Microgreens Without Soil: Coco Coir, Mats and Soilless Mix

You can grow microgreens without soil using pre-moistened coconut coir, a hydroponic grow mat or a soilless seed-starting mix. Utah State Extension says: “Use a seed-starting mix, coconut coir, or a hydroponic grow mat,” and Penn State Extension adds that you can also grow microgreens on seeding mats. These media are lightweight, tidy and easy to pre-moisten, and they suit a counter, a windowsill or a small shelf. This guide covers each soilless option, how to prepare it and the steps from sowing to harvest.

Soilless choices at a glance

Penn State describes the common media as peat-based mixes, coconut coir and fiber mats. Oregon State and Clemson also describe moistened potting soil, so the choice is yours.

Coconut coir

Coir is lightweight and holds moisture. Utah State’s instruction is to pre-moisten it to the consistency and feel of a wrung-out sponge, which gives the seed a damp but not dripping bed. Pre-moistening before the tray is filled means every seed lands on an evenly damp surface.

1. Pre-moisten the coir until it feels like a wrung-out sponge (USU).
2. Fill a clean shallow tray with drainage holes to an even depth.
3. Sow seed thickly and evenly.
4. Cover for the blackout stage.
5. Uncover and give light, then water from the bottom.

Grow mats and seeding mats

A mat is the tidiest option. Penn State notes that one advantage of seeding mats is that harvested microgreens are cleaner than when grown in potting soil. Penn State also says that after distributing the seed on the surface, it is not necessary to cover seed with medium, and it could be better not to, so the sprouts stay clean. A mat sits in the tray, the seed goes on top, and the shoots grow up and are cut above the mat.

Virginia Tech’s tray work uses hemp and burlap grow mats for broccoli and radish microgreens. Mats can be a good fit if you like a short setup and a simple cleanup.

Soilless seed-starting mix

A soilless mix is the most familiar soilless route. UMD says to use any lightweight soilless mix in the container or tray, and Penn State describes 1020 greenhouse trays or 10- or 20-row seedling trays filled with soilless potting media. Moisten it before filling (Clemson, for potting soil, and USU, for coir, both start with damp medium). Level it, sow, and cover.

Seed, tray and rate

The medium is only one part of the setup.

• Tray. Shallow with drainage holes, clean and food-suitable (PSU). Pie tins and takeout trays work too (USU).
• Seed. Sold for microgreens or marked untreated (Clemson). Penn State’s rate is 10 to 15 g on a standard 12 x 20 inch tray; the seed calculator scales it to the variety and tray.
• Soak. Optional and by species. UMD says 6 to 12 hours speeds germination for pea, sunflower, corn, chickpea, beet, chard, cilantro, mung bean and buckwheat.

Blackout, light and water on a soilless tray

The method after sowing is the same as on any medium.

• Blackout. Utah State stacks a second tray on top with a weight so the seed stays in contact with the medium. Clemson and UMD use a clear cover to hold warmth and moisture. See the blackout method.
• Light. Bright window or 16 to 18 hours under grow lights (grow lights for microgreens).
• Water. Bottom watering suits mats and coir very well. Set the tray in shallow water until the medium is wet, then lift it out and let it drain (PSU). See watering microgreens.
• Temperature. 70 to 75 F (UMD), or 68 to 72 F (Purdue).

Harvest and the next tray

Cut when the first true leaves show, or at 2 to 4 inches (USU, Purdue), with clean scissors just above the medium. The roots stay in the mat or coir. For the next crop, use fresh medium: Clemson says to refill containers with fresh media and sow new seed, and Penn State notes that fresh medium for each cycle is the better start. UMD allows a 50:50 mix of old and new medium. Microgreens do not regrow after cutting (OSU), so every tray is one harvest. See harvesting microgreens.

Which soilless medium should you pick?

• Choose a mat for the cleanest harvest and the simplest tray.
• Choose coir if you like a loose, absorbent bed and are happy to pre-moisten it.
• Choose a seed-starting mix if it is what you already have; it works like any lightweight medium.

You can compare all the options, including potting soil, in growing medium for microgreens. For the full method from tray to table, read how to grow microgreens, and for supplies see growing microgreens at home.

Time to harvest

Soilless trays follow the same timeline as any other. Microgreens go from seed to harvest in 7 to 21 days (PSU, Cornell). Arugula takes 6 to 8 days, radish 8 to 12 and pea shoots 8 to 10 (USU). The harvest planner turns a sow date into a cut date, and the fastest microgreens collection lists the quick ones.