Can You Leave Grow Lights On 24 Hours A Day?

(Last Updated On: October 10, 2021)

Providing your favorite plant with ample light can be tricky if you don’t have enough open space to put your plants. It’s also the same case if you live in an area where the sunlight is as rare as it gets during certain seasons. 

So, using a grow light makes a lot of sense for many people, as it provides the necessary light for your indoor plants for steady growth.

But many people have a common question. Can you leave grow lights on 24 hours a day? In a basic sense, it seems that it would be best for your plant. But in reality, keeping the grow light on for 24 hours will significantly weaken your plant and cause stress and make it susceptible to various diseases.

We need to discuss a lot more to that question to understand better how a grow light works and how to use it properly. So without further ado, let’s get into the nitty-gritty details.

Advantages of 24-hour Lighting

Advantages of 24-hour Lighting

There are certain advantages to leaving the grow lights on 24 hours every day. The primary benefit is that it provides your plant with more light than it ever needs, so grow lights will ensure faster plant growth. 

Some of the benefits are described in detail below:

Faster Growth During Vegging

Grow lights ensure faster plant growth during the vegging process. It had been noticed previously that if you accidentally left grow lights on during veg for 24 hours straight, not only did it make the plants grow faster, but it also started developing the buds much sooner than seen on the normal light. 

Although there might be some disadvantages, a 24 hour light cycle during veg far outweighs them.

Don’t Need Grow Light Timers

Another benefit of using grow lights for indoor plants is that you don’t need to maintain grow light timers, as you will keep the grow lights on 24 hours every day. 

Disadvantages of 24-hour Lighting

Disadvantages of 24-hour Lighting

If you’re looking for steady and proper growth for your plant, maintaining a light dark cycle is an absolute must for the plants. Too much light will create a whole lot of problems for your plants if you’re not careful. 

Here are some of the disadvantages you have to face if you keep the grow lights on 24 hours.

Plants will Get Weaker

Up to a certain period, keeping the grow lights on for longer is good for plant growth. 

But if you decide to keep the lights on 24 hours continuously for many days, the plants eventually will stop growing and get weaker. Their growth will be reduced to a halt, and they will be way more susceptible to all sorts of diseases.

Late Flowering

Leaving your grow lights on for 24 hours a day is an absolute catastrophe during the flowering period.

Cannabis plants will have difficulty switching to flowering if the light and dark cycle is not appropriately maintained. You have to reduce the lights to 12 hours a day if you want proper flowering.

Reverting to Vegging State

Too much light will bring disastrous results to plant growth. If you leave grow lights on during the flowering phase for the plants, there’s a high chance that those plants will go back to the vegetative phase once again. 

Moreover, after a plant has been reverted to the vegetative phase, it’s tough to revert it again to the flowering phase. So, it is not recommended to keep the grow lights on during flowering at all.

Additional Stress, Costing, and Heat

Leaving grow lights on 24/7 means wasting a lot of electricity that the plants are not utilizing. You don’t need more lights to keep your plant growing. Plants need to rest, especially the vegetable bearing plants. 

Using more lights means you will generate more heat, electricity bills, and stress to your electricity lines that will not be helpful for your plants at all.

What Happens If You Leave Grow Lights On 24 Hours A Day?

What Happens If You Leave Grow Lights On 24 Hours A Day?

In a basic sense, plants aren’t that different from regular animals and us humans. They also need to sleep, and they need breaks from light exposure. 

For example, weed plants can absorb light all day. If the grow light is kept on all day, they’ll engage in the photosynthesis process without any breaks. So on a more positive side, keeping the grow light on will result in maximum growth for weed plants.

But like everything else, there are inevitable consequences to this. If a cannabis plant is kept at constant light during the flowering phase, they are likely to encounter a high amount of stress. This excess stress will lead to unhealthy plants, increased susceptibility to disease and pests, and certain death.

If you somehow accidentally left grow lights on overnight, it won’t kill your plant at the spot. Weed plants that are in a vegetative stage will be mainly fine for a day or two. Although 24 hours of light is not recommended, it won’t cause that much harm to your plants if you maintain the light cycles afterward.

But if somehow the plants are in the flowering state, it’ll cause a big problem and even decrease the life cycle and cause stunted growth for the plant. 

Cannabis plants are especially very sensitive to constant light exposure when they are in the flowering state. More than 12 hours a day lighting will confuse the plant into a vegetative state yet again. If this occurs, it will take a very long time, more than two weeks, until they start flowering again.

Weed plants can grow to the optimal situation if kept at around 12 to 16 hours of light every day, regardless of whether the light is natural or artificial. Twelve hours of light each day is the sufficient light amount for any kind of plant.

When you determine growing plants in LED growing light, it is essential to provide the optimal light output to the plants. Different kinds of plants will need a different set of light and dark periods to offer more yield.

Can Grow Lights Stay On 24 Hours A Day?

Grow lights provide artificial lighting for maintaining light dark cycle crucial for plant growth and development. Grow lights like LED and fluorescent can produce a light spectrum that is beneficial to plant growth.

LED bulbs may cost more than fluorescent ones. But in the long run, the price difference between their usage goes in favor of LEDs. That’s because if you keep the LED lights on 24 hours, it’ll spend half the electricity to keep the fluorescent bulb running. 

Though modern grow lights can keep running for 24 hours with no problem, it is not recommended at all for producing plants that are healthy and live long. If you accidentally left grow lights on overnight, it could harm your plants, depending on the type and stages.

In general, you shouldn’t keep growing lights for 24 hours because plants need a light dark cycle for proper development. Plant cycles go through photosynthesis and rest phases periodically. For proper respiration and metabolic works, getting the plant 18 hours of light at maximum is suggested.

If you’re unsure about how much light your plant needs for optimal growth, check out different websites and instruction manuals available on the internet. 

Ideal Amount Of Light Exposure For Plants

Ideal Amount Of Light Exposure For Plants

The ideal light cycle for most flower plants, including marijuana plants, is very low. They don’t need much light for long-term sustenance. To produce plants that have healthy flowers and leaves, you need around 20 to 40 moles of light every day. For maximum yield, it is suggested to get as close to 40 moles as possible.

You don’t need 24 hours of light every day as it will negatively impact the plant’s health. To ensure both light and temperature needs, you need around 12 to 18 hours of light every day. If you run 18 hours of grow light to maintain the light cycle, the lights will deliver 617 μmols for a second and eventually add to 40 miles per day.

When you need to switch to 12 hours of light per day for growth, the need for light grows up to 926 μmols per second to add up to 40 mols per day.

It might not be possible for you to reach 40 miles per day. A more realistic light cycle is around 20 moles. For that, you need 308 μmols of light for 18 hours and 462 μmols for 12 hours.

People want to reach the lighting needs with their low amount of lights. So they like leaving grow lights on 24 hours straight to achieve that. You’re much better off getting grow lights that can reach higher light outputs than keeping them on all day long.

Important Factors to Consider While Keeping the Grow Lights On

There are certain essential factors that you must consider if you plan to keep the grow lights on for indoor plants.

Plant Types

Considering the light exposure that different plants need, they can be divided into three groups.

  • Low-light plants: These types of plants need less light energy than regular ones. You can keep them under LED grow light for around four hours a day.
  • Medium-light plants: Medium-light plants need higher light exposure than low-light plants. They are the standard type. Most plants are medium when it comes to the light cycle. You need to provide them with six to eight hours of light every day.
  • High-light plants: High-light plants need a lot of light exposure and good light intensity for proper growth. You need to keep them at around 14 to 16 hours light if you plan on growing indoors.

Growth Phases

To keep the natural process of plant growth ongoing, you need to balance both light and dark cycles. The different phases of plants might require a diverse light spectrum and enough light sources to complete the growth cycle successfully.

  • Seedling State: The most optimal light exposure for the seedling states is to keep them in 16 hours of light and then 8 hours of dark. Although seedlings need a lot of light to ensure maximum growth, it is crucial to keep in the dark to maintain the plant’s health.
  • Vegetative State: Vegetative phases need enough light for sustaining the growth cycle and respiration process. One light cycle that is ideal for the vegetative state is the 18 hour light followed by 6-hour darkness. It will provide a solid stem, an ample amount of leaves, and a proper rest period.
  • Flowering State: Proper amount of light in this phase will force plants to grow healthy buds. This stage is crucial for cannabis growers, as the marijuana plants are quite delicate in the state. The optimal cycle for the stage is to have 12 hours of light and 12 hours of darkness.

Grow Light Types

Different types of grow lights cover different light spectrums. Moreover, they support different growth phases and photosynthesis. So choosing a proper grow light for your plant is reasonably necessary.

  • LED grow light: LED grow lights are very good at maintaining the light dark cycle for your indoor plants. They provide plants with more light, cover a wide range of light spectrum, and consume low energy. 

The only negative aspect of LED grow lights is their price point, higher than ordinary lights. They can also cause harm to the human eye.

  • Regular light: LED light bulbs are very cheap, and they last for long periods of time. But they can’t provide a good light intensity to the plants. They have a pretty narrow wavelength, ranging from infrared light to normal light. Also, they produce unstable energy and can get heated quickly.
  • Fluorescent light: These types of light aren’t recommended for plant growth, but in dire situations, they’ll work. You just need to keep them around six or seven inches from the plants. 

A downside to these lights is that they can get heated pretty quickly, resulting in light burn in plant leaves.

  • HID: HID, also known as the high-density discharge light, is recommended for people on a tight budget. The plant receives less light compared to LEDs, which can be detrimental to indoor gardening. 

A good solution is to use a mirror or reflective surface on the bottom of the plant so that the plant is constantly exposed to a higher amount of light.

  • Metal Halide Light: These types of lights are great for indoor growers, as they can produce more light per watt. They aren’t great for growing cannabis, though, as they produce a lot of heat, hamper the rest period and halt the biochemical process needed for the flowering phase.

Temperature and Humidity

To produce both healthy and viable plants for future development, you need a proper light cycle. A suitable light cycle ensures the necessary temperature and humidity required for plant growth. 

The ideal temperature for plant growth is 80 degrees Fahrenheit. And to continue normal respiration, the humidity must be around 40%.

Distance Between Grow Lights and Plants

The distance between grow light and plant depends on the type of bulb used and the different stages of plant growth. A quick note to remember is that the higher the light’s wattage, the more distance should be made between the plant and the light.

Following is a chart for knowing the optimal distance between the light and the plants.

Type of Grow LightDistance
LED12-24 inches
Fluorescent10-19 inches
HID25-60 inches
Metal Halide Lights25-30 inches

Frequently Asked Questions (FAQ)

What happens if you leave grow lights on 24 hours a day?

At first, the plants will grow more because of the increased photosynthesis. But over time, this will cause stress in the plants and make them vulnerable to various diseases.

Can grow lights be on 24 hours a day?

Yes, modern grow lights can be on 24 hours a day, but this is not an optimal lighting setup for plants.

How many hours a day should grow lights be on?

Grow lights should be on for six to twelve hours per day, depending on the plant and stages of plant growth.

Can grow lights start a fire?

LED or fluorescent grow lights have almost zero to no possibilities of starting a fire.

Final Words

Depending on the plant type and growth stages, the grow lights can be kept for around 6 to 18 hours. But like animals, plants also need dark phases for rest and maintain the plant cycle. 

Experts never recommend keeping grow lights on 24 hours straight every day. It is also important to calculate the amount of light a plant needs every day to maintain the sufficient light needed for growth.

To summarise – can you leave grow lights on 24 hours a day? The answer is yes. But you should not do so.

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