A seedling is a young, newly sprouted plant that has exhausted the energy reserves stored within its original seed. Once the first tiny leaves, known as cotyledons, unfurl, the plant immediately transitions to relying on photosynthesis for growth and must receive light to survive. A lack of adequate light intensity or duration will quickly cause the seedling to stretch and become thin and weak, a condition called etiolation. To prevent this legginess and ensure robust stem and root development, indoor seed starting requires providing high-quality light and careful management of the light-dark cycle.
The Optimal Daily Light Cycle
Seedlings require a significant period of light exposure each day to fuel their rapid growth during the vegetative stage. The ideal daily schedule for most young plants falls within a window of 14 to 16 hours of continuous light. This duration mimics the long, sunny days of late spring and early summer, encouraging the maximum rate of photosynthesis necessary for building plant structure. Maintaining this consistent cycle is simplified by using an automatic timer, which removes human error and ensures the plants’ internal clock remains regulated.
Once the seedling emerges from the soil, the need for light is immediate. The 14 to 16-hour light period should begin as soon as the first sprout is visible above the soil line, shifting the plant from its stored-energy phase to the energy-producing phase. The remaining 8 to 10 hours of the 24-hour cycle must be a period of uninterrupted darkness for proper development.
Why Seedlings Require Darkness
Seedlings actually require a predictable dark period for performing essential metabolic functions. This dark phase is when the plant primarily engages in respiration, a process where it uses the sugars created during daytime photosynthesis to build cell walls, roots, and sturdy stems. Without this dedicated rest period, the sugars produced would not be efficiently converted into new plant structure.
A continuous 24-hour light cycle can also lead to photoinhibition, which is a stress response where the photosynthetic apparatus becomes overwhelmed and damaged. Constant exposure prevents the plant from repairing the oxidative damage that occurs naturally during photosynthesis. The necessary period of darkness allows the plant to regenerate key photoreceptors, resetting its internal clock and preventing energy depletion. This crucial dark period supports the plant’s natural circadian rhythm, influencing root growth and overall hardiness.
Light Intensity and Positioning
The intensity and proximity of the light source are equally important for preventing etiolation. Seedlings must receive a high concentration of light energy, which means grow lights need to be positioned very close to the top of the plant canopy. For common fluorescent fixtures, such as T5 or T8 bulbs, the ideal distance is often only 2 to 4 inches above the seedlings to deliver sufficient intensity.
Light Emitting Diode (LED) fixtures are highly efficient and produce less heat, but the appropriate distance varies widely depending on the fixture’s power. Starting with LEDs positioned 12 to 24 inches away and adjusting based on the plants’ reaction is a safer approach. If seedlings begin to stretch, the light source is too far away and must be lowered immediately. Conversely, if the leaves appear bleached, curled, or scorched, the light is too intense or too close and should be raised to prevent damage.
Preparing Seedlings for Outdoor Conditions
Before indoor-grown seedlings can be successfully transplanted into a garden, they must undergo a process called “hardening off.” This gradual transition acclimates the tender plants to harsher outdoor elements, including stronger light, wind, and temperature fluctuations. The light from a grow lamp is significantly less intense than full outdoor sunlight, which also contains strong ultraviolet (UV) radiation.
The hardening off process typically spans one to two weeks and involves slowly increasing the plants’ exposure to the outdoors. On the first day, seedlings should only be placed in a shaded, protected area for a couple of hours. Over the following days, the duration of outdoor time is extended, and the plants are slowly moved into areas that receive dappled sunlight, then finally direct sun. This methodical process allows the foliage and structure to toughen, reducing the risk of sunscald and transplant shock.