A seedling is a young plant that has recently sprouted from a seed, representing the most vulnerable and growth-intensive stage of its life. For indoor growers, relying on supplemental grow lights is necessary because ambient household light is not strong enough to fuel healthy development. Successful seedling growth depends on providing the energy required for photosynthesis, the process by which plants convert light energy into chemical energy for food. Controlling the duration of artificial light is the most important factor for ensuring young plants develop strong stems and robust foliage rather than becoming weak and spindly.
The Ideal Daily Light Duration
The goal of using grow lights is to mimic the long, bright days of late spring and early summer, when most plants naturally experience their fastest growth. For the majority of common garden vegetables and flowers, such as tomatoes, peppers, and brassicas, this translates to a daily light cycle of 14 to 16 hours of continuous light. This duration provides enough time for the seedling to accumulate the necessary “Daily Light Integral” (DLI), the total amount of light energy received over a 24-hour period. Providing less than 14 hours often results in poor, stretched growth, a condition known as etiolation.
The remaining 8 to 10 hours of the day must be a period of complete darkness. The 14- to 16-hour duration is a reliable standard for most indoor setups using well-positioned LED or fluorescent fixtures. Fixtures should be kept just a few inches above the seedlings to maximize light delivery. Using an automatic timer is highly recommended to ensure the light cycle is precisely repeated every day, which helps maintain the plant’s internal biological clock.
Why Seedlings Need Darkness
The dark period is an essential phase for critical metabolic functions that cannot happen in the light. During the light period, the plant focuses on photosynthesis, creating sugars for energy. It is during the subsequent darkness that the plant switches to respiration, utilizing those stored sugars to fuel growth and energy reallocation.
The 8- to 10-hour resting period is necessary for the plant to transport and process the carbohydrates produced throughout its structure. This process is crucial for developing strong, thick cell walls in the stems and leaves, resulting in sturdy, healthy plants. Continuous 24-hour light is detrimental because it leads to stress, prevents the plant from repairing cellular damage, and disrupts hormonal regulation. The plant’s circadian rhythm relies on the consistent alternation of light and dark to properly regulate growth hormones.
Adjusting Duration Based on Plant Type and Stage
While 14 to 16 hours is a general guideline, specific plant species and their developmental stage may benefit from slight adjustments. Most common vegetable seedlings are considered “long-day” or “day-neutral” plants, meaning they thrive under the extended daylight of summer. For these types, the standard long light cycle is ideal for maximum vegetative growth.
Some growers may increase the light duration to 18 hours during the rapid vegetative growth phase to accelerate development, but this is often unnecessary for typical home garden seedlings. As seedlings mature and form their first “true leaves,” the duration should be kept consistent until they are prepared for the outdoor environment.
An important transition occurs when preparing seedlings for transplanting outside, a process called “hardening off.” During hardening off, the light duration and intensity are gradually reduced to acclimate the plants to natural light cycles and conditions.
Recognizing Symptoms of Light Stress
Observing your seedlings is the most practical way to determine if the current light duration or intensity is appropriate for your specific setup. Symptoms of too little light are characterized by etiolation, where the plant stretches excessively. This results in a pale color, thin and weak stems, and widely spaced leaves. The seedling appears to be reaching toward the light source, indicating a need to increase the light duration or increase the light intensity by lowering the fixture.
Conversely, signs of too much light or duration manifest as photodamage or light burn. The most common symptoms are leaves closest to the light source turning yellow or bleached white, often with the leaf veins remaining green. In severe cases, the leaf edges may appear dry or crispy. If these symptoms appear, the light duration should be reduced or the light fixture should be raised away from the plant canopy.