A seedling is a young plant that has recently emerged from a germinated seed, consisting of its first root, stem, and initial leaves. Starting seeds indoors requires supplemental lighting because the low intensity and short duration of natural sunlight passing through a window, especially during late winter and early spring, is often insufficient for proper growth. This lack of light causes the tender plants to stretch and become weak, a condition known as etiolation. To cultivate robust, garden-ready transplants, the quality of the light—its color or spectral composition—is more important than the quantity of light provided.
The Science of Light and Growth
Photosynthesis, the process plants use to convert light energy into chemical energy, is driven by light in the visible spectrum, specifically the 400 to 700 nanometer (nm) range. Chlorophyll, the primary photosynthetic pigment, does not absorb all colors of light equally, which forms the basis of the Photosynthesis Action Spectrum. Chlorophyll absorbs light most effectively in two distinct regions: the blue-violet range (around 430–470 nm) and the red-orange range (around 640–670 nm).
Light outside these peaks, particularly green light (around 500–600 nm), is largely reflected, which is why plants appear green to the human eye. Beyond the light used for energy conversion, plants possess specialized photoreceptors that act as light sensors, governing development. Phytochromes, for instance, respond to red (around 660 nm) and far-red (around 730 nm) light, regulating processes like seed germination and stem elongation.
Cryptochromes are another class of photoreceptors that primarily sense blue light (around 400–500 nm), controlling developmental responses such as leaf expansion and the suppression of stem growth. The balance of light colors received determines the plant’s overall shape and structure, a process called photomorphogenesis.
Optimal Light Spectrum for Seedling Development
The color of light most beneficial for producing a sturdy, healthy seedling is blue light, typically in the 400–500 nm range. Blue light is directly responsible for preventing the undesirable stretching or “legginess” that often plagues indoor-started plants. By signaling to the cryptochromes, blue light promotes compact growth, encourages the development of short, stocky stems, and stimulates the formation of thick, dark green leaves.
This light spectrum is particularly important because it mimics the high blue content of full sun, signaling to the young plant that it is not shaded by a canopy of other leaves. Without sufficient blue light, the plant will waste energy elongating its stem in a futile search for a brighter light source. While red light (600–700 nm) is essential for fueling photosynthesis and is the most efficient wavelength for energy conversion, an excessive amount of red light relative to blue light can encourage stem elongation.
For robust seedling development, the light spectrum should feature a balanced mix with sufficient blue light. Plants grown under a high proportion of blue light are typically shorter, denser, and better prepared for transplanting into the garden.
Practical Light Choices and Setup
Translating these biological requirements into a practical setup for the home grower leads to a preference for full-spectrum white LED fixtures. These lights combine red, green, and blue light diodes, often coated with phosphors, to emit a balanced white light that includes the high blue content necessary for seedlings. Full-spectrum lights with a high Correlated Color Temperature (CCT) are recommended, rated between 5000 Kelvin (K) and 6500 K.
This high Kelvin rating indicates a cooler, bluer light output, signifying a greater proportion of blue wavelengths suitable for young plants. While specialized “blurple” fixtures, which emit a mix of only red and blue light, are also effective, the white light option allows gardeners to observe the true color of their seedlings for early detection of issues.
The intensity of the light is measured using Photosynthetic Photon Flux Density (PPFD), and seedlings typically require a PPFD in the range of 100 to 300 micromoles per square meter per second (µmol/m²/s) once true leaves appear. To achieve this intensity, the fixture’s distance from the seedlings is a primary consideration, and lights need to be positioned closer than one might expect. For most LED fixtures, this distance is often between 12 and 24 inches, though lower-power fixtures may need to be as close as 6 inches. The duration of light, or photoperiod, should be set to run for 14 to 16 hours per day to provide sufficient energy for growth while still allowing the necessary dark period for metabolic processes.