Coco Coir vs. Reusable Grow Medium: Sunflower Microgreens Case Study

February 3, 2023

As many of our OG YouTube subscribers know by now, one of our favorite grow mediums for growing microgreens has been coco coir, especially for our nutrient experiments, where we compare nutrients against each other. We'll even set up grows where two trays get regular filtered tap water (the control group) and are compared against a particular nutrient to see how it performs. When we run these types of experiments, we generally use coco coir as the grow medium.

We choose coco coir as our go-to for nutrient experiments because it has very little nutrition of its own. That gives us control to add our own nutrients, whether soil additives or hydroponic nutrients, without too much nutrient exposure disrupting the plant's development.

Black Oil Sunflower microgreens at full harvest height grown by On The Grow

Coco coir has also given us wonderful germination and growth because of its water absorption and retention, with or without nutrients, as long as you use a good brand of coco coir.

For a while, coco coir was always in our top 5 growing mediums we suggest, especially for new growers learning how to grow microgreens. But we now have a new favorite grow medium, and it happens to be a product we sell ourselves: our reusable silicone grow medium.

If you're brand new to growing, start with our Beginner's Guide to Growing Microgreens or grab the printable Beginner's Guide PDF.

Quick Answer: Which Performs Better, Coco Coir or a Reusable Grow Medium?

In our high-density seeding case study, the two trays on our reusable grow medium averaged 816.95 grams of harvest weight, compared to 606.3 grams for the two coco coir trays, when we raised our Black Oil Sunflower seeding density by 20%. That's about 35% more harvest weight on the reusable medium. The coco coir trays also developed mold at the higher density, and one tray was unusable. The reusable medium trays showed no mold and had white root systems. This was one test with one crop, run on our original 316 stainless steel reusable medium, and it reflects our grow space and our methods. Results will vary in your space. At our normal, lower sunflower seeding density, coco coir still works well for us.

Important Note Before You Read: About the Reusable Grow Medium

We ran this case study in 2021 on our 316 stainless steel reusable grow medium, which we've since phased out in favor of our reusable silicone grow medium. We haven't repeated this exact test on silicone, so treat the results below as findings from stainless. The 1.2mm silicone version is food-grade platinum-cured silicone with a fiberglass mesh core, FDA compliant, LFGB Tested & Compliant, and rated for 20 to 30+ grow cycles with proper care. Silicone is easier for us to handle, clean, and store. If you're curious about stainless, see our stainless steel reusable grow medium guide. For the silicone medium itself, see our reusable silicone grow medium launch blog and our reusable grow medium guide.

The Reusable Grow Medium vs. Coco Coir: Setting the Stage

A reusable grow medium (silicone or stainless) is completely inert, with no nutrient content, which makes it a good medium for nutrient trials. The main learning curve compared to coco coir, which absorbs and holds moisture, is that the reusable medium doesn't absorb water. It depends on you watering correctly during germination and in the days leading up to harvest. It's easier than you'd expect once you learn how.

For which varieties work best on a reusable medium, see our microgreen seed compatibility guide. Sunflower is one of the large seeds with aggressive roots, so it's a good test crop for this comparison.

Now to the point of this post: how the reusable grow medium performed against coco coir for Black Oil Sunflower microgreens when we raised our normal sunflower seeding density by 20%.

Experiment Overview

Test overview comparison chart of coco coir vs reusable grow medium for sunflower microgreens

Grow Method

We tested two trays of Black Oil Sunflower microgreens on coco coir against two trays on the reusable grow medium. Each setup used three Bootstrap Farmer 10x20 trays: a no-hole tray as the reservoir, a mesh tray to grow in, and another no-hole tray on top to trap moisture and hold the weight.

Each tray received 150 grams of Black Oil Sunflower seed from the same bag. After weighing, we rinsed the seeds to remove debris, since sunflower seed tends to be very dirty, which can encourage pathogens. We did not soak any of the seeds. We seeded directly on top of the grow mediums.

After seeding, both groups got the same amount of regular filtered tap water from a hand pump sprayer to start germination. We stacked each group two trays high, put a top tray on each stack, and placed a 15 lb paver on top of each. For current weight guidance, see our 4-tray weight experiment.

The trays sat on a seedling heat mat with a thermostat (on our Amazon storefront) set to 78°F, so it never goes above that. We used the heat mat because it was getting cooler in Texas at the time, and it helps keep germination up.

Over the following days we treated both groups the same, misting twice a day with filtered tap water. After 6 days under weight, we moved every tray into the light for the first time. For more on that transition, see our weight to blackout timing guide.

Grow space temperature and humidity readings during sunflower microgreen experiment

This grow took longer than usual because of the colder weather in North Texas at the time of the test. As winter came on, our grow space got cold, and cooler temperatures slow germination, growth, and water uptake. During this test the grow space ran from about 61.6°F at the minimum to 86°F at the maximum, with humidity between 36% and 76%.

After un-stacking, we staggered the trays on the same shelf in the order reusable grow medium, coco coir, reusable grow medium, coco coir. This kept the comparison as even as we could without letting light output or the fans on one side skew the results.

Sunflower microgreens shedding their black hulls as yellow cotyledons emerge over a mesh tray

Every tray then received the same hydroponic nutrient (Masterblend 4-18-38) by bottom watering for 5 days, along with 5 days of light from three Barrina 20W T5 lights (also on our Amazon storefront). Our lights run on a timer set to 17 hours on and 7 hours off. For our full lighting setup, see our complete microgreen lighting guide.

To mix the Masterblend 4-18-38, we followed the label directions and used 1 tsp Masterblend, 1 tsp calcium nitrate, and 1/2 tsp Epsom salt in a 3-gallon bucket, pH balanced to 5.5 to 6.0. For dilution ratios for every fertilizer we recommend, grab our Free Fertilizer Ratio Guide for Microgreens PDF.

What Did We Notice During the Grow?

Every time we water, we look closely at the trays for clues about how the crop is developing. During germination, both groups germinated very well at about the same rate. A couple of days in, the coco coir trays appeared to be germinating a bit better than the reusable medium trays.

Near the end of the weighted germination period, we saw multiple spots of mold on the coco coir trays around the seed hulls the sunflowers were pushing off. We removed those seeds with tweezers and treated the whole tray with our DIY organic fungicide spray after misting with plain water. On the reusable medium trays, there was only one area where we had to remove bad seeds, plus a small corner where seeds weren't germinating, which we put down to less water retention.

We gave the reusable medium trays the same treatment, so both groups were handled consistently. At this point the reusable medium looked less prone to disease than the coco coir. For more on identifying and preventing mold, see our complete mold guide.

Sunflower microgreens side-by-side comparison on reusable grow medium versus coco coir

Once the trays went into the light and began getting nutrients by bottom watering, everything kept developing well. At first the coco coir trays still led, as they had during germination. By harvest day, though, both coco coir trays had fallen behind. Both reusable medium trays sat almost 1/2 inch taller. (Keep in mind the coco coir also started with a 3/4 inch height advantage, since coco coir sits higher in the tray than the reusable medium.)

Apart from that first side-by-side impression, everything else looked the same, so it was time to harvest. That's where it got interesting.

Harvest Day

All the trays had developed true leaves, so we harvested them. This is where we saw the biggest differences between the two groups. We'll start with the coco coir trays.

Sunflower microgreen tray 1 grown on coco coir at harvest with weight comparison

Tray 1: Microgreens Grown on Coco Coir

Harvesting Tray 1, we saw nothing that stood out as unhealthy. The whole tray looked great. It had a harvest weight of 624.8 grams. The root structure was well developed except for one spot with no root growth, and the roots were slightly yellow, possibly from the coco coir itself.

Sunflower microgreen tray 2 grown on coco coir at harvest showing mold development

Tray 2: Microgreens Grown on Coco Coir

This is where things went from good to not good. Tray 2 had a lot of mold, along with sunflower stems browning halfway up, a sign of pathogens like damping off. We did not keep the product from this tray.

Microgreen mold close-up comparison on coco coir grown sunflower microgreens

Tray 2 had a harvest weight of 587.8 grams. Its roots were well developed with the same slightly yellow color as the first tray. The product itself was unusable because of the excessive mold.

Average harvest weight from the coco coir group: 606.3 grams.

Tray 1: Microgreens Grown on the Reusable Grow Medium

Sunflower microgreen tray 1 grown on reusable grow medium at harvest

Harvesting Tray 1 on the reusable medium, we saw no signs of mold and no browning stems. It was a healthy-looking grow, and we could see root hairs throughout the tray. It had a harvest weight of 783.7 grams. The roots were long, like you'd see in a Kratky system, and pure white. The only spots without roots were areas where nothing was growing.

Sunflower microgreen tray 2 grown on reusable grow medium with healthy white roots

Tray 2: Microgreens Grown on the Reusable Grow Medium

Tray 2 looked just like Tray 1: no mold, no browning stems, and healthy white roots. It had a harvest weight of 846.2 grams.

Average harvest weight from the reusable grow medium group: 816.95 grams.

Harvested tray of sunflower microgreens grown on reusable grow medium showing full canopy

The Numbers: About 35% More Harvest Weight

Microgreen harvest weight comparison chart between coco coir and reusable grow medium

Average microgreen harvest weight comparison, reusable medium about 35% higher than coco coir

Comparing the group averages gave a 210.65 gram difference between the two coco coir trays and the two reusable medium trays, or about 35% more harvest weight from the reusable medium. Part of that gap comes from the unusable coco coir tray, which dragged down the coco coir average. Without it, the gap would be smaller. With only four trays, we treat this as one data point, not proof.

We also looked at cleanup. In our hands-on opinion, both groups took about the same time. The coco coir trays were so rooted into the coir and the mesh tray that they were harder to pry apart, and the coir sometimes tore. The reusable medium trays took a little effort to pull away at first, then within about 45 seconds we had the roots and stems removed with our Microgreen Medium Scraper. After that, the medium just needed to dry, get brushed off, get sanitized, and go back into rotation. For full cleaning steps, see our cleaning and sanitization guide.

Sunflower microgreens with thick white roots growing through a mesh tray under grow lights

Conclusion

In this test, both reusable medium trays at a 20% seeding density increase came out ahead of both coco coir trays, in harvest weight and in overall crop health. Seeing that side by side was striking. This is one test with one crop, so we'll keep testing to see whether the trend holds or we just got lucky.

Even so, the consistency within each group stood out: poorer growth on both coco coir trays and healthier growth on both reusable medium trays. Some customers have also told us they had fewer mold issues on our reusable medium than they'd had with other mediums like jute and hemp mats. Once you learn how to use a reusable grow medium, it can hold its own against other mediums.

That doesn't mean we'll stop using coco coir. We still like it. In this test it didn't handle the higher seeding density well, so when we use coco coir, we stick with our usual lower sunflower seeding density of 125 grams rather than the 150 grams used here.

We no longer sell coco coir directly. If you'd like to try it, here's the coco coir brand we recommend on Amazon.

Black Oil Sunflower microgreens growing on reusable grow medium with healthy canopy

Run Your Own Comparison and Keep Track

If you want to run a test like this yourself, the key is recording the same details for every tray. In Microgreen Tracker, our grow-tracking web app, you log each tray with its seed, sow weight, medium, harvest weight, and notes, so you can compare trays side by side later. You can start a free 7-day trial to see if it fits your setup.

Sunflower Black Oil tray detail in the Microgreen Tracker app showing day 3 of 8, seed sown, water used, and cost

Frequently Asked Questions About Coco Coir vs. Reusable Grow Mediums

Which is better for microgreens, coco coir or a reusable grow medium?

Both work well, and they suit different situations. Coco coir is forgiving for beginners and has worked well for us at lower seeding densities. A reusable silicone grow medium can be reused for 20 to 30+ grow cycles (1.2mm) and gives cleaner harvests with less root debris. In our one high-density sunflower test, the reusable medium trays gave about 35% more harvest weight, and the coco coir trays developed mold. That test used our stainless steel medium, so it's a data point and not a guarantee. Run your own comparison for your space.

Does coco coir cause mold on microgreens?

Coco coir doesn't inherently cause mold, but it holds a lot of moisture, and that, combined with high seeding density, poor airflow, or overwatering, can lead to mold. In our case study, one of the two coco coir trays at the increased density developed enough mold to be unusable. Reusable mediums don't absorb water, so they depend on your watering, and in this test they stayed cleaner at the same density.

Can you reuse coco coir for microgreens?

We don't recommend it. Roots tangle into the fibers, separation is tedious, and even after sanitizing there's still a pathogen risk. Fresh coco coir from a compressed brick is affordable enough that starting fresh each grow is practical. Compost the spent coir. See our used grow medium guide for disposal options.

How long does the reusable silicone grow medium last?

Our 1.2mm reusable silicone grow medium is rated for 20 to 30+ growing cycles with proper care, and the 1.7mm for 15 to 30+. Both carry a 1 year warranty covering manufacturing defects, material failures, and premature deterioration under normal use. Replace it when you see visible tears, deep cuts, or surface wear. Cosmetic staining from red or pigmented seeds doesn't affect how it grows. Hand wash only.

Does the reusable grow medium need fertilizer for microgreens?

Yes. Silicone is inert and contains no nutrients, so your microgreens get nutrition only through the water you provide. We recommend Ocean Solution 2-0-3 at 0.5 oz per gallon, pH balanced to 5.5 to 6.0. This test used Masterblend 4-18-38, and General Hydroponics MaxiGro is another option. See our fertilizer comparison for the full breakdown.

Can you seed microgreens denser on a reusable grow medium?

In this one test, we raised Black Oil Sunflower from 125 grams to 150 grams per 10x20 tray. The reusable medium trays stayed clean at that density, while the coco coir trays developed mold. That's one crop at one density increase, so don't assume it applies to every crop. Overcrowding can still cause problems on any medium. See our growing mistakes guide.

Why do coco coir trays have yellow-tinted roots?

The slightly yellow roots we saw on coco coir trays may come from the coir itself transferring color. The roots on the reusable medium were white. We harvest above the root line either way, so root color doesn't affect the harvest. See our post on why we don't eat microgreen roots.

Can I trust these results?

This was one case study, with one variety (Black Oil Sunflower) at one density increase (20% above normal), run in a grow space that swung between about 61.6°F and 86°F. Other varieties may behave differently, and your environment, water, and watering habits will change the outcome. We encourage you to run your own comparisons before choosing a medium.

Is the reusable silicone grow medium worth the cost?

It depends on how often you grow. A reusable medium costs more up front than a bag of coco coir, but you clean and reuse it across 20 to 30+ grow cycles instead of composting a medium every grow. If you grow regularly, that can make sense. If you grow only occasionally, coco coir or soil may fit better. Our ranking of hydroponic grow mediums can help you compare.

Where can I learn more about growing on a reusable grow medium?

Start with our reusable silicone grow medium launch blog, then see our seed compatibility guide for which varieties work best. Our cleaning and sanitization guide covers maintenance, and our Video Library has tutorials for specific varieties.

Final Thoughts

We're off to do more experiments and share them with y'all, so stay tuned. If you've run your own coco coir vs. reusable medium comparisons, we'd love to hear your results.

Curious about other growing supplies we use? Check out our Amazon storefront for our recommended growing equipment, or browse our Learn hub for more guides.

Happy growing!

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This post contains affiliate links, including links to Amazon. On The Grow may earn a small commission on qualifying purchases at no additional cost to you. We only recommend products we use and trust. For details, see our Affiliate Disclosure.

The results in this post come from our own first-hand testing in our own grow space, on the stainless steel version of our reusable medium, with a small number of trays and one crop. Results vary with your environment, seed, water, and methods, so we encourage you to run your own tests. Microgreen Tracker accounts, subscriptions, billing, and support are handled at microgreentracker.com, not onthegrow.net.

Published: November 18, 2021
Updated: November 14, 2026
All content is property of On The Grow®, LLC