How Long To Keep Grow Lights On Indoor Plants?

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

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

The same with making sure that your plants are not overly exposed to light.

So, once your grow lights are already set up, the next question you may have is “how long do I keep the grow lights on the plants?

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

You’ve set up your grow room, and you know it’s important for plants to be exposed to light, be it natural or artificial light.

However, do you know how long you leave the lights on?

The answer really depends on what kind of plant you are trying to grow.

Low-light plants need about four hours of light per day, medium-light plants would need about six hours per day and high-light plants may need to be exposed to light for up to 14–16 hours a day.

Keep in mind, if you’re using artificial lights, the plants will require longer hours of light exposure versus “natural light” or sunlight.

Plants Need Dark Time for Respiration Process

In general, most plants need at least 8 hours of darkness for respiration to happen.

If you are growing vegetative plants, such as cannabis, they don’t really require time off from light exposure and thus can be exposed to grow lights for 24 hours.

It is important to establish a routine for when the grow lights will be turned on and off to avoid confusing your plants.

While it’s fine for cannabis plants to be exposed to light for 24 hours, we still suggest that you give them a few hours of dark time as this will help them through their flowering stage.

Full Spectrum

How Long to Keep Grow Lights on 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.

Seedlings require 14–16 hours of light every day. Do not leave your grow lights on all day.

Seedlings need some dark time to go through their preparation cycle.

Importance of a Consistent Schedule for Your Grow Lights

You need to plan to ensure that your plants or crops are exposed to light at the same time every day.

Some simple solutions include using an alarm to let you know when to turn on and shut the grow lights off or plugging your grow lights into an analog timer that will turn them on and off automatically.

For more experienced indoor growers and if you don’t mind spending extra, you can use a light timer that you can incorporate into your grow light system.

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

How Much Light is Necessary to Grow Plants?

Light is essential to your plant’s growth as it is required for plants to undergo photosynthesis. Light works along with carbon dioxide and water in a process called photosynthesis.

It doesn’t really matter whether your plants get light from 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.

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 plants, 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, you can still save them by reducing their light exposure. 

It’s important to be aware of any yellow spots on your plants.

What Happens When Plants are Not Getting Enough 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 Plants With 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 some things that you should keep in mind.

Getting Full Spectrum of Light

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

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 shades 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 the warmth needed or the red spectrum.

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 plant’s 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.

light intensity Needed to optimize Plant growth

Measuring Light Intensity on Plants

Speaking of heat, 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.


For more information, read our guide on how far grow lights should be from plants.


Some growers have their grow lights installed in a way that they can still adjust the distance between their plants, but some just leave the distance as it is by using mainframes 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, you 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.

How long can plants stay under a grow light?

Low-light plants require approximately four hours of light per day, medium-light plants require approximately six hours per day, and high-light plants may require exposure to light for up to 14–16 hours per day.

Can you overuse grow lights?

For most plants, you can overuse grow lights. Most plants need at least 8 hours of darkness for respiration process to occur. Without respiration cycle, the plant cannot create energy for growth.

When should I turn on my grow lights for Seedlings?

As soon as the first seedlings begins to sprout, turn on your grow lights. Many plants seedlings grow quickly, and will reach for a light source such as the sun or grow light as they emerge.

What color grow light is best?

Violet-blue light with wavelengths ranging from 425 to 525 nanometers promotes photosynthesis, better chlorophyll absorption, and ultimately growth for your plant. When your plant is flowering, red red light in the 600-725 light spectrum is best.

Conclusion

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 to know what type of plant you have so that you can 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. Good luck and have fun growing indoors!

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