Grow lights are artificial light sources designed to promote plant growth by mimicking the light spectrum and intensity of natural sunlight. They work, provided they are correctly chosen and set up to deliver the necessary light energy for photosynthesis. Grow lights allow indoor gardeners to cultivate a wide variety of plants regardless of natural light availability, extending the growing season and enabling control over the plant’s environment.
The Science: How Plants Use Light
Light is the energy source that powers photosynthesis, the process plants use to convert carbon dioxide and water into the sugars they need for growth. Photosynthesis is driven by light within the Photosynthetically Active Radiation (PAR) range, which spans wavelengths from 400 to 700 nanometers (nm). Plant pigments, primarily chlorophylls, absorb these specific wavelengths to fuel the conversion of light into chemical energy.
The color of the light spectrum influences different aspects of plant development. Blue light, typically around 400–500 nm, is absorbed by chlorophyll-b and helps regulate plant morphology, promoting compact, sturdy growth and strong stems. Without sufficient blue light, plants tend to stretch and become “leggy” as they search for a light source.
Red light, centered around 600–700 nm, is absorbed by chlorophyll-a. This wavelength is particularly important for the flowering and fruiting stages of a plant’s life cycle. By adjusting the ratio of red and blue light, growers can manipulate a plant’s growth to encourage leaf production or stimulate blooms.
Understanding Grow Light Technologies
The three main types of grow lights available to home growers differ in their technology, efficiency, and heat output.
Light-Emitting Diode (LED)
LED fixtures have become the most popular choice due to their high energy efficiency and long lifespan. While the initial purchase cost for LED systems can be higher than other options, their low power consumption and minimal heat generation make them cost-effective over time. Modern LEDs can also offer a customizable spectrum, allowing the grower to fine-tune the light to specific growth stages.
Fluorescent Lights
Fluorescent lights, such as T5 high-output tubes or Compact Fluorescent Lights (CFLs), are a budget-friendly option with a lower initial cost. These lights produce less intense light and generate very little heat, which makes them ideal for starting seeds or for low-light plants like African violets. Fluorescent bulbs are typically placed closer to the plant canopy, but they are less efficient and require replacement more frequently than LEDs.
High-Intensity Discharge (HID)
HID lights, which include Metal Halide (MH) and High-Pressure Sodium (HPS) lamps, offer very high light intensity suitable for large, light-demanding plants. HID fixtures are less energy efficient than LEDs and produce a substantial amount of heat, which often necessitates additional cooling equipment. While HIDs are cheaper to purchase upfront, their bulbs have a much shorter lifespan, typically around 10,000 to 18,000 hours, and the running costs are higher due to energy consumption and cooling requirements.
Optimal Placement and Timing
Effective use of a grow light depends on delivering the correct intensity of light for the required duration. Light intensity decreases rapidly as the distance from the source increases. Growers use Photosynthetic Photon Flux Density (PPFD), which quantifies the number of PAR photons hitting a specific area per second.
The ideal distance between the light and the plant varies based on the fixture type and the plant’s growth stage. For seedlings, a lower intensity is required, and LED lights are placed 24 to 36 inches away to achieve a target PPFD of 100 to 300 micromoles per square meter per second. As plants mature into the vegetative stage, the light source is moved closer, to about 18 to 24 inches, to provide a higher intensity of 400 to 600 micromoles per square meter per second.
The duration the light stays on forms the plant’s photoperiod. This duration contributes to the Daily Light Integral (DLI), which is the total amount of light energy a plant receives over a 24-hour period. Most common indoor plants thrive with a photoperiod of 12 to 16 hours of light per day, but they also require a dark period for metabolic rest. Insufficient DLI will cause plants to grow weak and pale, while excessive light can lead to leaf burn or bleaching.