Microgreens are the edible seedlings of vegetables and herbs harvested just after the first true leaves emerge, typically within one to three weeks of planting. Due to their high density and very shallow root systems, precise moisture management is the most important factor for success. Unlike traditional garden plants, microgreens cannot tolerate inconsistent moisture levels; a brief period of being too dry or too wet can quickly destroy an entire crop. Maintaining a specific and consistent moisture level is paramount to ensuring healthy growth and preventing common issues like mold or rot.
Watering Schedules Based on Growth Stage
The frequency of watering is not constant but shifts significantly between the initial germination phase and the active growth phase. During the blackout or germination period, which typically lasts two to five days, the tray relies on the initial saturation of the growing medium before seeding. Additional watering is generally not necessary during this time, as the seeds are kept in the dark and under weight. The exception is if the growing medium surface appears visibly dry, especially around the edges, in which case a very light misting may be applied to prevent the seeds from drying out.
Once the microgreens are exposed to light and enter the active growth stage, their water demand increases rapidly, necessitating a shift to daily or even twice-daily watering. The most reliable way to gauge water need is the “lift test,” where the grower lifts the tray to judge its weight. A tray that feels surprisingly light indicates the growing medium has dried out and requires immediate watering, while a heavy tray suggests sufficient moisture. Specific varieties also influence frequency; large-seed varieties with high biomass, such as peas or sunflowers, will consume water much faster and may require twice-daily watering in their final days, whereas smaller, slower-growing types like broccoli or basil often only need water once per day.
Recommended Watering Techniques
Top watering, which involves pouring water directly onto the seedlings, is generally discouraged for established microgreens because it can lead to two major problems. The forceful application of water can damage the delicate stems, and the residual moisture on the dense canopy of leaves promotes the growth of mold and fungus, potentially causing the disease known as damping off. The only time top watering is appropriate is with a fine mist during the initial blackout phase to remoisten the surface without disturbing the seeds.
The preferred technique for established microgreens is bottom watering, or sub-irrigation, which delivers moisture directly to the root zone while keeping the leaves dry. This method uses capillary action, where water is drawn upwards through the growing medium from a reservoir tray placed underneath the growing tray. The growing tray, which must have drainage holes, is placed inside a tray without holes, and clean water is added to the bottom tray. The water level should be high enough to allow the growing medium to wick up the moisture, which usually takes about 10 to 30 minutes, and any remaining excess water should then be drained off to prevent the roots from sitting in stagnant, waterlogged conditions.
Environmental Variables That Change Water Needs
The optimal watering schedule is never static because the rate at which microgreens consume and lose water is heavily influenced by the surrounding environment. Evapotranspiration (water evaporation and leaf transpiration) is directly accelerated by several external factors. Higher ambient temperatures increase the metabolic rate of the plants and speed up evaporation, meaning a tray grown at 75°F will need water more frequently than one grown at 65°F.
Airflow and humidity significantly influence moisture replenishment needs. High airflow, typically provided by circulating fans, is beneficial for preventing mold but increases surface evaporation, thereby drying out the growing medium faster. Conversely, a low-humidity environment, such as a dry winter room, will pull moisture from the trays more quickly than a humid one, demanding a more frequent watering schedule. Stronger grow lights also increase the rate of transpiration in the plants, causing them to draw up more water from the roots to support photosynthesis.
Diagnosing Water Stress in Microgreens
Recognizing the visual symptoms of water stress allows growers to quickly apply corrective action. Underwatering first manifests as a general wilting or drooping of the delicate stems and leaves because the plant cells lack turgor pressure. If the drought stress continues, the edges of the leaves may begin to turn brown and become crisp to the touch as the plant attempts to conserve its remaining moisture. The immediate remedy for an underwatered tray is a thorough bottom watering soak to allow the dried-out growing medium to fully rehydrate.
Conversely, overwatering creates an anaerobic environment where the roots cannot access oxygen, which can cause a condition that mimics underwatering. An overwatered tray will also wilt, but the leaves will feel soft and limp, and the lower stems may appear yellowed or translucent. Excess moisture encourages the development of fuzzy white mold on the soil surface and can lead to “damping off,” where the stem collapses at the soil line due to fungal infection. The corrective action for overwatering is to immediately stop applying water and significantly increase the airflow around the tray to encourage rapid surface drying before the next watering cycle.