A seedling is a young plant that has recently emerged from its seed and is transitioning from relying on stored energy reserves to producing its own food through photosynthesis. Light is the primary fuel for this process, necessary for strong, healthy growth and preventing weak, elongated stems. While growers often assume that more light equals better growth, the physiological limits of the plant mean that light can become excessive, causing stress and damage. Understanding when light becomes a liability, rather than an asset, is important for indoor seed starting.
The Limits of Photosynthesis
Plants conduct photosynthesis by capturing light energy within specialized organelles called chloroplasts to convert carbon dioxide and water into sugars. As light intensity increases, the rate of photosynthesis rises proportionally, but only up to a maximum level known as the light saturation point. Beyond this threshold, the plant’s internal machinery, particularly its enzymes and carbon fixation cycles, cannot process the light energy any faster, and adding more light provides no additional benefit.
When light energy exceeds the plant’s capacity to utilize it, the surplus energy can initiate a damaging process called photoinhibition. This phenomenon specifically targets Photosystem II (PSII), a protein complex in the chloroplasts that is highly susceptible to light-induced stress. The excess energy excites molecules within PSII, causing them to react with oxygen and generate highly unstable molecules known as reactive oxygen species.
These reactive oxygen species inflict damage upon the photosynthetic apparatus, hindering the plant’s ability to efficiently harvest light. While plants have mechanisms to repair this damage, prolonged or excessively intense light can overwhelm these defenses, leading to chronic stress and a reduction in overall growth rate.
Recognizing Overexposure Damage
The first signs of light overexposure often appear on the leaves closest to the light source, which receive the highest intensity. A common indicator is leaf bleaching, where the green color fades to a pale yellow or white hue. This discoloration occurs because the excessive light has destroyed the chlorophyll pigment, making the leaf unable to photosynthesize effectively.
Another distinct symptom is the physical deformation of the leaves, which may begin to curl downward and inward. This curling is a defensive response by the plant, attempting to reduce the total surface area exposed to the overwhelming light intensity. In severe cases, the leaf edges or tips may become brown and crispy, indicating direct tissue damage similar to sunburn or scorching.
Chronic light stress often results in stunted growth despite consistent watering and nutrient supply. If the cotyledons or first true leaves remain small, or if the seedling stops adding new foliage while other factors are ideal, excessive light intensity may be the cause. Recognizing these visual cues early is important to preventing irreversible damage to the young plant.
Controlling Light Intensity and Duration
Managing light for seedlings involves regulating both intensity and duration. Seedlings generally thrive when the light intensity, measured as Photosynthetic Photon Flux Density (PPFD), is kept within a range of 100 to 300 micromoles per square meter per second (μmol/m²/s). For most home setups, the most straightforward way to control intensity is by adjusting the distance between the light fixture and the top of the seedlings.
The intensity of light decreases rapidly as the distance from the source increases, following the inverse square law. Fluorescent lights are typically positioned 2 to 4 inches above the canopy, while powerful LED fixtures may need to be 12 to 24 inches away to achieve optimal intensity. As the seedlings grow, the light fixture must be raised weekly to maintain a consistent distance and intensity.
Controlling the duration of light exposure is equally important, as seedlings require a dark period for metabolic processes like respiration and recovery from the previous day’s photosynthesis. A photoperiod of 14 to 16 hours of light per day is generally recommended for initial growth stages. Providing more than 18 hours, or continuous 24-hour light, is unnecessary and can induce stress, inhibiting growth and health.