How Long To Keep Grow Lights On Indoor Plants?

Grow lights are an excellent tool for fulfilling light necessity when it comes to growing crops or plants indoors, but they can also be harmful to the plant if you’ll overexposing them.

So, once your grow lights are already set up, it’s important that you know how long to keep grow lights on plants.

You always need to make sure that the plants you are growing indoor are still getting enough light to make sure that they grow nicely.

The same with making sure that your plants are not overly exposed on light. It’s okay if you don’t know how long your grow lights should be really on as this is something that requires research and understanding.

If you’re just new to growing plants indoors with artificial light, then you’re on the right page!

We’ll discuss what you need to know to maintain the good health of your starting seeds, gardening inside, or houseplants when it comes to lighting.

Why is Light Necessary When it Comes to Growing Crops or Plants?

Light is essential to your plant’s growth as this is required for plants to undergo photosynthesis. Light works along with carbon dioxide and water.

It doesn’t really matter whether your plants get light from the sunlight or any artificial light.

Photosynthesis is when plants make use of carbon dioxide, water, and light to produce glucose. Glucose is what plants use to have energy.

Thus, they would grow healthy and avoid getting wilted. Basically, photosynthesis needs to take place in order for plants to produce their own food.

Light is the key, but of course, you need to know how long you should be putting your plants under it.

How Long to Keep Grow Lights on Plants During the Day?

Now that you know why it’s important for plants to be exposed to light be it natural or artificial light let’s get to answering your question of how long you should really have your grow lights when on growing plants or crops indoors.

The answer really depends on what kind of plants you are trying to grow. There are some plants that require just a few hours, while some are fine even if they are exposed to light 24/7.

What you have to keep in mind, however, is that if you’ll use artificial lights or grow lights, the plants you’re trying to grow will require longer hours of light exposure compared to using natural light or sunlight.

How long to keep lights on my seedlings?

During seeding or when you’re just starting and trying to germinate seedlings, some think it’s okay to have the grow lights on for 24 hours. This is really not true. In fact, some seeds would have a hard time to germinate when exposed to the sun.

Generally, plants require some dark time, and it still has something to do with photosynthesis.

So while the plants are producing glucose because of light, carbon dioxide, and water when the lights are on, during the time that the lights are off is when they consume the glucose or energy.

This is what helps them grow healthy. This process is called respiration.

Do plants grow faster with 24 hours of light?

Full Spectrum

Most plants need at least 8 hours of darkness for respiration to happen. Some plants would require 12-16 hours of light exposure to make sure that respiration completely takes place.

It really depends so make sure that you do your research when it comes to the needs of the plants you will grow.

If you are growing vegetative plants, they don’t really require time off from light exposure. This means that these plants won’t really mind if you leave your grow lights on for 24 hours.

Here are some examples of plants and crops that are fine with light exposure the whole day:

  • Spinach
  • Lettuce
  • Orchids
  • Cacti
  • Coleus
  • Dieffenbachia

While it’s fine for these plants to be exposed to light for 24 hours, we still suggest that you give them a few hours of dark time. This will help you lessen your expenses when it comes to the electricity consumed by your grow lights.

For plants that bear fruits or flowers, do keep in mind that they need time off from the light or dark time. This is required to force these plants to push through their flowering stage.

Usually, dark loving plants would only need to be exposed to light for 12 hours. So if your grow space, tent, or box is enclosed and doesn’t have any windows where sunlight could pass through, then you need to shut your lights off for about 12 hours.

8If you’re wondering what type of plants or crops need their dark time, here is a list you may use as your guide:

  • Poinsettias
  • Kalanchoe
  • Spider Plants
  • Dracaena
  • Tomatoes
  • Creeping Fig
  • Peppers
  • Maidenhair Fern
  • Parlor Palm

Those are just a few plants that require some dark time. Keep in mind that during the hours that your grow lights are off, make sure that there will be no light interruption.

You will also need to establish a routine when the grow lights would be turned off to avoid confusing your plants.

Tips to Make Sure that You’re Not Overly Exposing Your Crops to Light

Tips To Make Sure That You’re Not Overly Exposing Your Crops To Light

You need a strategy when it comes to ensuring that a routine is established when exposing your plants or crops to light.

You can go basic and just use an alarm to let you know when to turn on and shut the grow lights off. If you don’t mind spending extra, you can use a light timer that you can incorporate into your grow lights system.

This means that you can just set a timer as to when the grow lights should automatically turn on and off. This is a video instruction that teaches how to set up an Analog Grow Light Timer.

It’s best to know if the plants you chose to grow are considered as low, medium, or high-light plants. Low-light plants would only need about four hours of light exposure, while medium-light plants would need about six hours.

When it comes to high-light plants, some need 14-16 hours of exposure to artificial lights. If you have no clue as to what high, medium and low-light plants are, you can always check the box of the seeds you got.

If there’s no indication of how much light the plants that you have a need, here is a guide that you may use:

High-light Plants

  • Araucaria Plant
  • Schefflera Arboricola
  • Ponytail Palm Tree
  • Yucca Plant
  • Ficus Benjamina
  • Ming Aralia Plant
  • Ficus Pandurata
  • Codiaeum Petra
  • Chrysalidocarpus Lutescens
  • Aloe Vera
  • Coleus
  • Burro Tail

Medium-Light Plants

  • African Violet
  • Bird’s Nest Fern
  • Boston Fern
  • Dumb Cane
  • Moth Orchid
  • Moon Valley Pilea
  • Chinese Money Plant

Low-Light Plants

  • Maidenhair Fern
  • Staghorn Fern
  • Philodendron
  • Snake Plant
  • Asparagus Fern
  • Weeping Fig
  • Elephant’s Ear
  • Fittonia
  • Bear’s Paw Fern
  • Earth Star

Those are just a few plants that fall under the categories listed above. Take note of this to help you decide how long you should be turning your grow lights on and off.

What Happens When Plants are Over-Exposed to Light?

Plant Under Light

Plants that are exposed to light for too long will produce more energy than they can handle.

It could destroy your plant so if you leave them under your grow lights for too long, you’ll just end up with dead and wilted plants. A sign that a plant is absorbing too much light is yellow spots on its leaves.

However, if you see these yellow spots on the leaves of your plants or crops, this can still be corrected by reducing their light exposure. 

Just make sure to do this when it’s not yet too late!

What Happens When Plants are Not Properly Exposed to Light?

To directly answer this question, your plants won’t survive as they won’t have the time to go through photosynthesis to produce enough energy to thrive.

If you’re an indoor grower with many plants and crops, it is highly suggested to get a backup generator that could help your grow lights power up if any power outages would last for more than just a day!

Other Things to Consider When Growing Crops and Plants by Using Grow Lights

Light Spectrum

While the length of light exposure greatly matters when growing plants indoors, there are also a few things that you should consider when it comes to using grow lights.

Here are things that you should keep in mind.

Light Spectrum

Since sunlight just only seems to deliver one light color, you probably are thinking that it’s okay to use just one shade or spectrum of lights.

This could do but if you really want your plants to grow healthy and of quality, you’ll need to understand how different Color Spectrum Affects Plant Growth.

You may use fluorescent bulbs with white and yellow shade to deliver what the plants need.

While you may also use incandescent bulbs for your grow room setup, it’s important for you to know that this type of light only delivers to the warmth need or red spectrum.

Next Read: Full Spectrum LED Grow Lights For Indoor Plant Growth

Aside from that, it produces more heat and can easily die out.

There are LED grow lights you can get that have literal red and blue lights to cater to your plants’ needs when grown indoors.

This is a good option as LED doesn’t really produce much heat. This could also help you maintain the humidity level in your grow room.

Measure light intensity to optimize the growth and health of your plants

Measuring Light Intensity on Plants

Speaking of heat, the light intensity is also something to keep in mind when using grow lights.

You can control the light intensity in your grow room by adjusting the distance between your grow lights and crops. Here is a complete guide about How Far Should Grow Lights Be From Plants?

Some growers have their grow lights installed the way that they can still adjust the distance between their plants, but some just leave the distance as it is by using main frames to support their grow lights.

The light intensity will really depend on the type of plant you have. The knowledge of low, medium, and high-light plants will come in handy when it comes to knowing what intensity your plants need.

For medium-light plants, they only need about 250 feet of candles. This means that that’s the light and heat intensity they need.

For high-light plants, they’ll need at least 1000 feet of candles to thrive. If you have a low-light plant, just the light exposure is already sufficient.


Growing plants indoors will really require proper planning and equipment. If you want to be successful in growing crops or plants indoors, you’ll need to really invest your time and effort.

It’s not easy to establish a routine when it comes to the light exposure of the plants you are growing if it’s not automated so you really should plan accordingly.

The best thing that you need to do first is knowing what type of plant you have so that you could cater to its needs.

Light intensity and time exposure are just a few crucial things to keep in mind when growing plants so make sure that you have yourself ready!

Growing plants indoors can be an enjoyable hobby and even provide a living. It shouldn’t be stressful and hard. Knowing what things to consider is always a must. Enjoy growing indoors and good luck!