Watering is one of the hardest things to get right when caring for microgreens. Overwatering can cause root rot or damping off in your microgreens, leading to death. Underwatering can cause your microgreens to dry out, wilt, and die. Hitting the goldilocks amount is difficult because measuring the exact amount of water you’re dispensing is hard. To make matters worse, plants don’t always need the same amount of water from day to day or from crop to crop. Tendril peas and radish use water faster than basil and amaranth. Arugula on day four will need less water than arugula on day ten. For these reasons, watering is one of the hardest skills for new farmers to perfect. Today, though, we will talk about the differences between top watering and bottom watering. When is the best time for each method, and what are the benefits and drawbacks of each? We'll end with a talk on automated watering and how it could be a long-term fix for your watering issues.
Top watering is as it sounds, adding water from above the microgreens. Farmers will almost exclusively top water when planting microgreens because the seed needs to get water to germinate. A common tool for top watering is called a Watering Wand. A watering wand on the “shower” setting softly distributes water over a large area, which is efficient but doesn’t disturb the seed bed. After microgreens germinate and go under lights, it’s much less common to see top watering. Why? First, it’s messier. Top watering invariably gets at least a little water on the racks, lights, and floor. Second, microgreens are planted close together. This reduces airflow within the canopy and between the plants. Reduced airflow is linked to higher instances of mold and disease. What’s also linked to higher instances of mold and disease is higher humidity levels. Humidity levels rise when water evaporates. Water vapor can come from three sources. It can come from the soil, from the water you applied to the leaves while top watering, and from the plants as they respire. This is called evapotranspiration. You don’t have much control over soil evaporation or evapotranspiration. However, top watering will dramatically increase the humidity immediately around your microgreens. Third, your microgreens only absorb water through their roots. Top watering applies water at the wrong end of the plant, although it will eventually trickle down into the soil for absorption. Finally, if your microgreens are wet at harvest, you’ll see a dramatic reduction in their shelf life. Water is the antithesis of good shelf life with leafy greens. In general, avoid top watering after placing your microgreens under lights.
Bottom watering is as it sounds, adding water from beneath the microgreens. In the case of bottom watering, farmers usually add it to an entirely separate tray. Bottom watering works through displacement and capillary action. You add water to a tray beneath the one holding your soil and developing microgreens. When you lower the top or microgreen tray to nest with the bottom tray, there's not enough space for the water. The water has no choice but to displace up through holes in the top tray and into the soil. Once water enters the soil in the top tray, capillary action disperses it throughout the rest of the soil. Of course, when your microgreens grow, their roots will stick out into the bottom tray. With longer roots, the microgreens can absorb water from the soil and the bottom tray. Bottom watering is less common when planting because it requires more infrastructure to administer. We’ll talk about automated watering next, but bottom watering when planting would closely resemble an automated watering system. You would need to flood each tray with water to ensure the seeds get wet.
Bottom watering is best reserved for after you put your microgreens under lights. Bottom watering avoids or mitigates all the problems mentioned above. With less water hitting the canopy, the sensitive foliage is less likely to mold. With less water evaporating from the canopy surface, the humidity levels will be lower. By applying water directly to the roots, you’ll also use less water overall, saving on utility bills.
Although automated top watering does exist, automated watering is usually in the form of bottom watering. It’s also possible to automate in different ways and at different scales. You can “automate” watering by using flood tables, which allows you to water 8, 16, or 16+ trays at a time. This is more of an efficiency gain than an explicit automation. In theory, you could use smart sensors to detect low soil moisture. You could connect those sensors to a smart device. The smart device would control solenoid valves, which would manage the water flow into your flood trays. Automating your watering is expensive and you need specialized knowledge to build these systems correctly. However, automating your watering can reduce many of the problems associated with over or underwatering your crops. An automated system can release a set volume of water. All trays in a flood table would get the same amount. Check out Boston Microgreens' semi-automated system and Living Earth Farms' fully automated system in the videos below.
What Does “Grown-to-Order” Microgreens Mean?
Selecting the Best Growing Medium for Your Microgreens: Pros & Cons
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