Microgreens are the young, edible seedlings of vegetables and herbs, harvested shortly after the first true leaves emerge, while the initial cotyledon leaves are still fully developed. They offer a concentrated burst of flavor and nutrients, and their rapid growth cycle makes them ideal for indoor cultivation. While microgreens start their journey in complete darkness to encourage uniform germination, light is necessary for their brief final growth stage to ensure a high-quality harvest.
The Role of Light in Microgreen Development
Light serves as the primary energy source for microgreens, driving photosynthesis once the seedlings emerge from the soil medium. Light energy is converted into chemical energy, which is used to build biomass and support overall growth. Without sufficient light, the young plants cannot develop the necessary strength or structure for harvest.
The light spectrum is responsible for the microgreen’s desirable vibrant green color through the synthesis of chlorophyll. Blue light wavelengths, in particular, play a significant role in promoting this chlorophyll production. This pigment is what captures light for photosynthesis, making it fundamental to the plant’s survival and growth.
Light controls the physical structure of the microgreen through a process called phototropism. Exposure to light signals the stems to grow upright and strong, which is desirable for harvest. Seedlings grown without adequate light will exhibit etiolation, resulting in weak, pale, and spindly stems that are excessively long as the plant desperately reaches for a light source.
Timing the Introduction of Light
The life cycle of a microgreen is divided into two distinct phases, each with specific light requirements. The first phase is the blackout or germination period, which occurs immediately after the seeds are sown. During this time, light is intentionally withheld by covering the trays.
This initial dark period encourages uniform germination and causes the seedlings to stretch, resulting in a taller, more uniform crop. The lack of light also encourages the establishment of robust root systems before the plant focuses energy on above-ground growth. This dark period typically lasts between two and four days, depending on the microgreen variety.
The transition to the light phase begins once the seedlings have reached a certain height and firmly anchored themselves to the growing medium. Introducing light too early, before the roots are established, can lead to uneven growth, while waiting too long results in overly stretched, pale, and weak stems. Once the cover is removed, the seedlings must be immediately placed under bright light to stop stretching and begin greening up.
Choosing the Right Light Source and Intensity
For indoor microgreen cultivation, artificial lighting is often preferred over natural sunlight because it offers consistent intensity and duration, which are difficult to achieve on a windowsill. Light-Emitting Diodes (LEDs) are a popular choice due to their energy efficiency, low heat output, and the ability to customize the light spectrum. Fluorescent fixtures, such as T5 or T8 bulbs, also work well and are a cost-effective alternative.
Microgreens thrive under a full-spectrum light that mimics natural sunlight, though the red and blue wavelengths are the most photosynthetically active. Blue light is particularly useful for encouraging vegetative growth and chlorophyll development, while red light helps to regulate stem elongation. A photoperiod, or duration of light exposure, of 12 to 16 hours per day is generally recommended for optimal growth.
Light intensity is measured in photosynthetic photon flux density (PPFD) and should be moderate to high for microgreens. A common target intensity falls within the range of 200 to 400 µmol/m²/s. To deliver this intensity without causing damage, the light fixture must be positioned correctly, typically just a few inches above the canopy of the plants. Maintaining a distance of four to six inches is often sufficient with lower-intensity fixtures, but this must be adjusted to prevent heat from scorching the delicate leaves.
Practical Impact on Flavor and Nutrition
The amount and quality of light a microgreen receives directly affects its flavor and nutritional profile, which are the primary reasons for consumption. Adequate light exposure is necessary to activate the production of secondary metabolites, which are compounds responsible for the vibrant flavors, pungency, and aromas characteristic of microgreens. Insufficient light results in a bland, watery taste, while properly lit microgreens possess a concentrated, vibrant flavor.
Light also significantly boosts the concentration of numerous health-promoting compounds. Higher light intensity, for example, is linked to elevated levels of antioxidant activity and total phenolic content in the harvested greens. Specific wavelengths, particularly blue light, can enhance the accumulation of compounds like anthocyanins, which are responsible for the rich red and purple hues seen in some varieties.
Optimized light exposure increases the concentration of certain vitamins, such as ascorbic acid (Vitamin C). A microgreen grown under insufficient light will be pale, visually unappealing, and nutritionally inferior. Conversely, a harvest exposed to the correct light intensity and spectrum yields a crop that is visually vibrant, texturally firm, and packed with maximum nutritional value.