Why Do Microgreens Need Weight? Our 4-Tray Experiment

May 8, 2023

One of the most persistent questions we get from microgreen growers is this: why do microgreens need weight on top of them during germination, and how much weight should you actually use?

It's a great question, and one we wanted to answer with real data rather than just opinion. So we ran a side-by-side experiment with four trays of kale microgreens and documented everything on video. In this updated guide, we're sharing exactly what we learned, why weight matters, how much to use, and how to dial it in for different crop varieties.

Brand new to microgreens? Pair this blog with our beginner's guide to growing microgreens for the full foundation.

Quick Answer: Why Do Microgreens Need Weight?

Microgreens need weight during germination for three reasons: it traps humidity around the seeds, presses them into firm contact with the grow medium so they stay moist, and creates resistance that helps seedlings shed their seed hulls and develop stronger stems. Most varieties do well with 5 to 7 lbs of weight on a sprouting tray and 10 to 15 lbs on a 10x20 tray. Some varieties like amaranth need little to no weight (just an empty tray on top), while peas and sunflower benefit from heavier weight of around 15 lbs.

The Three Reasons Microgreens Benefit from Weight

Before we get into the experiment, here are the three main reasons we put weight on top of microgreens during germination. When all three of these are working together, you get stronger germination, more even growth, and a much cleaner-looking harvest.

Trapping humidity: Humidity is critical for seed germination. Weight on top of a cover tray helps trap moisture around the seeds and keeps the environment consistent through the germination process.
Pressing the seeds into the medium: The weight presses the seeds down into firm contact with the grow medium. This contact is what keeps the seeds consistently moist, which is exactly what they need to germinate evenly across the tray.
Creating resistance: Resistance gives the seedlings something to push against as they grow. This serves two purposes: it helps the plants shed their seed hulls (cleaner harvest, less debris) and it builds a stronger base on the stems because the plants are working against pressure.

Our Side-by-Side Experiment: 4 Trays, 4 Weight Methods

To show this in action, we ran a four-tray experiment using kale microgreens on coco coir, all grown in sprouting trays under identical conditions. The only thing we changed between trays was the weight setup on top.

You can watch the full experiment in our YouTube video below, but here's the breakdown of what we tested:

Tray 1: No weight, no cover, no humidity trap of any kind. Just open seeded medium.
Tray 2: No weight, but a wet paper towel laid across the top of the seeds to trap a little humidity.
Tray 3: An empty tray placed on top with no added weight. The only pressure on the seeds is the weight of the empty tray itself (about 3.2 oz).
Tray 4: An empty tray on top plus a 7-lb paver stacked on it.

Once we seeded the trays, we misted each one with water, placed them in their groups, and set them on a dark shelf out of the way. That wrapped up day one.

Four microgreen trays with various weights on top comparison setup

What Happened Each Day

Day 2: The seeds on trays 2, 3, and 4 all showed slight improvement. Tray 1, with nothing at all, lagged noticeably behind. We gave them all a light misting and returned them to the dark shelf.

Day 3: Trays 3 and 4 continued to make steady progress. Trays 1 and 2 were still struggling, with sparse germination compared to the covered trays.

Day 4 (the big tell): This is when the seedlings that have germinated normally move into the light phase. The difference between trays was dramatic.

Day 4 comparison of microgreens grown with various weights on top

Trays 1 and 2 had clearly germinated poorly, if at all. Trays 3 and 4 had both made it through germination cleanly, but only Tray 4 (with the added weight) had the even, uniform canopy we want to see. The seedlings on Tray 4 were pushed down evenly by the weight, which forced them to grow in sync. Tray 3's canopy was uneven, with seedlings sticking up at different heights.

That side-by-side really shows what weight does. It's not just about applying force, it's about creating the conditions that let the entire tray germinate together.

How Much Weight Should You Use?

Weight needs vary significantly by crop variety. Here's the general guidance we follow:

Sprouting trays: 5 to 7 lbs works well for most varieties.
10x20 trays: 10 to 15 lbs for most varieties. Peas and sunflower really benefit from the full 15 lbs.
Light-weight varieties: Amaranth, celosia, and other tiny-seeded crops do best with just an empty tray on top (no added weight). Too much pressure can crush them.
Heavier varieties: Peas, sunflower, wheatgrass, and corn all benefit from heavier weight because their bigger seeds need stronger pressure to stay in firm contact with the medium.

For variety-by-variety weight recommendations, our free Tray-Specific Microgreen Seeding Guide PDF lists exactly what to use for each crop, using W for Weight and ET for Empty Tray so you always know what each variety needs.

Don't have a paver or dedicated grow weight? Common stand-ins include bricks, dumbbells, free weights, a heavy book, or another grow tray weighted down with water. Just make sure whatever you use distributes evenly across the tray surface so you don't get uneven germination.

When to Take the Weight Off

Weight only stays on during the germination phase. Once your seedlings have pushed the weight or cover tray up about 1 to 2 inches, it's time to transition them out of weight and into blackout (or directly into light, depending on the variety).

The exact timing on this is one of the most common things growers ask us about. Lifting weight too early gives you weak, uneven stems. Leaving it on too long compacts the canopy and can crush tender seedlings. We covered the full timing breakdown in our weight to blackout transition guide, which is one of our most useful resources for getting the timing dialed in.

Weight on Reusable Silicone Grow Mediums

Weight works the same way on our reusable silicone grow mediums as it does on soil and coco coir. The only difference is that the silicone is firmer than a loose medium, so the seeds make solid contact right away. We still recommend using weight on reusable silicone for the same three reasons (humidity, contact, resistance), and we follow the same per-variety weight recommendations.

If you're growing on reusable silicone, check our seed compatibility guide for reusable grow mediums for the full list of which varieties work well on it.

Common Questions About Weight on Microgreens

Why do my microgreens grow unevenly without weight?

Without weight, the seeds don't make consistent contact with the grow medium. Some seeds make good contact and germinate, others sit loose on top and don't get enough moisture. The result is patchy, uneven germination across the tray.

Can I use too much weight?

Yes. Too much weight can compact loose mediums like soil and coco coir, restrict airflow, and even crush smaller-seeded varieties like amaranth. The sweet spot for most varieties is 5 to 7 lbs on a sprouting tray and 10 to 15 lbs on a 10x20 tray.

Do I need weight on every variety?

No. Some small-seeded varieties like amaranth, celosia, and certain mustards do best with just an empty tray on top, no added weight. Heavier-seeded crops like peas and sunflower benefit the most from added weight.

How long does weight stay on?

Weight stays on through germination only, typically 2 to 4 days. Once the seedlings push the cover and weight up about 1 to 2 inches, they're ready to move into blackout or directly into light depending on the variety.

What can I use as weight if I don't have a paver?

Bricks, dumbbells, free weights, heavy books, or another grow tray weighted with water all work. The key is even weight distribution across the tray.

Does weight matter on reusable silicone grow mediums?

Yes. Weight serves the same purpose on our reusable silicone grow medium as it does on soil or coco coir: trapping humidity, keeping seed contact firm, and creating resistance for stronger stems and cleaner seed hull removal.

Final Thoughts

If there's one takeaway from this experiment, it's that weight makes a measurable difference in both germination success and final harvest quality. The trays with no weight or no humidity trap (1 and 2) struggled. The trays that had at least an empty tray on top (3 and 4) germinated reliably. And the tray with added weight (4) produced the most even, uniform canopy.

You don't need to overthink it. Start with 5 to 7 lbs on a sprouting tray or 10 to 15 lbs on a 10x20 tray, lift the weight when seedlings push it up about 1 to 2 inches, and adjust per variety based on what we list in the seeding guide. That's the whole formula.

If you want to go even deeper on the timing and transitions, check out our Beginner's Guide to Growing Microgreens PDF, our book Becoming a Microgreen Master, or our full Microgreen Masterclass for growers who want everything we know in one place.

Have you tested weight vs. no weight on your own grows? What varieties did you try, and how did it shake out? We'd love to hear what you've learned.

CJ & Mandi, On The Grow

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Why Do Microgreens Need Weight? Our 4-Tray Experiment
Published: 2023
Updated: May 25, 2026
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