than blue light. But, in the vegetative phase, they rely mainly on blue light.
The flowering phase is also more light-demanding than the vegetative phase. So, plants need the highest quantity of light in the flowering phase.
It’s important to choose a grow light that suits your plants in each phase of their growth.
But before we talk about choosing a grow light, what is a grow light?
Many people ask if they can use regular lamps to grow plants. The short answer is no, you can only grow plants with grow lights.
Grow lights are lamps that are optimized for plant growth. Unlike regular lamps, they emit the right spectrum and the proper intensity of light that can fuel photosynthesis.
The most popular types of grow lights are:
- LED lamps
- Fluorescent lamps (FLs)
- HID lamps
Most indoor gardeners would tell you that LEDs are the best grow lights today.
They’re some of the most efficient lamps in the world. LEDs can emit more useful light and less heat than fluorescent and HID lamps while consuming the same amount of power.
The light spectrum of modern LED grow lamps is also greatly optimized for plant growth.
Most LED grow lights are very easy to use. You don’t need separate fixtures, ballasts, reflectors to run them.
I highly recommend LEDs to every indoor gardener, especially beginners who are attempting to grow plants under artificial light for the first time.
LEDs have a very long lifespan. They can live around 50,000 to 100,000 hours. This is equivalent to 5 – 10 years of continuous usage.
It’s important to mention that the light intensity of any lamp degrades in the last period of its life.
For that reason, you should stop using your grow light when it reaches the last quarter of its expected lifespan.
Refer to the information provided by the manufacturer of the grow light to know more about its lifespan and when to stop using it.
LEDs are widely available on the market today. You can find hundreds of unique brands of LED grow lights by doing a simple search on Amazon.
The competition between LED grow light brands is fierce nowadays.
As a result, the prices of LED grow lamps are becoming cheaper day after day.
Plus, to beat their competition, top-rated LED grow light brands provide a lot of information about their lamps and recommendations on how to use them.
Fluorescent lamps (FLs) are low-intensity to medium-intensity grow lights.
Most of the time, FLs are used to grow limited quantities of herbs, leafy greens, and low-light houseplants.
Fluorescent lamps are popular mostly because they’re cheap. Apart from that, they’re more difficult to handle and maintain than LEDs.
Standard fluorescent grow lamps emit light that leans to the blue side of the spectrum and for this reason, they work best for plants in their vegetative phase.
However, nowadays, you can find full-spectrum FLs suitable for both the vegetative and flowering phases.
Most fluorescent grow lights are tubes or compact lamps, and unlike most LEDs, they need light fixtures to function.
Compact fluorescent lamps (CFLs) are used in spotlight fixtures to illuminate individual houseplants. The best grow CFLs are the ones marked as full-spectrum.
Fluorescent tubes, on the other hand, are used in wide fixtures to give light for several plants growing in the same space.
The best fluorescent tubes for plants are the ones labeled as full-spectrum HO T5.
T5 lights are the most efficient fluorescent tubes because they have the smallest diameter. The smaller the diameter of a fluorescent tube, the more efficient it is.
HO means high output of light, which also means that the lamp has higher intensity of light compared to other FLs.
Because fluorescent lights are less intense than LED and HID lamps, you need to place them closer to your plants. But not too close that you accidently damage or burn the leaves.
Ever since LED grow lights came out, fluorescent grow lamps have been declining in popularity.
Today, few indoor gardeners use fluorescent lamps to grow plants.
Still, you may find fluorescent lamps suitable for you, especially that they’re very affordable.
However, keep in mind that a fluorescent lamp can generate light for around 20,000 hours. This is less than half the lifespan of LEDs.
So, if you’re using fluorescent lamps, you need to replace them more often than LED lamps.
Also, FLs consume more electricity than LEDs, so you’ll be paying more on your electric bill if you use FLs instead of LEDs.
High-Intensity Discharge (HID) lamps are very popular among professional indoor growers.
Those grow lamps are considered highly intense lights but they’re not as efficient as LEDs.
Gardeners who grow heavy-flowering and fruiting plants, especially cannabis, rely a lot on HID lamps.
Many cannabis growers believe that the highest yield can be only obtained when HID lamps are used.
The best HID grow lamps are called Metal Halide Lamps (MH) and High-Pressure Sodium (HPS) lamps.
MH lamps have a spectrum that leans to the blue side and is suitable for the vegetative phase of plants.
HPS lamps have a spectrum that leans to the red side and will make plants flourish during their flowering phase.
Many gardeners grow their plants under MH lamps in the vegetative phase, then they switch to HPS lamps when their plants flower.
To get the most out of your HID lamp, you need to use a fixture that is equipped with a reflector and a ballast.
Some indoor gardeners prefer using HID lamps because they put out a lot of heat.
So, if you’re growing your plants in a cold garage, HID lamps can keep your plants warm.
The price of an HID lamp with its fixture is higher than the price of a fluorescent lamp and comparable to that of some LEDs.
HID lamps are the hardest to handle, so they’re really not for everyone.
If you’re still a beginner, I recommend you avoid HID lamps until you gain solid experience in indoor gardening.
You saw above that different grow lamps have different characteristics.
One thing that varies from one grow lamp to another is the footprint (or coverage area).
What is the footprint of a lamp?
The Footprint and the Growing Space
The area in which you can grow your plants is determined by the footprint of the grow lamp.
A lamp’s footprint (or the coverage area) is the area it can light.
The distance between the illuminated surface and the lamp defines the dimensions of the footprint.
When you raise a lamp, you increase its footprint but decrease the intensity of light over that area.
Similarly, when you lower a lamp, you decrease the footprint but make the light more intense.
That’s a concept we all know from everyday life.
When you focus a flashlight on a wall, the further you go away from the wall, the wider the footprint of the light and the less bright it becomes.
So, to increase or decrease the footprint of a lamp, you need to adjust its hanging height. More on this in the upcoming sections.
The perfect footprint for your plants is the one where they can get the quantity of light they need.
For example, 250-watt HPS lamp can light 2.5 x 2.5 feet to 3 x 3 feet (0.75 x 0.75 meter to 0.9 x 0.9 meter) area of growing plants.
Most grow LED light manufacturers provide the recommended core coverage area (footprint) and hanging height of their lamps for both flowering and vegetative stages.
In case you want to grow a large number of plants, you’ll need more than one grow light and a large growing space.
It’s important to decide how many plants you want to grow and where you want to grow them before searching for a grow light.
If you’re growing a few houseplants, you will need separate light fixtures that can illuminate individual plants.
Those lamps are the easiest to work with.
Here’s what I am talking about:
In any other case, you’ll need more advanced lighting fixtures to grow your plants, such as Mars Hydro TS 1000.
If you want to grow plants on a large scale or on a highly professional level, you will need an entire room, or some grow tents to achieve your goals.
Grow tents give gardeners more control over the growing environment, therefore achieving better results.
As you saw above, artificial light, unlike sunlight, can vary in quality and intensity depending on the type of the lamp and its distance from plants.
In such a case, measuring light is important.
Understanding light measurements can help you choose the ideal grow light for your plants.
Although watt is not a measurement of light, it is often used by gardeners incorrectly to evaluate the intensity of a grow lamp.
Watts tell you how much electric power your lamp uses.
Technically, the more watts a lamp consumes, the brighter its light.
But that also depends on the type and the efficiency of the grow lamp.
For example, a 200-watt LED can generate the same amount of light as an HPS lamp that draws 1000 watts.
Having said that, you’ll see many LED grow lights labeled as 300W, 600W, 1000W…
Most of the time, those labels are not the actual wattage of the lamp but the wattage of the HPS lamp that has the same intensity as the lamp.
So, if a 200-watt lamp generates the same light intensity as a 1000-watt HPS lamp, the manufacturer might label it as a 1000-watt lamp.
Those labels are very misleading and tricky.
Always look for the actual wattage of the lamp to know how much power it consumes.
Also, avoid using watts as a light measurement and don’t trust brands that only provide the wattage of their lamps.
Watt is a unit of electric power.
Lumens and Lux
You might already be familiar with lumens and lux.
Those units are used to estimate the overall brightness of a lamp.
Lumens is the measurement of the quantity of visible light. Lux is the measurement of lumens over a defined area.
In principle, lumens and lux cannot measure the amount of useful light that a lamp puts out.
A lamp might be too bright in terms of lux or lumens, but the light it emits is not useful for plants at all.
To avoid the confusion, indoor gardeners use other measurement units (PPF and PPFD) that can estimate the quantity of useful light generated by a lamp.
Kelvin temperature is used to describe the spectrum of light that a lamp emits.
Keep in mind that in this case, kelvin is not a measurement of the actual temperature of the lamp.
Sometimes, manufacturers don’t provide the kelvin ratings of their lamps. Instead, they might label them as cool or warm.
A lamp that has a kelvin rating higher than 5000K produces more light in the blue range of the spectrum and is considered “cool”.
If a lamp has a kelvin rating below 3000K, it emits more red light than blue light and its spectrum is labeled as warm.
Again, cool and warm labels have nothing to do with the temperature of the lamp.
Full-spectrum grow lamps that produce sun-like light have a kelvin rating between 4000K and 4800K.
If a lamp is labeled as full-spectrum, that doesn’t mean it emits sun-like light.
A lamp that has a kelvin rating above 5000K may emit a small quantity of light in the red spectrum and therefore, it can be labeled as full-spectrum.
Note: since an LED lamp is composed of several diodes, you must check the kelvin rating of each type of diode used.
PPF and PPFD
PPF and PPFD are the most accurate light measurements for grow lamps.
I will not go into details about the science behind PPF or PPFD.
I just want you to know that PPF is the quantity of useful light that a lamp emits each second.
And PPFD is the quantity of useful light that your plants are receiving each second.
Professional grow light manufacturers will provide both the PPF and PPFD measurements of their lamps.
How can you use PPF and PPFD measurements?
If you want to know how bright and efficient a grow light is, you can look at its PPF (sometimes labeled as PAR or light output).
PPFD can also assess the brightness of useful light but has many other uses that you will learn about shortly.
Since light scatters, the PPFD readings vary from one position to another in the growing space.
The PPFD values are the highest at the center of the lamp’s coverage area because that’s where light is most intense.
As you move away from the center, the PPFD values decrease.
PPFD values are usually displayed in a chart like the one below.
In this chart, you can see how the PPFD values vary between the center and the sides of the coverage area.
That means it’s not enough to examine the PPFD values at center of the lamp’s coverage area.
Some grow light brands will only tell you about the PPFD measurement at the center of the lamp’s footprint.
Don’t fall for that!
If you want to grow many plants under one light fixture, you need to know the PPFD measurements of the light all over the coverage area.
You don’t want the plants at the center to be growing taller and bigger than the plants located at the sides.
Now that you know all about the basics of growing plants with lights, let’s talk about choosing a suitable grow light for your plants!
Section 2: Choosing a Suitable Grow Light
It’s very important to take your time to search for a grow light that is suitable for the plants you want to grow.
Most grow lights cost a good amount of money, so mistakes at this stage are expensive.
In this section, I will teach you how to choose the perfect grow light for your plants by examining various important factors.
Intensity & Efficiency
The intensity (or brightness) of a grow light is determined by the amount of light it can put out.
Usually, the higher the intensity of a grow light, the higher its price.
You should search for a grow light that can give your plants the minimum light intensity they need.
Since the light intensity of a grow lamp is measured in PPF or PPFD, it’s important to learn about the light requirements of your plants in terms of these units.
It’s not enough to know if the plant you want to grow is full sun, partial sun, or partial/full shade. You must also know the exact quantity of light it needs.
Here is a brief list of the PPFD (in mol/m²/s) requirements for plants:
1. Full sun plants need a minimum of 400 to 600 mol/m²/s
2. Partial sun plants need a minimum of 150 to 200 mol/m²/s
3. Full shade and other low-light plants need around 100 mol/m²/s
This list is not very accurate, but it’s still very helpful. More exact numbers depend on the number of hours of light that your plants receive per day.
How can you use those PPFD values in your search for a grow light?
Suppose you want to grow tomatoes indoors.
Tomato plants are full sun. They need at least 400 to 600 mol/m²/s of PPFD.
So, you need to find a grow light that has PPFD measurements around 400 to 600 mol/m²/s.
Take a careful look at the PPFD charts of two different grow lights. Which grow light do you think is better for your tomato plants?
If you chose the second grow light, you’re correct.
The first grow light is less uniform than the second one. You can see that the PPFD values at the corners of its coverage area are not enough for tomato plants.
If you’re looking to grow plants sustainably and economically, you also need to examine the efficiency of a grow lamp.
The efficiency of a grow lamp is the amount of useful light it can emit for every watt of electric power it consumes.
So, to find the efficiency of a grow light you must divide its PPF value (or lumens) by its wattage. The resulting value is called the PPF efficacy rate.
Any grow light that has a PPF efficacy rate above 1 μmol/J is considered efficient.
Many modern LED grow lights can generate 2 μmol/J and more.
Manufacturers of high-quality grow lights usually provide the PPF efficacy rate of their lamps.
Besides examining the intensity of a grow light, you must examine its spectrum.
To find information about the spectrum of a grow light, you must check its kelvin rating or spectrum label.
Most LED grow lights are labeled as full-spectrum but they mainly emit blue and red light.
As you saw earlier with HID lamps, the ratio of red to blue light usually varies from one grow light to another.
Some grow lights emit more red light than blue light, and they are used for heavy-flowering and fruiting plants.
Other grow lights have a spectrum that leans to the blue side. These lamps are more suitable for leafy greens and herbs.
But, the newest LED grow lights have a balanced spectrum that is ideal for all species of plants and all growth phases.
They also emit sun-like light that doesn’t dazzle the eyes, like the purple/pink light emitted by traditional LED grow lights.
As a starter, I recommend you go for those modern full-spectrum grow lights.
Section 3: Using Lights to Grow Plants
Using grow lights is a straightforward job. But it can get sophisticated when you want to grow plants on a large scale or in a more controlled environment.
In this section, I’ll discuss the fundamentals of using grow lights.
Distance from Plants
Before you run your grow light, you must adjust the distance between it and the growing plants.
The ideal distance is determined by the light requirements of the plants, their growth phase, and the intensity and the type of the lamp.
If the grow light is too close to your plants, it can burn them.
The more intense the grow lamp, the wider the distance should be between it and your plants.
Typically, you should maintain the greatest distance between your plants and the lamp when the plants are still seedlings.
The lamp should then be lowered a few centimetres when the plants enter their vegetative phase of growth.
Plants need the highest intensity of light when they are in the flowering phase, and this is when the distance between the plants and the lamp should reach its minimum.
The minimum recommended distance between plants and a grow light is when they get the greatest quantity of light without being damaged or burned.
Grow light manufacturers provide the recommended hanging height of their lamps for each phase of growth.
You should stick to the recommendations provided by the lamp manufacturer to be able to use the device properly.
Remember, when you lower a light, its footprint decreases.
So, the footprint of the light should become smaller during the flowering phase of your plants than during their vegetative phase.
How Long Should You Keep Your Light On?
One of the most fundamental questions in indoor gardening is how long should grow lights stay on each day?
The short inaccurate answer is that plants need between 12-18 hours of artificial light a day.
The more accurate answer depends on two metrics: PPFD and Daily Light Integral (DLI).
Daily light integral (DLI) is the quantity of light that your plants receive (or need) each day.
You can calculate the number of hours of light your plants need each day if you have the PPFD measurements of your grow light and the DLI requirements of your plants.
Below is a brief list of the DLI requirements of plants:
- Full sun plants need between 20 to 30 mol/m²/d
- Partial sun plants need 5 to 10 mol/m²/d
- Full shade and other low-light plants need a maximum of 5 mol/m²/d
Check this online DLI calculator to estimate the number of hours of light you need to meet the DLI requirements of your plants. To use it, insert the minimum PPFD measurement of the grow light you’re using. In the time box, start with 8 hours and keep increasing the number of hours gradually until you get a DLI value that is suitable for your plants.
You won’t succeed in indoor gardening if you don’t provide your plants with a suitable growing environment.
No matter where you’re growing your plants (tent, room, or closet), always keep an eye on the following environmental conditions:
Every plant has an optimal temperature for growth. But, most plants will thrive at temperatures between 66°F and 75°F (19–24°C).
Most times, growers don’t need to regulate their indoor temperatures because they’re already in the optimal range.
But, in other instances, the temperature might be too low or too high for the growing plants.
There are many methods to control temperatures indoors and it’s up to you to choose the one that suits you the best.
If you’re growing in a tent and using a lamp with high wattage, you need to install fans to keep the air flowing and prevent heat from building up in the growing environment.
In case you’re growing in a room, keep a close eye on the fluctuations in temperature around your plants.
Air conditioners, radiators, heaters, fans, etc. can help you manage the temperature in the growing space.
Some heating methods, such as air conditioning, can make the air dry. They decrease the humidity levels in the atmosphere, which is problematic for your plants.
Note: HID lamps emit a lot of heat. So, you should use LEDs instead of HID lamps if you want to have more control over the temperature in the growing space.
The optimal humidity levels for mature plants are between 40 and 70 percent.
Most often, the humidity levels indoors are in that range. You can check the relative humidity in your growing space using a hygrometer.
Keep in mind that plants need less humidity during their flowering phase than their vegetative phase.
To increase the humidity around your indoor plants, you can mist them few times during the day or water them more regularly.
If you’re growing in a tent or dedicated grow room, you can control the humidity using a humidifier or dehumidifier.
It’s important to keep the air flowing in the growing space to prevent diseases and pests from propagating in your plants’ environment.
If you’re growing a small of quantity of plants in a well-ventilated room, you probably won’t need any fan to make the air circulate.
In all other cases, however, you need at least one box or oscillating fan to keep the air flowing in the growing space.
Remember that the air needs to be completely replaced every 3 to 5 minutes in the growing space.
When you are gardening in a tight space, such as a grow tent, or growing many plants in one room, you need an advanced ventilation system.
Advanced ventilation systems rely on exhaust and intake fans, and carbon filters to deal with the odor.
Setting up the ventilation system is usually simple when you’re using LEDs but it’s more complicated with HID lamps. Check this guide for tips on how to set up a ventilation system in your growing space.
If you want to grow your plants in a very tight place or if you want to increase the yield, you need to increase the levels of CO₂ in the growing environment.
Supplementing some plants with CO₂ can highly increase their productivity.
But once you increase the levels of CO₂ in the growing environment, you need to increase the temperature above 78°F (25°C).
CO₂ generators can be expensive and difficult to have.
Check those DIY methods to generate CO₂ without purchasing a generator.
That’s all you need to know about growing plants with lights for now.
I recommend that you start experimenting with grow lamps as soon as possible.
Gardening, whether outdoors or indoors, is all about practice, and now that you have all the information you need, you should start growing your plants.
Don’t forget to share your thoughts and questions in the comments below!