Can You Leave Grow Lights on 24 Hours a Day?

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By Alex Harris

You can technically leave grow lights on your plants 24 hours a day to speed up plant growth, but you probably shouldn’t in most cases. While it’s enticing, it has severe disadvantages.

All light and no period of uninterrupted darkness are bad for plants for multiple reasons. First, it’s costly for you. But more importantly, it can lead to weakness, disease, and stunted growth, since it robs the plant of a balanced photosynthesis + respiration cycle.

Bellow, I’m going to go over the effects of 24 hour grow lights usage and how to better light your plants.

Also read: Grow Lights That Professional Growers Use

Length of Light-Dark Cycle in Plant Growth

light dark cycle in plant growth

Most plants have a 24-hour light cycle called a photoperiod. However, it can be short-day, meaning more than 12 hours of darkness (uninterrupted) and less than 12 hours of light.

There is also a long-day cycle, characterized by less than 12 hours of darkness (uninterrupted) and more than 12 hours of sunlight.

Advantages of 24-Hour Lighting

Unfortunately, there’s only one real advantage to leaving your lights on full time.

Faster growth

24-hour lighting leads to more rapid growth, which we all want at the end of the day.

Disadvantages of 24 Hour Daylight

There are several disadvantages that you need to take into consideration.

Photosynthesis and Respiration

24-hour lighting negatively affects the plant’s ability to respire or rest. In addition, continuous light keeps the plant working hard at all times, which means it can tire and become vulnerable to developing diseases.

Additionally, the lack of respiration means constant photosynthesis. But healthy plant growth requires both parts of the cycle (photosynthesis and respiration).

Extra Costs

Another downside to 24-hour light for your plants is the added cost. Grow lights run on electricity, and running them constantly can spike your utility bill.

And, grow lights themselves can get expensive, as you need multiple to light multiple plants. They can be as cheap as $20-$30 and as expensive as hundreds of dollars.

Added Stress on Equipment – While you can leave a grow light on 24 hours a day, this will significantly reduce the number of days you get out of one light bulb, which means you’ll have to buy more.

Added Heat

Intense and overly frequent light and heat from grow lights can damage a plant by drying it out, hindering photosynthesis processes. No photosynthesis means no growth.

Light Duration Factors to Consider

The ratio of light versus darkness you give to a plant depends on the type of plant (mentioned above) and the growing conditions.

Have you read: How Far Should Grow Lights Be From Plants?

Type of Plant

Long day plants – such as root veggies, peas, spinach, and chard – need more light than darkness. On the other hand, short-day plants – such as soybeans, rice, and onions – need more darkness than light.

Some “neutral” plants rely on their age to bloom rather than sunlight versus darkness in a day (tomato plant and corn, for example).

Growth Phase

The color wavelengths LED lights emit (blue and red) are helpful for seedling growth, the flowering stage and those just beginning to fruit.

Type of Light

The type of light exposure (halogen bulbs or LED) will affect indoor plant growth, too. LED is more powerful and still energy-efficient.

Temperature and Humidity

Humidity and moisture are necessary for photosynthesis and growth. However, keep in mind that grow lights may get hot and dry out your plants. So, it’s good to supplement humidity with a humidifier or watering from below.

Size of Grow Space

If you have a small grow space, you still need to make sure your grow light isn’t too close to the plant. LED grow lights need at least 12 inches of distance from the plant.

The intensity of the grow light changes based on its Watts, affecting how far it should be from the plant.

Frequently Asked Questions

Here are the most common questions that might spring to mind.

Does more light mean more yield?

No, more light doesn’t necessarily mean more yield unless it is in moderation. However, more light can help, as long as there is still time for the plant to sit in darkness.

How much light output do plants need?

Most plants need at least 8 hours of uninterrupted darkness each day. 12 to 16 hours of light is ideal, with more light necessary for flowering plants.

Will plants grow faster under 24 hours of light?

Yes, at first, they can grow quickly with 24-hour light. However, the stress this puts plants under can stunt their growth later by making them vulnerable to disease.

Can plants get too much artificial light?

Yes, plants can get too much artificial light because too much light keeps the plant from respiring. Unfortunately, this means it won’t get a chance to release sugars into the rhizosphere or microbes in the soil.

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

The photosynthesis and respiration cycle will be offset, and the plant will get dry and weak. Basically causing the flowering phase to fail.

Conclusion: Use Artificial Lighting Properly

Using an LED grow light or grow lamp 24 hours a day is not recommended because it can cause severe damage to plants. Consider a balanced light/dark cycle based on the plant’s photoperiod instead.

Further reading: The Difference Between Led Vs Fluorescent Grow Lights