Putting a grow light under a plant is an effective strategy to optimize indoor gardening. This technique, known as under-canopy lighting or supplemental lighting, ensures the entire plant receives light energy for photosynthesis. While plants naturally absorb light from above, providing light from below addresses limitations in light penetration, maximizing the plant’s total photosynthetic capacity.
The Rationale: Why Light from Below
The scientific reason for using under-lighting is to overcome light attenuation, which is the reduction of light intensity as it travels through the dense upper leaf canopy. When plants grow bushy, the upper foliage shades the lower leaves significantly, limiting the light available for photosynthesis in those lower areas. This shading causes lower leaves to become less productive or even senesce (die off). Supplemental lighting from below targets these shaded leaves, boosting their capacity to convert light into energy.
Light quality plays a role in light penetration, as certain wavelengths pass through the canopy more easily. Green light, which plants reflect the most, and far-red light transmit more deeply into the lower canopy layers than blue or red light. Introducing light from underneath provides a direct energy source to these lower leaves, which are otherwise starved of photons. This supplementary light works in conjunction with the main light source to utilize the entire leaf surface area efficiently.
Practical Applications and Setup
Under-canopy lighting is frequently used in controlled environment agriculture, such as multi-tiered growing racks or vertical farms, where space optimization is necessary. In these setups, the light fixture for plants on a lower shelf simultaneously serves as the under-light for the plants on the shelf above. This dual-purpose use makes the system efficient in terms of space and energy usage.
Low-growing plants, such as leafy greens, herbs, and microgreens, benefit from under-lighting because their dense, low profile creates a self-shading issue near the base. The fixtures must be mounted securely beneath the growing surface or shelf, often using slim, low-heat LED light bars to maintain a small vertical profile. The light source should be positioned close enough to provide an adequate Photosynthetic Photon Flux Density (PPFD) to the underside of the leaves, typically within six to twelve inches, depending on the fixture’s intensity.
Safety and Efficiency Considerations
Using electrical fixtures close to the growing medium and water sources requires careful attention to safety, particularly regarding heat and moisture. Although modern Light Emitting Diode (LED) fixtures produce less heat than older lighting types, they still generate warmth that needs management, especially in enclosed spaces like grow tents or stacked racks. Adequate airflow must be ensured around the fixtures to prevent heat buildup, which can cause plant stress or damage the equipment.
Electrical safety must be a priority since watering or hydroponic systems introduce the risk of splashing or spills. It is recommended to use a Ground Fault Circuit Interrupter (GFCI) outlet with any electrical equipment near water, as this device quickly shuts off power in case of a short circuit. For maximum light efficiency, reflective materials, such as white paint or Mylar sheeting, on the base surface can amplify the under-lighting effect. These reflective surfaces redirect scattered light back up toward the plant canopy, ensuring minimal light energy is wasted.