Coco Coir vs. Reusable Grow Medium: Sunflower Microgreens Case Study
As many of our OG YouTube subscribers know by now, one of our favorite grow mediums for growing microgreens has been coco coir, especially when we are doing any of our nutrient experiments where we compare various nutrients against each other to see how they perform. We'll even sometimes do grows where we take two trays using regular filtered tap water (control group) and put them against a particular nutrient to get an idea of how the nutrient performs. When we do these types of experiments, we generally use coco coir as the grow medium.
The reason why we choose this grow medium as our "go to" for nutrient experiments is because coco coir has very little nutrients of its own. This gives us the perfect opportunity to take control and add in our own nutrients (whether they are soil additives or hydroponic nutrients) without disrupting the plant's development from too much nutrient exposure, which can have adverse effects.
Coco coir has also always provided us with wonderful microgreen germination and growth since it has great water absorption and retention, whether we use nutrients or not, if we use the right brand of coco coir.
For a while, coco coir was always in our top 5 growing mediums we suggest, especially for new growers who want to learn how to grow microgreens. But we have a new favorite grow medium in town, and it happens to be a product we now 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 $3.99 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 reusable grow medium outperformed coco coir by a significant margin when we increased Black Oil Sunflower microgreon seeding density by 20%. The two reusable medium trays averaged 816.95 grams of harvest weight compared to the two coco coir trays averaging 606.3 grams, a 35% increase in yield. The coco coir trays also developed mold issues at the higher seeding density, with one tray becoming completely unusable. The reusable medium trays showed no mold and produced cleaner, taller plants with beautiful white root systems. Important note: this test was done on our original 316 stainless steel reusable medium. We've since transitioned to a reusable silicone grow medium as our primary product, which performs nearly identically and is easier to clean. For lower seeding densities, coco coir still works great.
Important Note Before You Read: About the Reusable Grow Medium
This case study was originally conducted in 2021 on our 316 stainless steel reusable grow medium, which we've since phased out in favor of our reusable silicone grow medium. The 1.2mm silicone version is virgin food-grade platinum-cured silicone with a fiberglass mesh core, FDA compliant per 21 CFR 177.2600, LFGB Tested & Compliant, and rated for 20 to 30+ grow cycles with proper care. In our experience, the silicone version performs equally well or better than the stainless steel did, and it's much easier to handle, clean, and store. The case study findings below still apply directionally to the silicone version. The reusable silicone grow medium handles higher seeding densities far better than coco coir. For a deeper look at the silicone medium specifically, see our reusable silicone grow medium launch blog.
The Reusable Grow Medium vs. Coco Coir: Setting the Stage
The reusable grow medium (silicone or stainless) is a completely inert medium with absolutely no nutrient content. This makes it the perfect medium for nutrient trials. The only learning curve compared to coco coir (which is absorbent and holds onto moisture) is that the reusable medium doesn't absorb water and is dependent on you watering it correctly during both germination and the days leading up to harvest. It's much easier than you would expect once you learn how to use it.
For a deeper dive on which microgreen varieties work best on a reusable medium, see our microgreen seed compatibility guide.
Now that we've discussed both mediums, let's get to the point of this blog: a comparison of how the reusable grow medium performed against coco coir for growing Black Oil Sunflower microgreens in a situation where we increased our normal seeding density for Sunflower microgreens by 20%.
Experiment Overview
Grow Method
In this microgreen experiment, we tested (2) trays of Black Oil Sunflower microgreens grown on coco coir compared to (2) trays of Black Oil Sunflower microgreens grown on the reusable grow medium. Both groups had the same tray setup, which was (3) Bootstrap Farmer 10x20 trays per setup: 1 no-hole 10x20 (to act as the reservoir), 1 mesh 10x20 (to grow in), and another no-hole 10x20 (used for trapping in moisture and adding weight on top).
Each tray from each group received (150 grams) of Black Oil Sunflower microgreen seeds from the same bag of seeds. After weighing out the seeds, we first gave them a rinse to remove any debris, since sunflower microgreen seeds tend to be very dirty, which can promote pathogens taking over. Once the seeds were rinsed, we did not soak ANY of the seeds. We instead seeded directly on top of the grow mediums.
After all the trays were seeded, we then gave them both the same amount of water using regular filtered tap water in a hand pump sprayer to ensure that they were watered enough to begin germinating. We then took each group and stacked them on top of each other. The coco coir tray was stacked two high, since there are only (2) trays in that group. Then we did the exact same for the reusable grow medium trays. Once both groups were stacked together, we put the top trays on top of both groups and each group then had a 15 lb paver placed on top of the trays.
The trays were placed onto a seedling heat mat with a thermostat (available on our Amazon storefront) that we set to 78°F. This way it never goes above that temperature and maintains the temp we set it to. We used a heat mat because it was getting cooler in Texas at the time of the test, and this helps to keep our germination rate up.
As the days followed, we continued to give each group the same treatment as they germinated, which was misting them twice a day with filtered tap water. After 6 days of being under 15 lbs. of weight, we determined that it was time to move them all into the light for the first time. For more on when to make this transition, see our weight to blackout timing guide.
In case you're curious, the reason why it took longer than usual for this grow was due to the colder temperatures in Dallas, Texas at the time of this test. As winter was coming, our grow space was getting cold, and cooler temperatures affect the germination and growth of microgreens, along with their uptake of water. During this test, our grow space was around 61.6°F minimum and 86°F maximum (humidity levels 36% min and 76% max).
Once we un-stacked all the trays, we staggered them on the same shelf in the order of reusable grow medium, coco coir, reusable grow medium, coco coir. This way we can try to get the test comparison as even as we can without risking the light output on the shelf or the fans on one side disrupting the results.
After all the trays were staggered onto the shelf, each tray received the exact same hydroponic nutrient (Masterblend 4-18-38), which they were given via bottom watering for an entire duration of 5 days total, along with all trays receiving 5 days total of light from the (3) Barrina 20W T5 lights (available on our Amazon storefront). Our lights are set on a timer that is set for 17 hours on and 7 hours off every day. For our full lighting setup recommendations, see our complete microgreen lighting guide.
As for mixing the Masterblend 4-18-38 nutrient for microgreens, we follow their directions and use 1 tsp Masterblend, 1 tsp calcium nitrate, and ½ tsp Epsom salt, which we then mix in a 3-gallon bucket and pH balance to the 5.5 to 6.0 range. For complete 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 go to water our microgreen crops, we like to examine what's going on in front of us to help us get a clue to how the crops are developing. During the germination stage for the sunflower microgreens, both trays in both groups were germinating very well and practically at the same rate. However, a couple of days into germination, the coco coir trays appeared to be germinating better than the reusable grow medium.
Nearing the end of the "weighted" period of germination, I noticed that on the coco coir trays I was seeing multiple spots of mold appearing around the seed hulls that the sunflowers were pushing off. I had to take a pair of tweezers to remove those bad seeds and treated the entire tray with our DIY organic fungicide spray after I first misted them with plain water. What was interesting is that on the reusable grow medium trays, there was only 1 area that I had to remove bad seeds from and then a small corner section where the seeds weren't germinating (due to less water retention).
I went ahead and did the same treatment as I did for the coco coir trays, which was treating the entire tray with an organic fungicide spray after I first misted them with plain water. This way it was consistent for both groups. At this point, it seemed like the reusable grow medium was less disease-prone when compared to the coco coir. For more on identifying and preventing mold, see our complete mold prevention and identification guide.
As time went on and the microgreen crops were put into the lights for the first time and started receiving hydroponic nutrients via bottom watering, they continued to all develop nicely. At first, it appeared that both coco coir trays were still in the lead, like they were during germination. However, by harvest day I noticed something interesting. Both coco coir trays that were originally ahead in growth had fallen behind both of the reusable grow medium trays, which now sat almost ½ inch taller than both of the coco coir trays.
(Keep in mind too that the coco coir also had a height advantage of 3/4 inch from the start, as the coco coir sits higher than the reusable grow medium in the tray.)
Other than the first impression of looking at the side-by-side growths of both groups, everything else looked the same, and it was time to move into harvesting the trays. This is where it got interesting.
Day 5: Harvesting Day
Since all the trays from both groups were developing true leaves on the sunflower microgreens, it was time to harvest them. This is when I really noticed some BIG differences with each grow medium group. Let's first start with the microgreens grown on the coco coir grow medium.
Tray 1: Microgreens Grown on the Coco Coir Medium
During harvesting Tray 1 of the sunflower microgreens that were grown on coco coir, I really didn't see anything that stuck out to me as being unhealthy. Overall, the entire tray looked great, and I was happy with it.
Tray number 1 had a harvest weight of 624.8 grams. When I looked at the root structure, I noticed that it was well developed, except for one spot where there was no root growth. The color of the root structure was slightly yellow, potentially due to the coco coir medium itself.
Tray 2: Microgreens Grown on the Coco Coir Medium
During harvesting Tray 2 of the sunflower microgreens that were grown on coco coir, this is when things went from good to not good. My 2nd tray had lots of mold along with sunflower stems that were browning halfway up the stem (a sign of pathogens like damping off). I ended up not keeping the product from this tray after harvesting it.
Tray number 2 had a harvest weight of 587.8 grams. When I looked at the root structure, it was well developed and had the same slightly yellow color as the first tray, which I assume could have been due to the coco coir medium itself. The bad news with this tray was the product was unusable from the excessive mold development.
- Our average harvest weight from the coco coir group was 606.3 grams.
Tray 1: Microgreens Grown on the Reusable Grow Medium
During harvesting Tray 1 of the sunflower microgreens that were grown on the reusable grow medium, I was not noticing any signs of mold, nor was I seeing any of the stems browning. In fact, it was a super healthy-looking grow, and I could see all of the beautiful root hairs throughout the tray.
Tray number 1 had a harvest weight of 783.7 grams. When I looked at the root structure, it looked incredibly healthy. The roots were long, like you see in a Kratky system. The only spots that didn't have root development were the areas where there were no microgreens growing. The coloration of the roots was pure gorgeous white.
Tray 2: Microgreens Grown on the Reusable Grow Medium
During harvesting Tray 2 of the sunflower microgreens that were grown on the reusable grow medium, I noticed the exact same things I was noticing on tray number 1, which was no signs of mold or the stems browning. Again, it was very healthy looking.
Tray number 2 had a harvest weight of 846.2 grams. When I looked at the root structure, everything looked just as happy, healthy, and white as the first group.
- Our average harvest weight from the reusable grow medium group was 816.95 grams.
The Numbers: A 35% Yield Increase
After finding the averages for both groups, we deducted them from each other to find the difference. What we found was a 210.65 gram difference between the average harvest weights of the two coco coir trays and the two reusable grow medium trays. That means there was a 35% increase of microgreens harvested from the reusable grow medium. Not to mention that the overall numbers would have been even higher for the reusable medium if we took away the harvest weight from "coco coir tray 2" that we couldn't use due to excessive mold.
Then the final observation was the ease of cleaning after harvesting the microgreens from the trays. In my hands-on opinion, both groups took practically the same amount of effort and time to clean. On the coco coir trays, they were so rooted into the coco coir and mesh tray that I found it more challenging to pry it apart from the tray since sometimes the coco coir would tear.
Whereas on the reusable grow medium trays, both took a little effort to pull away from the tray at first before easily coming off, and then within 45 seconds I had all the roots and stems removed thanks to our Microgreen Medium Scraper. All that was left for the reusable grow medium was to let it dry out, dry brush off anything remaining once dry, sanitize, and then reuse. For full cleaning protocols, see our complete cleaning and sanitization guide.
Conclusion
In the end, both trays that used the reusable grow medium to grow the sunflower microgreens at a 20% seeding density increase outperformed both coco coir trays. From harvest weight increases to overall crop health, it was quite amazing to see side by side. Of course, this is simply one test and one microgreen crop that we tested with, so we'll have to continue to do more tests like this to see if this trend continues or if we just got lucky.
Even if luck was on our side for this grow, seeing the consistency in poorer growth on both coco coir trays along with the consistency of healthier growth on both reusable medium trays was quite surprising. We've even had others who have bought our reusable grow medium and have reported not having mold issues like they would with other grow mediums such as jute mats and hemp grow mats. Once you learn how to use the reusable grow medium, it can perform just as good (or even better) as other grow mediums.
That's not to say that we will stop using coco coir, we still love it. It's just good to realize that coco coir can't handle higher seeding densities like the reusable grow medium can. When we decide to use coco coir, we should continue to stick with the lower seeding density we usually seed sunflower microgreens at, which is 125 grams rather than the 150 for this experiment.
Note: We no longer sell coco coir directly. If you'd like to try it for yourself, here's the coco coir brand we recommend on Amazon.
Frequently Asked Questions About Coco Coir vs. Reusable Grow Mediums
Which is better for microgreens, coco coir or a reusable grow medium?
Both work great for microgreens, but they shine in different situations. Coco coir is more forgiving for beginners and works well at lower seeding densities. The reusable silicone grow medium handles higher seeding densities far better, produces less mold at dense plantings, and saves money over time since you reuse it for 20 to 30+ grow cycles. In our high-density case study above, the reusable medium outperformed coco coir by 35% in harvest weight at a 20% seeding density increase. For dense seeding, reusable wins. For lower density grows, coco coir performs comparably.
Does coco coir cause mold on microgreens?
Coco coir doesn't inherently cause mold, but it can hold a lot of moisture, which (combined with high seeding density, poor airflow, or overwatering) can lead to mold issues. In our case study, one of the two coco coir trays at increased seeding density developed enough mold to be completely unusable. Reusable mediums tend to dry out faster between waterings, which reduces the moist conditions mold needs to establish.
Can you reuse coco coir for microgreens?
We don't recommend reusing coco coir for microgreens. While technically possible, the roots tangle into the fibers so heavily that separation is tedious, and even after sterilization there's still pathogen risk. Fresh coco coir is affordable enough (especially as compressed bricks) that starting fresh each grow is the practical choice. Compost the spent coco coir and reuse it in outdoor gardens. See our used grow medium guide for full 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, or roughly 18 to 24 months of regular use with proper care. Replace when you see visible tears, deep cuts, loss of flexibility, or surface degradation. Cosmetic staining from red or pigmented seeds doesn't affect performance.
Does the reusable grow medium need fertilizer for microgreens?
Yes, fertilizer is highly recommended when growing on a reusable silicone grow medium because the silicone is completely inert and contains zero nutrients. The only way your microgreens get nutrition is through the water you provide. We recommend Ocean Solution 2-0-3 mixed at 0.5 oz per gallon of water, pH balanced to 5.5 to 6.0. Other strong options include Masterblend 4-18-38 and General Hydroponics MaxiGro. See our best microgreen fertilizers blog for our full breakdown.
Can you seed microgreens denser on a reusable grow medium?
Yes, that's actually one of the biggest advantages we've found. In our case study, we increased Black Oil Sunflower seeding density by 20% (from 125 grams to 150 grams per 10x20 tray), and the reusable medium handled it beautifully. The coco coir trays at the same density developed mold issues, while the reusable medium trays stayed clean and produced 35% more yield. Higher seeding density on reusable medium means more yield per square foot of grow space.
Why do coco coir trays have yellow-tinted roots?
The slightly yellow root color we observed on coco coir trays is likely from the coco coir medium itself transferring color to the roots. The reusable grow medium produces pure white roots since there's no pigment or tannins to transfer. The yellow color on coco coir-grown roots doesn't affect the quality or safety of the microgreens, since we harvest above the root line anyway. See our blog on why we don't eat microgreen roots like sprouts.
Do I still need to do this experiment myself, or can I trust the results?
Our results were one case study with one variety (Black Oil Sunflower) at one specific seeding density increase (20% above normal). Different varieties may behave differently. We strongly encourage you to run your own comparisons in your specific grow space conditions if you're deciding between mediums. Your environment, temperature, humidity, and watering habits will all affect the outcome. That said, the consistency we saw between both reusable medium trays and both coco coir trays suggests the trend is real, not just luck.
Is the reusable silicone grow medium worth the upfront cost?
Yes, if you grow regularly. The reusable silicone grow medium has a higher upfront cost than a bag of coco coir, but it pays for itself over 20 to 30+ grow cycles. Coco coir gets composted after every grow, while the silicone medium just gets cleaned and reused. Over a year of regular growing, the reusable medium saves significant money and eliminates the ongoing waste of single-use mediums. For commercial growers and serious home growers, it's a no-brainer investment.
Where can I learn more about growing on a reusable grow medium?
Start with our reusable silicone grow medium launch blog for the product overview, then check our seed compatibility guide to see which varieties work best on reusable mediums. Our cleaning and sanitization guide covers all the maintenance basics, and our Video Library has detailed tutorials on growing specific varieties on reusable mediums.
Final Thoughts
Now 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 full list of recommended growing equipment.
Happy growing! 🌱
-CJ & Mandi
Related Blogs You Might Enjoy
- Microgreens vs. Mature Vegetables: Grow Time and Water Usage Compared
- How to Start Growing Microgreens: A Beginner's Guide
- How to Grow Broccoli Microgreens
- Meet Our Reusable Silicone Grow Medium
- Which Microgreen Seeds Work on Reusable Grow Mediums?
- How to Clean and Sanitize Microgreen Trays and Reusable Mediums
- Why Are My Microgreens Moldy? Mold vs. Root Hairs Guide
- What Is Damping Off and How to Prevent It
- Top Microgreen Growing Mistakes and How to Avoid Them
- What to Do With Used Microgreen Grow Medium After Harvest
- Best Microgreen Growing Advice and Resources
Written by: Mandi Vaughn
Edited by: CJ Vaughn
Published: November 18, 2021
Updated: May 23, 2026
All content is property of On The Grow®, LLC
Wow, thanks for doing this. I have a question: do you recommend the soft grow medium also for sunflower microgreens, or the stainless steel?
Cheers!