• On The Grow, LLC

Case Study: Coco Coir VS Reusable Microgreen Grow Medium - High seeding density

Updated: Nov 19


Black oil sunflower Microgreens growing on Reusable Microgreen grow medium_edited
On The Grow Farms Logo and Tag Line Do You Be-Leaf In Magic? Copyrighted

As many of our OG YouTube subscribers know by now, one of our absolute favorite grow mediums we like to use to grow Microgreens is Coco coir – especially when we are doing any of our nutrient experiments where we compare various nutrients against each other to see how they preform for growing microgreens. 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 – and when we do these types of experiments, we generally used Coco coir as the grow medium.


The reason why we choose this grow medium as our “go to” for nutrient experiments and just as our general favorite to grow with, is because coco coir has little nutrients of its own! This gives us the perfect opportunity to take control and add in our own nutrient whether they are soil additives or hydroponic nutrients without disrupting the plants developments from too much nutrient exposure, which can have adverse effects.


Black oil Sunflower Microgreens

Not to mention that aside from the ‘control’ aspect, coco coir has always provided us with wonderful Microgreen germination & growth since it has great water absorption and retention– whether we use nutrients or not – if we use the right brand of coco coir!


Coco coir will always be in our top 5 growing mediums we suggest, especially for new growers who want to learn how to grow microgreens. As of recently, we have a new favorite grow medium in town, and it happens to be a product we now sell ourselves (that’s how much we be-leaf it in). What we are referring to is the “Reusable Microgreen Grow Medium”


Our Reusable Grow medium is a 316 Grade Stainless Steel Mesh with 1-mm holes. Instead of having to constantly re-buy bags or bricks of grow medium like soil or coco coir – you can simply clean off our reusable growing medium, sanitize and then reuse it time and time again, if you take care of it. If you are curious about the Reusable Grow Medium (click here) to read our blog where we discuss it in more detail or (click here) to watch our playlist on YouTube where we share videos regarding it.


How to grow Basil Microgreens on Reusable Grow Medium - On The Grow

The Reusable Grow Medium is made from Stainless Steel, it is a completely inert medium and has absolutely no nutrient contents within the medium it’s self… which means it is the perfect grow medium for us to do nutrient trials with… the only “set-back” or learning curve rather, is that compared to coco coir which is an absorbent medium that holds on to moisture really well – the stainless steel is not an absorbent medium and is dependent on you watering it correctly during both Microgreen germination and in the days leading up to you harvesting the tray of microgreens. It’s much easier than you would expect once you learn how to use it and can be quite comparable to coco coir in the ease of use – in fact, check out our “How to grow Basil Microgreens on Reusable grow medium” video to see for yourself, and if you really want to see the process differences between coco coir and the RMGM, your in luck because we also have a “How to grow Basil Microgreens” video where we use coco coir!


Now that we have discussed both mediums we can get to the point of this blog, which is a comparison of how the Reusable Microgreen 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%. This was a test that we decided would make a great read for anyone who is interested in our findings.


Experiment Overview:


Microgreen Test Overview - Cococoir vs RGM.png

Grow Method:

In this Microgreen experiment we tested out - (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) BSF trays - 1 no hole 1020 (to act as the reservoir), 1 mesh 1020 (to grow in) and another no hole 1020 (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 where 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. For instance – 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 15lb paver placed on top of the trays. The trays were placed onto a seedling heat mat that has a thermostat we set to 78F – this way it never goes above that temperature and maintains the temp we set it to. We used a heat mat because it is getting cooler in Texas now 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.


Grow Space Temp and humidity

In case you’re curious, the reason why it took longer then usual for this grow was due to the colder temperatures in Dallas, Texas at the time of this test. As winter is coming, our grow space is 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 61.6F minimum and 86F 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 “Master Blend 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) Barinna 20W T5 lights on the shelf. Our lights are set on a timer that is set for 17 hours on and 7 hours off every day.


As for mixing the Master Blend 4-18-38 nutrient for Microgreens, we follow their directions and use 1 tsp Master Blend, 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 – 6.0 range.


What did we notice?


Now we can begin to discuss a few things that we saw during the germination and growth process – and then we’ll talk about what we discovered on harvest day.


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 Microgreen 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 days into germination, the Coco Coir trays appeared to be germinating better than the Reusable Microgreen Grow Medium.


Nearing the end of the “weighted” period of germination, I did begin to notice that on the Coco Coir trays I was seeing multiple spots of mold appearing around the seed hulls that the sunflowers were pushing off and I had to take a pair of tweezers to remove those ‘Bad seeds’, and treated the entire tray with an organic fungicide spray after I first misted them with plain water. What was interesting is 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 RMGM (reusable microgreen grow medium) was less disease prone when compared to the coco coir….


Sunflower Microgreens grown on Reusable Microgreen Grow Medium vs Coco coir side by sidde comparison

As time went on and the Microgreen crops where then 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…. and 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 each and 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 – and 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 I was coming across lots of mold along with Sunflower stems that were browning halfway up the stem (sign of pathogens like damping off) and I ended up not keeping the product from this tray after harvesting it.

Tray number 2 had a harvest weight of 587.8 grams – and 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 Microgreen Grow Medium

During harvesting Tray 1 of the Sunflower Microgreens that were grown on the Reusable Microgreen 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 – and 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… pure gorgeous white.

Tray 2: Microgreens Grown on the Reusable Microgreen Grow Medium

During harvesting Tray 2 of the Sunflower Microgreens that were grown on the Reusable Microgreen 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 – and 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 Microgreen Grow Medium group was – 816.95 Grams

Harvested tray of Sunflower Microgreens grown on Reusable Microgreen Grow Medium_edited.jpg
Microgreen Harvest weight Comparison.png
Average Microgreen Harvest weight Comparison.png

After finding the averages for both groups, we deducted them from each other to find the difference, and what we found was a 210.65 grams 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 Microgreen Grow Medium!!! Not to mention that the overall numbers would have been even higher for the RMGM 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, which 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 Microgreen 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 this awesome tool (click here). A that was left for the Reusable Grow Medium was to let it dry out in the sun, dry brush off anything remaining once dry, sanitize and then reuse.

Conclusion –


In the end, both trays that used the Reusable Microgreen 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 trays along with the consistency of healthier growth on both RMGM trays was quite surprising! We’ve even had others who have bought our Reusable Microgreen 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 microgreen grow medium it can preform 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 Microgreen Grow Medium can, and 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 then the 150 for this experiment. Now we’re off to do more experiments and share them with y’all, so stay tuned!!

Black oil sunflower Microgreens growing on Reusable Microgreen grow medium_edited

Want to learn EVERYTHING On The Grow has learned from all our years of growing and experimenting with microgreens? Then check out our Book below!

Mandi Warbington and CJ Vaughn of On The Grow wearing Micro-dose Merch

Written by: Mandi Warbington

Edited by: CJ Vaughn

Published: November 18, 2021

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