Grow lights are artificial light sources, typically LEDs, designed to replace or supplement sunlight by emitting specific wavelengths necessary for photosynthesis. Their default and most effective placement is directly above the plant canopy. While overhead placement is generally required for optimum growth, specific indoor setups and plant types sometimes necessitate alternative placements. The orientation of the light source significantly influences the plant’s biological response and the uniformity of the light it receives.
The Biological Imperative: Phototropism and Etiolation
The primary reason for overhead light placement stems from positive phototropism, the plant’s genetically programmed tendency to grow toward a light source. This response is regulated by the hormone auxin, which accumulates on the shaded side of the stem, causing cells there to elongate and bend the plant toward the light.
By placing the light directly overhead, the growth signal is distributed uniformly, encouraging straight, vertical stem growth. If the light source is positioned to one side, the plant will strain and stretch horizontally to reach it, resulting in a curved or uneven structure.
This stretching is a form of etiolation, a survival mechanism where the plant expends energy to grow taller in a desperate attempt to find adequate light. Etiolation results in a pale, spindly, and structurally weak plant. For a healthy, robust plant, a centralized overhead fixture provides uniform light intensity across the entire canopy, maximizing leaf surface exposure for photosynthesis.
The Physics of Light Distribution
The physical laws governing light intensity provide the practical reasons why proper overhead placement and distance are important. Light intensity from an artificial point source diminishes rapidly, a phenomenon described by the Inverse Square Law. This law states that intensity is inversely proportional to the square of the distance from the source.
If the distance between the light and the canopy is doubled, the light intensity reaching the leaves is reduced to one-quarter of its original strength. This exponential drop means that placing a fixture too high results in a significant loss of photosynthetic photon flux density (PPFD), the usable light energy for plant growth.
Conversely, placing the light too close can create “hot spots” of high intensity, potentially causing leaf burn or heat stress. Maintaining the correct distance is a continuous process because the plant canopy grows taller daily, constantly changing the light-to-plant distance.
Growers must balance maximizing light intensity by keeping the fixture close against maintaining an even light footprint without causing damage. In large-scale operations, automated lift systems are sometimes used to precisely manage this distance, ensuring plants receive consistent and optimal light levels as they grow.
Practical Applications of Supplemental Lighting
While overhead lighting is the standard for primary illumination, supplemental lighting is often beneficial or necessary. For tall, dense crops, such as mature tomatoes, the upper canopy often shades the lower leaves and branches, reducing their ability to photosynthesize.
Side Lighting
Side lighting involves placing linear fixtures vertically along the sides of the plants to illuminate shaded lower and middle sections. This improves light penetration and ensures uniform exposure across the entire structure, enhancing the development of lower buds and fruit. This method is often employed in combination with traditional overhead lighting to boost overall yield and quality.
Vertical Farming
In modern vertical farming setups, the physical stacking of plants on multi-tiered racks makes the standard overhead-only approach impractical. Light fixtures are positioned horizontally between the layers of plants, acting as the primary overhead source for the layer below. The use of thin, low-heat LED fixtures allows growers to place the lights very close to the plants, often within twelve inches, maximizing the number of layers in a small space.
Reflective Surfaces
Reflective surfaces, such as Mylar sheeting, also function as supplemental lighting by bouncing stray photons back towards the plant canopy. While reflected light is less intense than the direct source, it helps mitigate the effects of the Inverse Square Law by contributing to a more even distribution of light, ensuring the sides and lower parts of the plant receive usable light energy.