Microgreens are edible seedlings from vegetables and herbs, harvested shortly after the first true leaves appear. These young plants rely heavily on light to drive photosynthesis, converting light energy into the chemical energy needed for growth. After moisture and a suitable growing medium, light is the most important factor determining the success of a microgreen crop. Insufficient light causes etiolation, characterized by pale, weak, and overly stretched stems. Understanding specific light requirements is fundamental to cultivating robust, colorful, and nutrient-dense microgreens.
Light Intensity: Quantifying the Need
The measurable brightness of light reaching the greens is quantified using Photosynthetic Photon Flux Density (PPFD). This metric indicates the number of photosynthetically active photons that land on a square meter every second, expressed in micromoles per square meter per second (\(\mu\)mol/m²/s). For most microgreen varieties, the optimal light intensity range falls between 100 and 300 \(\mu\)mol/m²/s.
Home growers without a specialized PPFD meter can use the distance between the light source and the plants as a proxy for intensity. Most common LED or fluorescent grow lights should be positioned approximately six to twelve inches above the growing trays. Placing lights too far away causes seedlings to stretch as they search for light, resulting in leggy growth. Positioning the lights too close can lead to leaf bleaching or burning the foliage.
The total amount of light delivered over a 24-hour period is the Daily Light Integral (DLI), measured in moles of light per square meter per day (mol/m²/d). A DLI between 6 and 12 mol/m²/d is recommended for successful microgreen cultivation. This metric accounts for both intensity (PPFD) and duration (photoperiod), ensuring the plants receive enough total energy for their rapid growth cycle.
Optimizing the Photoperiod: Duration Requirements
The photoperiod defines the total number of hours of light exposure microgreens receive daily. Since microgreens are harvested young, they benefit from long light cycles to maximize photosynthesis. Most species thrive under a light duration of 12 to 16 hours daily, though many growers use a 16 to 18-hour cycle for faster development. This simulates the long days of summer, providing a consistent energy supply for rapid biomass accumulation.
A consistent dark period is necessary for the plant’s metabolic health following the extended light period. During the dark hours, microgreens carry out processes like respiration and the translocation of sugars produced during the day. Using a simple timer to automate a 16-hour “on” and 8-hour “off” schedule ensures consistency. A stable photoperiod maintains a predictable environment, promoting uniform and healthy growth.
The Impact of Light Color and Spectrum
The color, or spectrum, of light plays a role in shaping the physical characteristics and nutritional content of microgreens. Light in the blue (400–500 nm) and red (600–700 nm) wavelengths are the most significant for plant growth. Red light primarily encourages stem elongation and leaf expansion, contributing to overall biomass and yield. Blue light promotes a more compact growth habit, leading to shorter, stockier stems and thicker, darker green leaves.
A higher proportion of blue light enhances the production of phytochemicals, such as anthocyanins, which are responsible for the vibrant red and purple colors in varieties like amaranth and red cabbage. Blue light can also intensify the flavor profile, leading to spicier notes in brassica microgreens like mustard and broccoli.
For home growers, full-spectrum LED grow lights that mimic natural sunlight (marketed with a color temperature around 4,000 K to 6,500 K) are the most effective and energy-efficient option. While common household fluorescent lights (4,600K to 6,500K range) can be used, dedicated horticultural LEDs offer a better balance of red and blue light, leading to robust and flavorful harvests.
Lighting Strategy from Seed to Harvest
The lighting strategy for microgreens is divided into two phases: germination and active growth. The initial stage, lasting approximately two to four days, is the germination phase, which is conducted in complete darkness. During this “blackout” period, the seeds are covered and often weighted. This encourages the seedlings to push down their roots and stretch their stems, resulting in a more uniform stand. This darkness encourages the seedlings to grow tall as they search for light, providing a beneficial stretch before the leaves are exposed.
The second phase begins once the majority of the seedlings have sprouted and the cotyledons—the first embryonic leaves—have emerged. At this point, the trays are rapidly transitioned to the full recommended light intensity and duration. The lights should be set to the target PPFD of 100–300 \(\mu\)mol/m²/s for the 16-hour photoperiod to stop stretching and initiate robust photosynthesis. This quick transition from darkness to high light prevents the stems from becoming long and weak, ensuring the greens develop deep color and high nutrient content.