The short answer: fine, washed, low-EC coco coir is one of the two media that most commercial microgreen farms settle on, the other being a fibre mat (hemp or jute). Loose coir gives yields close to a peat-based mix for most crops, holds water well, sits at a pH microgreens like, and composts after harvest. Mats are cleaner at harvest and faster to handle but yield less per tray in every published trial that measured it. Pick coir when you sell by weight and have a bottom-watering setup; pick mats when harvest speed and a particle-free product matter more than grams per tray.
Key facts
- In a Penn State trial on pea and radish microgreens, coconut coir had pH 6.15, EC 1.33 mS/cm, 81.9 % water-holding capacity and 90 % total pore space, all close to the peat-perlite control (Poudel, Duenas and Di Gioia, 2023).
- On rapini, peat, a recycled textile mat and a jute-kenaf mat all yielded about 1,502 g/m²; a polyester mat yielded 13.1 % less. E. coli was found on the peat-grown crop only (Di Gioia et al., 2017).
- A SARE on-farm trial recorded 5.4 oz of broccoli per tray on a soilless mix against 2.7 oz on hemp mats, 1.7 oz on a felt mat and 1.5 oz on a cellulose pad (McEvilly, 2019).
- UF/IFAS recommends filling trays to 1/2 in. to 1 or 2 in. of sterile, loose, soilless medium, overhead misting only until germination and sub-irrigating afterwards (Treadwell et al., 2010, rev. 2013).
- Microgreens are produce commonly eaten raw and fall under the FSMA Produce Safety Rule; water that touches them must have zero generic E. coli per 100 mL (NCDA&CS, 2025; Penn State Extension, 2025).
Why coir suits a 7–21 day crop
A microgreen crop lives in its medium for one to three weeks and is cut above it. The medium has three jobs: hold enough water between irrigations, let air reach the seed, and not contaminate the harvest. Nutrition hardly matters; UF/IFAS notes that most crops need little or no fertiliser because the seed carries what the seedling needs.
Coir pith does the first two jobs well. Evans, Konduru and Stamps measured coir dust at 750–1,100 % of its dry weight in water, with a pH of 5.6–6.9. In the 2023 Penn State comparison, coconut coir had a water-holding capacity of 81.9 % against 82.8 % for peat-perlite, total pore space of 90.0 % against 88.2 %, and a bulk density of 0.10 g/cm³ against 0.13. The authors used two published benchmarks for an ideal medium: pore space above 85 % and bulk density below 0.4 g/cm³ (Abad et al., 2001), and pH 5.5–6.5 with EC below 0.5 mS/cm (Kyriacou et al., 2016). Coir and peat-perlite were the only two of the eight media tested that met the pH target. The third job, cleanliness, is where media differ most; see the food-safety section below.
Media compared from published trials
The table draws only on trials that measured the media side by side. Costs are from retail listings and are indicative.
| Medium | Yield (published trials) | Water use | Cleanliness at harvest | Cost per 1020 tray | After harvest |
|---|---|---|---|---|---|
| Loose coco coir | Radish equal to peat-perlite; pea about a third lower (Poudel et al., 2023) | High holding capacity, 81.9 % | Some pith on stems if cut low; sub-irrigate to keep canopy clean | One ~1 lb brick fills about three trays (Bootstrap Farmer) | Compostable; 8 weeks tray-to-sifted in summer for a peat mix (McEvilly, 2019) |
| Coir mat (needled coir, often rubber-backed) | Not quantified; grouped with mats | Used the most water of six media on arugula (Lake, 2018) | Clean, particle-free | Not published | Rubber backing slows decomposition (RusticWise) |
| Hemp mat | About half the weight of a soilless mix across four crops (McEvilly, 2019) | Lower than coir mat (Lake, 2018) | Clean; cut with electric knife | About $2–3 per 10×20 in. mat (RusticWise) | Compostable |
| Jute or jute-kenaf mat | Equal to peat on rapini, 1,502 g/m² (Di Gioia et al., 2017) | Dries faster than hemp (RusticWise) | Clean; yeasts and moulds similar to peat (Di Gioia et al., 2017) | About $2 per mat (RusticWise) | Compostable |
| Peat or peat-perlite mix | Highest pea yield; the reference medium (Poudel et al., 2023; McEvilly, 2019) | 82.8 % holding capacity | Highest Enterobacteriaceae and only medium with E. coli in the rapini trial | Majority of per-tray cost in the SARE trial | Compostable; non-renewable raw material |
| Polyester or felt mat | 13.1 % below peat on rapini; 1.7 oz vs 5.4 oz broccoli | Not reported | Clean; lowest yeast and mould counts | Not published | Industrial composting only or landfill |
| Rockwool | No microgreen trials found | High | Clean | Not published | Mineral fibre, not compostable |
Three caveats. The pea result is the only head-to-head where coir alone clearly lost to peat-perlite; the same trial found radish equal and antioxidant activity highest in coir for both species. The SARE trial gave every tray the same 1.5 cups a day, which favours media that hold water. The Colorado State study ranked rather than measured, so "most water" for the coir mat is a ranking, not a volume.
Rockwool rarely appears in microgreen trials. A spent slab is mineral fibre and cannot go to the compost heap, which is where most farms send tray waste.
Tray depth and how much coir you need
UF/IFAS gives the only extension figure for fill depth: partially fill the tray to 1/2 in. to 1 or 2 in. (roughly 1.3 to 2.5–5 cm) "depending on irrigation programs". Shallow fills save medium and dry faster; deeper fills buffer a missed watering. Most commercial growers on bottom-watered racks sit near the lower end.
A standard 1020 tray measures 25.4 × 50.8 cm, about 1,290 cm² of surface. At 1.3 cm that is about 1.7 litres of hydrated coir per tray; at 2.5 cm about 3.2 litres. Bootstrap Farmer's working figure is that one brick of just over a pound fills three trays, which matches a shallow fill from a 650 g briquette. For a farm running dozens of trays a day, 5 kg blocks hydrated in a tub are cheaper per litre than 650 g briquettes; briquettes are easier to batch exactly.
Do not compact coir in the tray. Level it, firm it lightly with an empty tray so the surface is flat, and leave it loose underneath. Seed sown at 1–2 seeds/cm² (the density used in the Penn State trial) germinates unevenly over a lumpy bed.
Hydration, EC and pH for microgreens
Microgreens are the most salt-sensitive crop coir is used for, because the whole plant is a seedling for its entire life. Kyriacou's ideal of EC below 0.5 mS/cm is stricter than most coir leaves the factory; the coir in the Penn State trial measured 1.33 mS/cm and still performed, but radish nitrate and pea yield may have reflected it. Buy fine-grade coir sold as low-EC or washed, ask for the EC figure and the extraction method, and read buffered vs unbuffered coco peat before deciding whether buffered material is worth the extra cost for a crop that is harvested before it can run short of calcium.
Hydrate with clean water, not the drain from a vegetable crop. If the batch tests above about 1 mS/cm by a 1:1.5 dilution, flush it once in the tub, drain, and retest. Coir pH of 5.6–6.9 needs no adjustment for a two-week crop. Measurement methods, meters and targets by method are in coco coir pH and EC management and are not repeated here.
Nutrition is optional. UF/IFAS suggests that long-cycle crops such as carrot, dill and celery, and fast ones that exhaust the seed quickly such as mustard and chard, benefit from floating the tray for 30 seconds in a solution of about 80 ppm nitrogen. Coir has almost no nitrogen, so basil, coriander and other herbs past day 14 are the trays to watch; herb feeding in coir is covered under herbs in coco coir.
Blackout and germination
Sow onto fully wetted coir, press the seed into the surface, and cover. The Penn State protocol was to saturate trays by sub-irrigation before seeding, let them drain, weight and cover them with black polyethylene, mist from the top daily for the first two days until the cotyledons formed, then sub-irrigate with tap water only. Weighting the cover (growers commonly stack an empty tray with a weight on top) keeps the roots pushing into the coir instead of lifting the seed mat.
Temperature drives the timing. Johnny's gives an ideal medium temperature of 75 °F (24 °C) until germination, then 60 °F (16 °C), with 65–75 °F (18–24 °C) generally favourable in the room; above 75 °F disease pressure rises and some crops germinate poorly. Bootstrap Farmer's rule of thumb is 2–3 days in the dark for quick crops and 3–4 for long-cycle crops such as pea and sunflower. Uncover when the stems are long enough to want light, usually at 2–4 cm, and move to the lit rack.
Coir rewets evenly and stays wet under a cover without crusting. Its risk is the same as peat's: a cover left on too long in a warm room gives fungal problems their start.
Watering: bottom watering only after germination
Every extension source agrees on this point. UF/IFAS: overhead mist is used only through germination; "after germination, trays should be subirrigated to avoid excess moisture in the plant canopy." Johnny's: bottom water or mist so the sowing mix does not splash onto seedlings, and never let the medium dry out. Penn State adds the hygiene reason: minimise overhead irrigation and keep hoses and nozzles off the floor.
In practice this means a solid 1020 tray under a slotted 1020 tray filled with coir. Pour water into the bottom tray, let the coir wick for 10–20 minutes, then pour off what has not been taken up. The SARE farm watered 1.5 cups (about 350 mL) per tray once daily; Bootstrap Farmer uses once a day as the benchmark. Both are starting points: a shallow fill at 24 °C under lights can need water twice a day.
Water quality matters as much as quantity. Under the Produce Safety Rule as summarised by NCDA&CS, irrigation water must be assessed annually, and harvest and post-harvest water, including water used to humidify the room or wash trays and hands, must have zero generic E. coli per 100 mL. Penn State adds that if a well or spring tests positive, stop using it and resume only after installing in-line treatment. Municipal water needs no testing, but keep the utility's results on file.
Food safety: why the medium is a hazard
Microgreens are regulated as produce, not sprouts. Sprouts are eaten whole with seed and root attached, often water-grown, and carry extra requirements under Subpart M of the rule; microgreens are cut above the medium after true leaves appear. University of Nevada, Reno recorded no known US foodborne illness outbreaks linked to microgreens between 1998 and 2017, against more than 50 for sprouts and baby greens and more than 350 for mature leafy greens. That record depends on growers keeping the medium out of the product.
Penn State Extension's guidance on media is specific. Avoid mixes with biological amendments of animal origin such as raw manure. Use coir, hemp mats or peat-based mixes from reputable suppliers that have been properly composted or heat-treated. Test for generic E. coli when in doubt. Keep the substrate from touching the edible parts at harvest. UNR asks growers to obtain a letter from the supplier confirming the medium is free of pathogens, or to use a soilless medium. Washed coir is a soilless medium with no animal inputs; the remaining risk is handling and water, not the material.
Published microbial data favour soilless media over peat. In the rapini trial, the peat-grown crop carried the highest Enterobacteriaceae count (log 5.46 CFU/g) and the only detectable E. coli (log 1.46); none was detected on the textile, jute-kenaf or polyester mats. Total aerobic bacteria did not differ by medium. Coir itself was not in that trial, so treat the result as evidence that low-organic-input media reduce one risk, not as proof about any one material.
Harvest, disposal and composting
Harvest at the first true leaf stage, usually about 2 in. (5 cm) tall after 7–21 days (UF/IFAS); Johnny's puts fast varieties at 10–15 days and slow ones at 16–25. Cut high enough above the coir that no pith travels with the stems. NCDA&CS recommends scissors by hand for crops in loose media and an electric knife for mats held vertically. Do not wash on the farm unless you must: Penn State and UNR both note that post-harvest washing appears ineffective and can add contamination. Pack in single-use clamshells and cool quickly; shelf life is 5–10 days.
What is left is a 1–2 cm slab of coir bound by roots. It cannot be reused for another microgreen crop: the root mat blocks sowing and carries the previous crop's microbial load. It composts readily. The Indiana SARE project, which set out to find a cheaper mat and ended up composting instead, found that a soilless mix went from tray dump to sifted, reusable material in eight weeks in summer and cut medium cost by more than $1.50 per tray; its author concluded the future lay in composting and reuse rather than in mats. Composted coir can go into nursery or field mixes; it should not go back into microgreen trays unless the compost has met the time-and-temperature treatment UNR specifies (at least 131 °F for 3 days in a static pile or 15 days turned). Rubber-backed coir mats and polyester mats go to landfill or, for some felt products, industrial composting only.
Buying fine-grade coir for microgreens
The specification is short. Fine grade (pith, not chips or fibre blends). Washed, low EC, with the EC figure and extraction method stated. No animal-origin additives. Supplied compressed, stored dry and clean, because a block that has been wet in transit has grown something. Unbuffered coir is sufficient for a crop harvested in two weeks. The same fine, low-EC grade is what nurseries use for plugs, and the hydration and feeding detail in coco peat for seed germination applies directly to microgreen trays.
CVM Fiber and Industries supplies washed, unbuffered coco peat in 650 g briquettes and 5 kg blocks from Tamil Nadu. If you run a microgreen or vertical farm and want to trial fine-grade coir against your current mats, contact us with your tray count and we will quote a sample lot.
FAQ
Is coco coir better than hemp mats for microgreens?
For yield, yes: in the only on-farm trial that weighed both, hemp mats gave about half the harvest of a soilless mix across broccoli, radish, mizuna and cabbage. For speed and cleanliness at harvest, mats win, because they can be lifted and cut with an electric knife with no loose medium. Farms selling by weight tend to use coir; farms selling to chefs by the tray often use mats.
How deep should coco coir be in a microgreen tray?
UF/IFAS gives 1/2 in. to 1 or 2 in. depending on your irrigation. On a bottom-watered rack most growers use about 1.5–2.5 cm, which is 2–3 litres of hydrated coir in a 1020 tray. Go deeper only if you cannot water daily.
Do microgreens in coco coir need fertiliser?
Usually not; the seed carries enough for a 7–14 day crop. Long-cycle crops such as basil, coriander, carrot and celery, and fast crops that exhaust the seed quickly, benefit from a light feed, for example floating the tray for 30 seconds in a solution of about 80 ppm nitrogen as UF/IFAS suggests.
Can I reuse coco coir after a microgreen harvest?
Not for another microgreen crop. The root mat and the previous crop's microbes make it unsuitable. Compost it; an on-farm study found spent soilless medium was ready to sift in about eight weeks in summer. Composted coir is fine for nursery or garden mixes.
What EC should coco coir have for microgreens?
As low as you can buy. The review most trials cite puts an ideal medium below 0.5 mS/cm; coir at about 1.3 mS/cm has performed acceptably in trials. Ask the supplier for the EC and the extraction method, and flush the batch once if it tests above about 1 mS/cm by a 1:1.5 dilution.
Why bottom water microgreens instead of misting?
After germination, water on the leaves raises disease pressure and spreads anything in the water across the canopy. Extension guidance is to mist only until germination and then sub-irrigate. Coir wicks well from below, which makes bottom watering easy.
Is coco coir safe for growing food under the Produce Safety Rule?
Yes, provided it comes from a reputable supplier with no animal-origin additives, is kept clean, and is not allowed to touch the harvested stems. Washed coir is a soilless medium named as acceptable by Penn State Extension and UF/IFAS. Water quality and worker hygiene are bigger risks than the medium itself.
Does coco coir change the taste of microgreens?
No published trial has reported a flavour difference. The Penn State trial did find the highest antioxidant activity and, in radish, the highest total phenols in coir-grown microgreens, which the authors linked to coir's lower EC. Nitrate in the harvest was also lower in coir than in peat-perlite for both crops.
Sources
- Poudel, P., Duenas, A.E.K. and Di Gioia, F.: Organic waste compost and spent mushroom compost as potential growing media components for the sustainable production of microgreens, Frontiers in Plant Science (2023)
- Di Gioia, F., De Bellis, P., Mininni, C., Santamaria, P. and Serio, F.: Physicochemical, agronomical and microbiological evaluation of alternative growing media for the production of rapini (Brassica rapa L.) microgreens, Journal of the Science of Food and Agriculture (2017)
- Lake, J.A. (advisor Davis, J.): Microgreen production: an evaluation of types of growing media, Colorado State University (2018)
- McEvilly, T.: Differential impacts of growing medium on microgreens productivity, SARE project report FNC17-1088 (2019)
- Treadwell, D.D., Hochmuth, R., Landrum, L. and Laughlin, W.: Microgreens: A New Specialty Crop, UF/IFAS Extension HS1164 (2010, revised 2013)
- Stoltzfus, J. and Fronk, L.: Ensuring Food Safety in Microgreens Production, Penn State Extension (2025)
- Moe, J.: Microgreens and Produce Safety, Desert Farming Initiative, University of Nevada, Reno Extension (2020)
- North Carolina Department of Agriculture and Consumer Services, Produce Safety Program: Microgreen Produce Safety Fact Sheet (2025)
- Evans, M.R., Konduru, S. and Stamps, R.H.: Source variation in physical and chemical properties of coconut coir dust, HortScience (1996)
- Kyriacou, M.C. et al.: Micro-scale vegetable production and the rise of microgreens, Trends in Food Science and Technology (2016), as cited by Poudel et al.
- Abad, M. et al.: National inventory of organic wastes for use as growing media for ornamental potted plant production, Bioresource Technology (2001), as cited by Poudel et al.
- Johnny's Selected Seeds: Microgreens, Key Growing Information
- Bootstrap Farmer: Top 5 Best Growing Mediums for Microgreens; How to Grow Microgreens at Home
- RusticWise: Best Growing Mats for Microgreens