How Long To Keep Grow Lights on Plants?

If you’re reading this, there’s a very good chance you already know that plants require light more than anything.

When plants get enough light with the correct intensity and color, they quickly grow and thrive, resulting in a good harvest.

But then, have you ever asked yourself how long you should let grow lights illuminate the plants? Actually, this will be determined on a case-by-case basis.

So, today I’ll spill the beans and let you know everything you should know about this question. I’ll do this by explaining other topics pertinent to this question.

Without much ado, let’s hit the ground running.

How Long Should I Keep Grow Lights on Plants?

The truth is, different seeds and plants won’t require the same light exposure and intensity. There are many factors that determine how long you should keep your grow lights on plants.

For example, what are you using the grow lights for? Is it foliage growth, vegetables, or flowering? And at what stage of growth are your plants?

In most cases, some plants need only 6 hours of light, while others need between 12 and 18 hours of light per day.

Here, we’ll focus on the popular types of plants you might want to grow indoors and their need for light:

Snake Plants, Tomatoes, and Peppers

how long to keep grow lights on plants

These are low-light plants that call for less light exposure throughout the day. Ideally, these will require no more than 6 hours of light per day.

Know that these low-light plants will benefit the most from a higher light intensity. You’ll want to keep a 2-3 feet distance between the light source and the plants for the best results.

Also, be sure to let lights shine on them for a shorter time. By so doing, you’ll give them adequate hours of darkness to consume energy, grow, and develop.

Chinese Money Plant, Dumb Cane, and Africa Violet

how long to leave plant light on

These are medium-light plants that require a little more light exposure, usually about 8-12 hours of light per day. If you grow these plants in areas that receive some sunlight, you better keep the light hours between 8 and 10 hours.

Other day-neutral plants consist of geraniums, coleus, and foliage plants. So, if you’re growing one of these plants from a nearby window, you’ll want to let them enjoy no more than 8-10 hours of artificial light.

Additionally, these plants should be kept at least 1 foot away from the grow lights. This is because they do best when set on higher light intensity.

Spinach, Lettuce, Orchids, and Aloe Vera

how long to leave grow lights on

Botanists consider these as long-day plants. That means they require the longest hours of light of all plants you can grow indoors.

Long-day plants may include most seedlings for veggies and garden flowers. For these ones, you’ll want to keep grow lights on for 16 to 18 hours per day.

Usually, these types of plants get pale and leggy if they don’t receive enough light exposure. For best growth, it would be helpful if you kept the grow lights at least 6 inches away from the plants.

As a general principle, be sure you’re in a position to switch your grow lights on and off. You might even want to try these grow light timers from Amazon. These are great if you’re going to grow vegetables and seedling flowers.

What If I Leave Grow Lights On For 24 Hours?

Leaving grow lights on for 24 hours a day is a mistake you don’t want to commit. Typically, plants need at least six hours of total darkness per day.

Darkness is essential for all types of plants. This is why getting an automatic timer for grow lights can make things a tad easier for you.

When it’s dark, plants not only take a rest, but they use that opportunity for respiration. This is where they break down energy to allow for growth and development.

Ideally, there are plants that can’t do without darkness per day, and there are those that don’t mind getting light 24 hours per day.

Here is a list of plants that can’t do without darkness:

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

These types of plants require that you keep grow lights off at least six hours a day for respiration to take place. You’ll want to balance active growth time and rest time for these plants to avoid confusing them.

Normally, prolonging active growth time for plants may affect many biological processes. For example, if some plants don’t get dark, their growth rate may delay.

They may even stop producing buds and fruits if they don’t get sufficient hours of darkness. That being said, some plants have no problem receiving light for 24 hours.

Plants and crops that don’t mind receiving light the whole day include:

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

While it’s OK to leave grow lights on for these plants, it’s still important to give them a few hours of total darkness.

This will not only help you reduce your electricity expenses, but it will also help these plants push through their flowering stage.

Keep in mind that plants that bear fruits or flowers will need dark time for at least 12 hours. So, be sure to turn your grow lights off for that duration of time.

What Kind of Light Do Plants Need?

When it comes to controlling the growth of a plant, the characteristic of light-matter the most. You’ll want to give plants the correct color and intensity so they can grow and thrive.

how long to leave grow light on
image: HomeStead

Light Quality

The color and type of light determine its quality. Usually, light has varying colors, and each one represents a different wavelength.

Plants don’t “see” light the same way we humans do. And that means that the colors we can see may not be helpful for them.

In a nutshell, plants appear green to our eyes because they reflect green light. Nonetheless, green light isn’t that useful to plants.

For plants to survive and thrive, they’ll need invisible light: blue, red, orange, and violet. The UV and infrared light from the sun may also be useful to plants under some circumstances.

Light Quantity

This is based on the amount of light that reaches the plants. It’s important to let your plants get more photons to allow for faster growth.

If you are growing plants that produce flowers and fruits, be sure to let them enjoy the correct light quantity. This is to allow them to convert water, CO2, and nutrients into energy, allowing for growth.

Some plants refuse to flower when they don’t get enough light. You might see a plant that appears to be flowering, but without intense light, it’ll fall halfway through the process.

Where Should I Position Grow lights?

For this, you’ll want to think about how a plant gets light in its natural habitat. If your indoor plants don’t get enough light, they’ll bend toward the light source, such as a window.

You’ll be required to place grow lights above your plants, so they can grow as well as they would in their natural habitat.

If you are growing plants indoors and there is a window, follow the rules below:

Bright Light

When it’s sunny, a southern or western-facing window provides bright light. As such, it would best to give your plants about six hours of artificial light to supplement the natural sunlight.

Also, don’t be tempted to move your plants closer to the window, even if it’s winter. Most plants don’t handle the cold and will most likely die.

how many hours of light for seedlings

Indirect Light

If your window is facing east, you’re most likely getting indirect sunlight. An east-facing window appears to have a curtain blocking the light from reaching plants.

As such, you’ll want to supplement your plants with more hours of light to help them grow and flourish. This can be around 6-10 hours of light depending on what you are growing.

Low Light

In winter, many grow rooms don’t get adequate light from the sun. This is especially true with north-facing windows that seem to be partially shaded.

If you can’t read a newspaper comfortably, your grow room is probably receiving low light. In that case, you’ll want to give your plants about 10-16 hours of artificial light to enable them to grow and thrive.

How Do I Ensure My Plants Get Enough Light?

Plants need to receive light every day at the same time. Therefore, you’ll want to invest in something that turns grow lights on and off automatically.

The best automatic timer does that perfectly, ensuring your plants get light or darkness when they need it.

With most grow light timers, you can set the light to turn on and off whenever you need it. And if you have a shoestring budget and can’t afford an automatic timer, an alarm will come in handy.

Final Thought

If you want to be successful in your indoor gardening endeavor, you must give your plants enough light. This means you need to understand your plant to know its light needs.

Hopefully, you now understand how long to keep lights on your plants. Enjoy doing indoor gardening!

Leave a Comment