How to Grow Microgreens in Trays Without Soil

Growing microgreens without soil is straightforward: you replace dirt with a clean, moisture-retaining mat or pad, spread seeds densely across the surface, and keep them moist under gentle light for roughly one to two weeks until the first true leaves begin to emerge. The soilless approach eliminates mess, reduces the risk of soil-borne fungi, and works in any indoor space with a windowsill or a simple LED fixture. What takes a bit more thought is choosing the right growing medium, managing water and nutrients, and handling food safety, since microgreens grown on any substrate can carry pathogens if seeds or irrigation water are contaminated.

Picking a Soilless Growing Medium

The most common soilless options for home trays are coconut coir mats, hemp mats, jute-kenaf fiber pads, and synthetic textile fiber pads. Each holds moisture differently and affects yield. Research comparing several alternatives found that textile fiber and jute-kenaf fiber produced fresh yields roughly equal to peat (around 1,500 grams per square meter for rapini microgreens), while sphagnum moss trailed by about 13%.¹ For practical purposes, that means most fiber-based mats will give you similar harvests as long as the mat stays uniformly moist.

Coconut coir is the most widely available option in garden centers and online. It comes as pre-cut pads sized for standard 10×20-inch nursery trays, and it holds water well without becoming waterlogged. Hemp and jute-kenaf mats are popular among growers looking for fully compostable materials. One thing to watch for: cannabis (hemp) mat without any nutrient solution has been associated with notably slower growth compared to other substrates, so if you go that route, plan on adding a dilute nutrient solution rather than plain water.²

You can also skip a fiber mat entirely and use a few layers of unbleached paper towel or burlap. Paper towels work fine for fast-germinating seeds like radish and broccoli, though they dry out quickly and need more frequent misting. Burlap gives roots something to grip but can harbor mold if airflow is poor. For a first attempt, a pre-cut coir or hemp mat is the simplest choice because it fits the tray, requires no cutting, and provides consistent moisture.

Setting Up the Tray

You need two standard nursery trays: one with drainage holes and one solid tray underneath to catch water. The perforated tray holds your growing medium, and the solid tray acts as a reservoir. This two-tray system lets you bottom-water, which keeps the leaf surface dry and discourages mold. Place your mat inside the perforated tray, wet it thoroughly until it is saturated but not pooling, and let excess water drain into the tray below.

Substrate depth matters more than you might expect. Research at Cornell evaluating multiple growing conditions found that fresh weight of microgreens increased as substrate depth rose from roughly 1.8 cm up to about 5.8 cm.³ Most commercial mats are only about 1 to 1.5 cm thick, which is on the thin side. If your yields seem underwhelming, try doubling up two mats or using a thicker coir block that you can press to about 3 to 4 cm deep. The extra depth gives roots more room to anchor and access moisture, which translates to sturdier, heavier shoots at harvest.

Seed Selection, Density, and Pre-Soak

Almost any vegetable, herb, or grain seed can be grown as a microgreen, but some are far easier than others on a soilless medium. Radish, broccoli, sunflower, pea, and mustard are reliable first choices because they germinate fast and produce thick, flavorful shoots. Smaller seeds like amaranth and basil can be trickier on mats because they tend to roll into clumps. Basil seeds produce a sticky mucilage coat when wet, which actually helps them cling to the mat surface, so they are an exception to the “small seeds are hard” rule.⁴

Seeding density is a balancing act. The same Cornell research found that total fresh weight per tray increased as seed density rose, but the weight per individual plant decreased, meaning each shoot got smaller and thinner as crowding increased.³ For home growers, a single even layer where seeds are close but not stacked on top of each other is the sweet spot. A rough guideline: about 10 to 12 grams of small brassica seeds (broccoli, kale, radish) per standard 10×20-inch tray, and 60 to 80 grams for larger seeds like sunflower or pea.

Pre-soaking large seeds like sunflower and pea for 8 to 12 hours speeds germination by a day or more. Smaller seeds generally do not need soaking. If you are concerned about food safety, a hydrogen peroxide soak before planting can reduce pathogen loads on seed surfaces without hurting germination. Research on seed decontamination found that a hydrogen peroxide and peroxyacetic acid soak followed by a gentle heat treatment significantly reduced Listeria on seeds, while germination rates remained statistically the same as untreated seeds.⁵ For a simple home approach, soaking seeds in a 3% food-grade hydrogen peroxide solution for about 10 minutes, then rinsing well, is a reasonable precaution.

The Blackout Phase

After seeding, cover the tray with another upside-down tray or a sheet of cardboard to block light. This “blackout” period typically lasts two to four days. During this time, the seeds germinate and the emerging shoots push upward against the cover, which encourages strong, elongated stems. The darkness also keeps the growing medium moist longer because there is no light driving evaporation.

Check underneath every day. You are looking for shoots about 2 to 3 cm tall with pale yellow leaves. Once they reach that height and most seeds have sprouted, remove the cover and move the tray to light. If you see fuzzy white strands at the base of the stems, do not panic. Those are root hairs, not mold. Root hairs are fine, single filaments growing off each rootlet. Mold, by contrast, appears as a cottony or web-like mass and usually has a musty smell. Improving airflow with a small fan is the best prevention for actual mold.

Watering and Whether You Need Nutrients

Bottom-watering is the standard method for soilless microgreen trays. Pour water into the solid bottom tray and let the mat wick it up through the drainage holes. This keeps leaves dry and minimizes disease. Check the bottom tray once or twice a day; the mat should feel consistently damp but never waterlogged. If you see standing water in the bottom tray that has not been absorbed within 30 minutes, pour some off.

Whether you need a nutrient solution depends on the species and your goal. Many fast microgreens like radish and broccoli will produce a perfectly good harvest on plain water alone, because the seed itself contains enough stored energy to fuel the first week or two of growth. But if you are growing slower species, aiming for maximum size, or pushing for a second cut from the same tray, a dilute hydroponic nutrient solution helps. For lettuce microgreens grown hydroponically, research suggests an ideal nutrient solution with a pH around 6.3, electrical conductivity around 1.8 millisiemens, dissolved oxygen of about 6 mg per liter, and a temperature near 18°C.⁶

You do not need a lab to hit these targets at home. A basic hydroponic nutrient concentrate mixed at quarter to half strength, combined with a cheap pH test kit from a garden store, gets you close enough. The dissolved oxygen number is worth noting because stagnant water in the bottom tray can run low on oxygen, especially in warm rooms. A study on leafy vegetables in floating hydroponic systems found that dissolved oxygen below about 5 mg per liter was markedly limiting for plant growth, and yield reductions started appearing around 6 mg per liter.⁷ For a simple tray setup, this just means changing the water in the bottom tray daily rather than letting it sit for days, and keeping air circulating in the room.

Lighting After the Blackout

Once the cover comes off, the microgreens need light to green up and develop flavor. A south-facing windowsill can work in bright months, but you will get more consistent results with a basic LED grow light positioned about 15 to 30 cm above the tray. Twelve to sixteen hours of light per day is a common range for indoor microgreens. The color spectrum of the light actually influences both growth and nutrient content. A study on Brassica oleracea (the cabbage family, which includes kale, collard, and cabbage) found that a combination of red and blue LED light was most effective for overall shoot and root growth, while blue light alone was particularly good at boosting certain beneficial compounds called glucosinolates, increasing one key compound by 61% compared to a white-light control.⁸

For home growers, this does not mean you need expensive tunable spectrum lights. A standard “full spectrum” or “daylight” LED shop light from a hardware store covers a broad enough range to produce healthy, green microgreens. If you happen to have a light with adjustable red-blue ratios, leaning toward a mix of both will give you the best of both worlds: good growth plus enhanced nutritional compounds. Temperature also plays a role. The Cornell research found that the number of days to harvest decreased in a straight line as air temperature rose from 14 to 22°C.³ Room temperature around 18 to 22°C is a comfortable range for both you and the plants.

Harvesting

Most microgreens are ready to harvest 7 to 14 days after seeding, depending on the species and temperature. The typical harvest point is when the cotyledons (the first pair of seed leaves) are fully open and green, and the first true leaf is just starting to emerge. Cut the stems just above the mat surface using clean scissors or a sharp knife. Harvest the whole tray at once if possible, because partially harvested trays tend to develop mold on the remaining damp stems and exposed mat.

Avoid washing microgreens if you can, because the added moisture shortens shelf life. If you bottom-watered and kept the leaves dry throughout the grow, they should be clean enough to eat straight from the tray. If you do need to rinse them, use cold water and dry them as gently and thoroughly as possible with a salad spinner or by spreading them on a clean towel.

Keeping Microgreens Fresh After Harvest

Microgreens are delicate and lose quality fast at room temperature. Research on garden cress microgreens found that weight loss after harvest roughly doubled when storage temperature rose from 4°C to 12°C, and modified atmosphere packaging helped slow that decline.⁹ At home, the practical takeaway is to refrigerate them immediately, ideally in a loosely sealed container with a dry paper towel to absorb excess moisture. Stored this way, most microgreens stay crisp for about five to seven days. Leaving them on the counter cuts that to one or two days at best.

Another option is to harvest only what you need and leave the rest growing in the tray. A living tray in the fridge will keep longer than cut greens, though growth will slow dramatically in the cold. Some growers keep a tray on the counter under lights and snip servings as needed, treating it like a living salad bar.

Food Safety Without Soil

One of the reasons people choose soilless growing is the perception that it is cleaner than soil. That is partly true: you eliminate soil-borne organisms and the gritty residue that comes with dirt. But going soilless does not eliminate food safety risk entirely. Research has confirmed that there is a high risk of foodborne pathogens transferring to the edible part of microgreens when seeds, irrigation water, or soilless substrates are contaminated.¹⁰ A separate study found that pathogen populations transferred to microgreens from contaminated soilless substrates and seeds at levels that were not significantly different from each other, meaning the contamination source (seed versus mat) mattered less than whether contamination was present at all.¹¹

The practical steps to minimize risk at home are straightforward:

• Buy from reputable seed suppliers: seeds sold specifically for sprouting or microgreens are more likely to have been tested or treated for pathogens than bulk agricultural seed.
• Use clean water: municipal tap water is generally fine. If you are on well water, consider a pre-soak seed sanitization step as described earlier.
• Sanitize trays between grows: a dilute bleach solution or hydrogen peroxide wipe between batches prevents buildup of bacteria and fungi on the plastic surfaces.
• Keep it cool: warm, stagnant conditions in the tray encourage bacterial growth. Good airflow and fresh water changes help.

Do Soilless Microgreens Taste Different?

Some growers swear that soil-grown microgreens have richer flavor, and there is a kernel of truth to the claim, though the picture is more nuanced than “soil equals better.” A sensory study comparing microgreens from local farms (both soil-grown and hydroponically grown) with commercial microgreens found that the local farm samples scored significantly higher on smell, appearance, taste, and overall liking. Interestingly, though, the difference between soil-grown and water-grown microgreens from the same local farm did not reach statistical significance.¹² That same study found no significant difference in total phenolic content between commercial and local samples, or between soil and hydroponic growing methods from the same farm. Freshness and handling appear to matter far more than the growing medium when it comes to both taste and antioxidant content.

This is reassuring if you are growing at home without soil. Your microgreens go from tray to plate in minutes, which is fresher than anything you can buy at a store. The flavor advantage of freshness likely outweighs whatever subtle differences the growing medium might contribute. That said, the nutrient profile does shift somewhat depending on the medium. Peat-grown microgreens, for instance, tend to accumulate higher concentrations of certain minerals and roughly double the nitrate content compared to those grown on textile or jute-kenaf fiber mats.¹ Whether that matters to you depends on your goals. Higher nitrate is not necessarily desirable from a health perspective, so fiber mats may actually be the better option on that front.

Species That Thrive on Mats and a Few That Struggle

Not every microgreen is equally well-suited to a soilless mat. Here is a rough breakdown based on how well different seed types anchor and grow on fiber pads:

• Easiest: radish, broccoli, kale, mustard, arugula, and cabbage. These brassicas germinate in one to three days, root into the mat quickly, and reach harvest height in 7 to 10 days.
• Moderate: sunflower and pea. Both need a pre-soak, produce thick shoots with substantial flavor, and take 10 to 14 days. Their large roots grip the mat well, but they drink a lot of water, so bottom-tray refills are more frequent.
• Tricky: basil, cilantro, and beet. Basil’s mucilage coat helps it stick, but it germinates slowly and unevenly. Cilantro seeds are hard-shelled and benefit from cracking (gently rolling with a rolling pin before soaking). Beet seeds are actually multi-germ clusters that produce several sprouts per “seed,” which leads to dense, tangled clumps that can trap moisture and mold.
• Not recommended for beginners: carrot and celery. Both have extremely slow germination, tiny seedlings, and low yields on mats. They are better suited to soil or at least a thick coir substrate where roots can develop over a longer period.

Reusing Mats and Managing Waste

After harvest, the mat is full of root mass and cannot be reused for another crop. Coconut coir and jute-kenaf mats are compostable and break down readily in a home compost pile along with the root network. Synthetic textile mats are not compostable and go in the trash, which is worth considering if you plan to grow continuously. Some growers skip mats entirely for this reason and use a thin layer of vermiculite or perlite in the perforated tray. These mineral substrates are reusable after a quick rinse and sanitization, though they lack the wicking properties of fiber mats and need more careful water management.

The two-tray system itself lasts indefinitely with proper cleaning. After each grow cycle, scrub both trays with hot soapy water or a dilute bleach solution, rinse thoroughly, and let them dry before reloading. This prevents biofilm buildup that could harbor bacteria across successive crops.