Why Should You Weigh Down Your Microgreens During Germination?
Aug 5th, 2024
Written by Garrett Corwin
Introduction
Why do microgreen farmers weigh down their germinating trays? I once worked on a small outdoor farm growing mixed vegetables. We never used pavers to weigh down germinating crops. So, why do we do it with microgreens? What are the benefits, and is it necessary?
If you're new to growing microgreens, most growers will use 10-15 pounds of pavers or bricks to weigh down a stack of trays during germination. It will look like a stack of freshly planted trays with an empty tray on top, and then a paver on the top of that tray. Weighing down crops during germination has three main benefits: it retains moisture, improves rooting, and helps shed seed hulls. Unfortunately, we found no scientific studies comparing weighted and unweighted germination in microgreen production. Therefore, this article will explain the benefits of weighing down your crops using the first principles of farming.
Moisture Retention
Moisture is a seed’s trigger to germinate. Likewise, a seed that has begun germinating but dries out during the process will die. Stacking your trays will trap the moisture from the planting step. Moisture retention will prevent the seeds from drying out during their 3-10 day germination window. Weighing down the stack of trays helps to lock in moisture, especially around the edges. Let’s say you have a stack of five germinating trays. The weight from the upper four trays will provide ample weight on the bottom tray. As you move up the stack, fewer trays are weighing down on any given tray. Without a paver on top, the topmost tray has nothing but an empty 10”x20” tray to cover the delicate seeds. More air can enter and dry out those seeds. You can germinate seeds without weight and get decent results. However, adding weight improves the germination rate by preventing the seeds from drying out. Aside from preventing too much air penetration, the added weight will improve seed-to-soil contact. Greater seed-to-soil contact will result in overall better hydration of the seed, which improves germination.
Before moving on, we must address a myth: watering your trays during germination. Only water mid-germination if you underwatered during planting. If you added enough water when planting, and you weigh down your germination stacks, you never need to add water to your seeds during germination. I can say this with full confidence, having grown 40+ varieties and 20,000+ trays. The situation is a little different for crops that germinate under a humidity dome, like basil but we won't go down that path.
Rooting
When a seed starts to germinate, the first part to emerge is the tip of the root structure called the radicle. Think of the radicle like the tip of a pen, it’s pointed but not razor-sharp. The radicle will try to dig into the soil to find moisture and nutrients, as well as branch out to create a root system. Most microgreen varieties are planted directly on the soil surface with no additional soil to cover the seeds. If the radicle can’t find adequate moisture by, say, digging down into wet soil, it’ll dry up and the plant will die. If that pen-tip of a radicle tries to dig into the soil without having a force to push against it, it won’t make it very deep. The weight of the seed itself is not enough to counteract the digging force of the radicle. What we're talking about is Newton's Third Law of Motion: every action has an equal but opposite reaction. The radicle digging down is the action, the weight of a paver creates the equal but opposite reaction. In field agriculture, seeds are either sown into the ground or small seedlings are transplanted. Either way, the seed is buried in the soil. So, the radicle is surrounded by wet, nutritious soil when it emerges. It’s both already in the desired environment and there is a weight of soil above it to push against. Neither of these is the case with most microgreens. So, to facilitate the digging down of the radicle, we weigh down the stack of trays. As the radicle emerges and digs into the soil, the weight above will prevent the seed from being pushed upward. In other words, weighing down your germinating trays creates a stronger root system.
Seed Shedding
Every microgreen grower has different preferences for how to grow certain crops, but I’ll tell you mine. Although we just discussed how seeds are generally not covered with soil when growing microgreens, there are a few exceptions. We cover cilantro, beet, chard, and a few others with a thin layer of soil during the planting process. Why? During germination, most seed hulls will either fall off on their own, or the customer may not notice them due to their small or soft nature. Of course, sunflower is an exception. We want to remove as many seed hulls as we can from our cilantro, beet, and chard trays. They can be unsightly, unpleasant, or even dangerous to eat. Try munching down on a handful of chard seed and tell me you didn’t fear for your teeth. They’re hard. The extra soil will help to soften the seed hull during germination by trapping moisture around it. However, the soil and the weighted tray above the seeds will also act as an abrasive to help tear the seed hull away from the leaves. As a seed germinates and the seed leaves or cotyledons begin to emerge, the plant begins to unfold, starting with the hypocotyl arch. The hypocotyl arch, as seen below, is the leading point of the plant, and then the cotyledons unfurl from there. As the cotyledons unfurl, the top layer of soil provides resistance against the seed hull. If the soil doesn't strip away the hull, the tray will. The weighted tray pressing down on the seeds and soil beneath it provides another chance to pull off the hull. As the cotyledons continue to unfurl, they'll rub against the bottom of the top tray. Adding weight to the top tray increases the pressure against the plants, thereby improving the chance of hull shedding.
Stronger Stems
Although there is no empirical evidence to suggest that weighing down your microgreens will result in stronger stems, it could be possible. Just as humans who lift heavy weights develop stronger muscles, plants forced to fight against a downward force could develop stronger stems.
Conclusion
For the vast majority of microgreen varieties that are stacked during their germination period, we suggest weighing them down with 5-20 pounds. We use 14 pound landscaping pavers from Home Depot. Weighing down your trays will improve the germination rate and rooting success of your crops.
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