Leaving Grow Lights on 24 Hours: Good or Bad Idea?

Grow lights were created to mimic the effects of natural sunlight used to grow outdoor plants. Outdoor plants grow using sunlight and are limited to the amount of time spent in the sun versus in the dark. So the question is: will your plants grow better if exposed to both light and dark periods or if you leave LED grow lights on 24 hours a day?

Can You Leave Grow Lights on 24 Hours a Day?

This debate continues across Internet forums, blog posts, and community sites like Reddit. Each person has their own opinion and might not want to hear different viewpoints. Instead of listening to the guy we know down the street, we prefer to consult with the Director of Plant Microbial Interaction at Mammoth Microbes, Karuna Chourey, PhD.

According to Dr. Chourey, you should NOT constantly run grow lights for 24 hours a day. Below is an interview with Dr. Chourey where she helps to finally answer the question we’ve all debated for decades. Pardon the not-super-great audio quality.

According to Dr. Chourey, plants are living things just like humans. Humans need to rest or sleep in order to grow, and plants are no different.

When the lights are on, the plants are producing sugars and going through photosynthesis. When the lights go off, the plant releases all the sugars to the rhizosphere and to the beneficial microbes in the soil. This means that leaving grow lights on for 24 hours can essentially starve a plant from fully utilizing a rest period that it needs to grow healthier plants.

Is Bigger Better?

Those who argue for the 24-hour grow light cycle claim that they are seeing larger plants than if they turned the lights off. We are not going to disagree with that, but the question becomes: is bigger better? According to Dr. Chourey, you may very well notice larger plants, but don’t expect them to be better plants. As she states in the video, plants that use a 24-hour grow light are larger but are weaker and lower quality. Basically, they are all show and may need to be ‘propped up’ as they are subject to a range of obstacles and diseases.

Quality Versus Quantity

Dr. Chourey agrees that those who grow with a 24-hour grow light may reach the end of the bloom cycle 20% faster than those giving plants a dark period. If you are at all interested in the quality of your plants, you will want to create a natural light cycle for your plants that includes a “night” mode.

Basically, the question now becomes: are you more concerned with the quantity you are producing, or the overall quality of the plants? This can help you decide which light cycle is best for your specific use case.

can you leave grow lights on 24 hours a day

References & Data

Don’t just take Dr. Chourey’s word for it. She provided other examples of research completed by scientists looking to answer the same question:

For millions of years, plants have grown using a light and dark cycle. Thanks to modern technology, we have the ability to change that. But should we? As Dr. Chourey pointed out, the 24-hour grow light might not be as good as it seems. You may find your plants grow bigger and faster on a 24-hour grow light system, but the overall quality of the plant will suffer. Not to mention, leaving your lights on all the time will also increase your electric bills.

Ultimately it is your decision. Would you rather have quality or quantity? The choice is yours, and we recommend running your own tests because results may vary by plant. Experimenting at home can help you decide what’s best for your plants and growing goals.

High Tech Gardening