What’s the Best Time of Day to Run Grow Lights?

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I gardened for years before I started using a grow light. When I finally did invest in a grow light–the SpiderFarmer SF1000D – it revolutionized my growing experience. My plants started growing exponentially, but I quickly realized that you can have too much of a good thing. While some growers have differing opinions on the best time to run a grow light, most all agree that plants do need a dark period. 

The best time of day to run grow lights is during normal daylight hours. Running a grow light for 14-16 hours followed by an eight-hour rest period that aligns with natural cycles is ideal for seed-starting, but you can run grow lights any time of day as long as you can keep a consistent schedule. 

Greenhouses are perfect for gardeners who have access to that luxury, but for some growers that just isn’t the reality. Fortunately, quality grow lights are becoming more and more accessible, making it possible for gardeners to start seeds and grow vegetables indoors!

Keep reading to understand why normal daylight hours are the preferred time to run grow lights, and why keeping a consistent schedule is even more important. 

Best time of day to run grow lights

There is no truly right or catastrophically wrong answer to this common question–but it is critical to choose a schedule that works for you. Most gardeners use grow lights with the intent to transplant seedlings outside, so it makes sense to keep to typical daylight hours, like between 6 AM to 10 PM or 5 AM to 11 PM. 

Some gardeners run grow lights during the daylight hours, and others run their grow lights through the evening hours into night. There are advantages and disadvantages to each approach, but either one will only work for your plants if it works for you. 

Other periods to run grow lights 

If running grow lights during normal daylight hours doesn’t work for you, it doesn’t have to. Many gardeners have success running grow lights during other times. Whatever you decide, just keep your schedule consistent and your plants will be more than happy.

Use a timer

The most important factor in using grow lights is to keep your schedule consistent–no matter which schedule you choose. Make it easier on yourself and invest in a timer – this one from VivoSun is inexpensive and works great. Outlet timers keep your lighting schedule perfectly, so there is no room for error.

Running lights at night

There are many gardeners that run their grow lights at night, for various reasons. Growers who live in areas with extreme temperatures might benefit from leaving the lights on through cold nights since plants are warmer when they are under lights. Similarly, growers in regions of the world with hot days might choose to run their lights at night to keep their plants from getting too hot during the day. 

Still, other gardeners run grow lights at night because their electric costs aren’t as expensive during the off hours. There’s nothing wrong with running grow lights at night, so long as you give your plants a dark period to rest–so even if you’re awake and about, make sure your indoor plants are resting in a dark place for at least four hours. 

Only run grow lights through the night if you plan on keeping your plants inside for their entire life cycle. If you’re starting seedlings with the intention to transplant them outdoors, keep the seedlings on a light schedule that closely resembles that natural one that you’ll be introducing them to. 

If you find yourself in a situation where you do need to acclimate your plants to a different lighting schedule, alter the hours slightly over a period of a few weeks. Adjust your plants’ schedule by an hour each day until you have transitioned them to a new schedule. 

How long to run grow lights

There are many opinions about how long you should run your grow lights, but what works well for one gardener might not work for someone else. Use the following recommendations as a guideline, and watch your plants closely to see how they react. 

Whether you use an LED grow light or a fluorescent bulb, make sure that it’s white light–or full-spectrum–for seedlings and mature plants. 

If you see signs of leaf scorch or plant sunburn, reduce the lighting and move the light further away from your plants. If your plants aren’t putting out new growth or the leaves are yellow, you might want to increase their time under the light and maybe bring the light a little closer. 

If your seedling leaves are pointing up or growing quickly, it’s a good sign that your plants are thriving. If your seedlings or new plant growth looks a little leggy, rotate the container every few days to correct it. 

Seed starting

Most experts recommend running a grow light for at least 12 hours and up to 16 for seedlings. Seedlings are more sensitive to light than more mature plants, so keep your grow light far enough away from the seedlings to burn the young plants. 

If you’re using LED lights, that means to hang your lights at least two feet above the tops of your seedlings, and move the grow light up as the seedlings grow. If you’re using fluorescent shop lights, you can keep those lights much closer–between four and six inches above the plants. Again, make sure to move your fluorescent bulbs up as the seedlings grow taller. 

Vegetative growth

Mature plants that are in a vegetative state can stand a little more light and thrive with 14 to 18 hours of artificial light each day. Keep your fluorescent bulbs no more than 12 inches away from your plants, and if you’re using an LED light, hang the light between one and two feet from the tops of your plants.

How much water a plant needs is directly dependent on how much light that plant is getting, so check your plants under grow lights more frequently and water as needed. You might find, as I also learned, that plants dry out much quicker under a grow light than they did in other areas of my home. 

If you can, bottom water your seedlings and plants to avoid burning wet foliage under the powerful lights. 

Dark periods

One common mistake that growers make is running grow lights 24/7. While it may seem that more light contributes to faster growth, plants, just like people, need a period of rest. 

This dark period, regardless of how long or short, is a critical time in plant development. When plants aren’t preoccupied with metabolizing light through photosynthesis, they can direct their attention to root development and towards moving stored energy through their stems and foliage.

Conclusion

Grow lights are an incredible tool that more and more gardeners are embracing. You no longer have to have a greenhouse to start healthy seeds at home. You no longer need to have garden space to grow your own vegetables, if you have enough grow lights and indoor space! 

Use the information in this article as a framework, and do your own research from there! After all, gardening is an experiment as well as an experience–make it yours!