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Many gardening enthusiasts face an important question during the seedling stage—— How many hours of artificial light do my seedlings really need?
If the light is too short, the seedlings may grow tall and weak. If the light is too long, the plants might get stressed.
To solve this problem, I found a gardening blogger who did an experiment. He grew four types of plants under four different light durations and shared very useful experiences.
This article will use his experiment data and observations to help you decide scientifically how many hours of grow light your seedlings need.
In this experiment, the blogger set up four different light environments: 6 hours, 12 hours, 18 hours, and 24 hours of light per day. He wanted to compare how the length of light affects seedling growth.
The plants grown were arugula, broccoli, tomatoes, and peppers.
He used the same seedling soil and the same type of grow lights — three 40-watt LED light strips running at the same time — to ensure the light quality was consistent.
Each environment had the same number of seed trays, and each cell had 4 seeds, making a total of 48 seeds per light environment.
The only variable was the hours of light; all other conditions stayed the same.
The blogger emphasized that this setup helped him clearly see how light duration affects germination rate, seedling quality, and overall growth.
Arugula showed a very stable germination rate, with almost all groups having 9 to 12 seedlings. The length of light had little effect on arugula’s germination.
Broccoli also had a good germination rate, about 9 to 10 seedlings, but the seedlings were leggy (thin, long, and weak), especially in the 6-hour and 12-hour light groups where legginess was most obvious.
Tomatoes had a high germination rate of 11 to 12 seedlings. The seedlings in the 12-hour and 18-hour light groups grew strong and healthy, with no legginess issues.
Peppers had a germination rate between 8 and 11 seedlings, showing overall good performance.
Summary: Germination rates were mostly unaffected by light duration, but seedling legginess was clearly linked to the number of hours of light.
Under all light conditions, broccoli seedlings showed legginess.
The 6-hour light group performed the worst, with thin, tall, and weak-looking seedlings.
Although the 18-hour and 24-hour groups were a bit better, the overall condition was still not ideal.
The blogger thinks that the light quality might not be suitable for broccoli and plans to run a “light fixture comparison experiment” later to test different types of lights.
The 6-hour light group showed slow growth and legginess;
The 12-hour group had moderate growth;
The 18-hour group grew vigorously with wide, healthy leaves;
The 24-hour group still grew but leaves began to curl, showing signs of stress caused by too much light.
The blogger noted that 24 hours of light gives the plants no “rest time,” which can cause negative effects.
Tomato seedlings under 6 hours of light were very leggy and showed the worst growth;
The 12-hour group performed the best.
The blogger said, “This is the light duration I recommend most for seedlings after many years. Plants are strong and healthy with almost no legginess.”
The 18-hour group was similar to the 12-hour group, with good growth;
The 24-hour group looked like it was growing, but leaves showed yellowing and dry edges—typical signs of light stress.
The 6-hour group did okay with no clear legginess;
The 12-hour and 18-hour groups grew more vigorously with greener leaves;
The 24-hour group’s growth slowed down and leaves became unhealthy.
Germination stage
The length of light exposure has almost no effect on germination rates. Other factors like temperature and humidity are better for ensuring successful germination.
Seedling growth stage
Too little light (such as 6 hours) causes seedlings to become leggy and weak.
Too much light (24 hours) leads to plant stress, shown by leaf curling, yellowing, and sometimes stopped growth.
The best light duration is between 12 and 18 hours, which helps most seedlings grow healthy without legginess.
Mind and Soil finally chose a daily cycle of 16 hours of light and 8 hours of darkness. He explained:
“16 hours of light meets the plant’s photosynthesis needs and also mimics a natural day-night rhythm, giving plants time to ‘rest’ and recover. From my many years of experience, this duration works best for most vegetable seedlings.”
For every gardening enthusiast, supplementing light is not just about timing but also about the quality and intensity of the light source.
This experiment clearly shows how light duration affects seedlings in many ways. We recommend starting with 12 to 18 hours of light and adjusting based on your own equipment and growing environment.
From custom light planning, to tailored quotes, and everything in between, our team of horticulture experts are always ready to assist.
Vanten LED
Guzhen, Zhongshan, Guangdong, China
WhatsApp: +86 180 2409 6862
Email: info @ vantenled.com
We are a professional LED plant light manufacturer, committed to using technology to enhance the maximum potential of the lamp, continuously maximizing the benefits for growers, and saving energy for the planet.
