How to Grow Microgreens in a Sprouting Tray: Soil, Coco Coir & Reusable Medium Guide

If you've watched our YouTube channel before, you may have seen us using some trays that are a little different from the typical 10x20 trays. So what are they?

Technically, they're called "sprouting trays" — and they weren't originally designed for growing microgreens at all. They're meant for sprouting, which is a completely different growing method. But over the years, we've figured out how to use them really well for microgreens, and at this point they've become one of our favorites.

In this blog, we're going to walk you through exactly how to grow microgreens in a large sprouting tray using three different grow mediums: soil, coco coir, and our reusable silicone grow medium. We'll cover tray specs, seeding rates, watering tips, and the specific quirks that make these trays a little different from what you may be used to.

---

Why Use Sprouting Trays for Microgreens?

There are two big reasons people reach for these trays: cost and space.

Sprouting trays are generally more affordable than standard 10x20 trays, which makes them a great option if you want to grow more without spending a lot upfront. And because they're smaller, they're easier to fit into tight spaces — a shelf, a countertop, or a small grow tent.

Don't let the size fool you though. We've consistently pulled solid harvest weights from these trays, especially once you dial in your watering. The benefits absolutely outweigh the challenges in our experience.

---

Tray Specs: Know What You're Working With

Before you get started, it helps to know what you're actually dealing with. The Large Sprouting Tray Set includes a white mesh tray and a green solid tray that work together throughout the entire grow.

Tray dimensions:

• External tray size: 12.75" × 9.5" × 1.875"
• Mesh tray internal size: 11" × 8" × 1"
• Green tray internal size: 11" × 8" × 1.5"
• Mesh hole size: 2.5 mm
• Soil/coco coir capacity: approximately 3–4 cups

The white mesh tray holds your grow medium and seeds. The green solid tray acts as your bottom-watering reservoir once you move to light — and flips upside down to serve as your blackout dome earlier in the grow. The 2.5 mm mesh holes are large enough to grow bigger seeds like peas, wheatgrass, and mung beans directly on the tray without any medium at all, and the included silicone reusable grow medium covers smaller seeds like broccoli that need a finer surface.

One thing worth knowing upfront: the green tray has a small notch in it, which means it doesn't create a fully sealed environment when using it as a lid. That's fine — just don't rely on it alone to retain humidity. Watering twice daily during germination is what keeps moisture where it needs to be, and making sure there are no fans pointed directly at the trays during germination.

---

Choosing Your Grow Medium

One of the first decisions you'll make is which grow medium to use. Here's a quick overview of each option before we get into the step-by-step:

Soil — Great for beginners. It's forgiving, retains moisture well, and most varieties do very well in it. Use a seed-starting mix and fill the mesh tray about ¾ full (roughly 3 cups). No fertilizer needed during the grow — the soil already has what the plants need.

Coco Coir — Similar to soil in how it's used, but it's a more neutral medium with no nutrients on its own. The big advantage is that you can add hydroponic nutrients when you switch to bottom watering, which can give your crop a boost. Also a great choice if you want a cleaner, less earthy setup.

Reusable Silicone Grow Medium — Our sprouting tray-sized silicone medium is pre-cut to 8" x 10" and fits right into the white mesh tray. No soil, no mess — just rinse, reuse, and grow again. It works great for most varieties and is especially popular with folks who want a long-term, sustainable setup. We'll cover this option in its own section below.

---

Step-by-Step: Growing with Soil or Coco Coir

Step 1 — Prep your tray

Place the white mesh tray on top of the green solid tray. Fill the mesh tray with soil or coco coir until it's about ¾ full — approximately 3 cups. Break apart any clumps and remove any large twigs if present (microgreens don't like growing on sticks). Spread the medium evenly across the tray and lightly tamp it down with your hands. Don't pack it tight — just a gentle press to level it out.

---

Step 2 — Choose your seeds and measure

Not sure where to start? We suggest beginner-friendly varieties like broccoli, kale, collard, radish, peas, and beans for your first few grows.

For seeding amounts, the large sprouting tray is roughly half the size of a standard 10x20 tray, so use about half the amount you would for a 10x20 tray. Here are some general starting points:

• Broccoli, kale, radish, and other small seeds: ~10–20 grams
• Sunflower: ~65 grams (no soak needed — always rinse before planting to remove hull debris)
• Peas: ~120 grams

You can also reference our free Tray-Specific Seeding Guide PDF for variety-by-variety seeding rates adjusted for sprouting tray sizes.

---

Step 3 — Seed evenly

Using your hands or a seed shaker, spread the seeds as evenly as possible across the grow medium. Try not to group seeds together in clumps. Even coverage = even germination.

---

Step 4 — Water in

Use a spray bottle or hand pump sprayer (best) to water the seeds and medium generously. You want everything moist — not soaking, not dripping, just thoroughly damp. Too much water at this stage is one of the leading causes of mold and slow germination.

This is our favorite misting device for all mediums.

---

Step 5 — Cover and add weight

Remove the green tray from underneath, knock out any medium that may have fallen into it, and place it on top of the seeds as a cover.

At this point you have two options: add a 5–7 lb weight on top of the green tray, or leave the tray as-is and let it act as the weight on its own. The amount of weight needed varies by crop variety — we have this listed in our free Tray-Specific Seeding Guide PDF along with suggested timing, using W = Weight or ET = Empty Tray so you always know what each variety needs. Weight can be really beneficial for even germination and seed hull removal — we actually did a whole video on this topic if you want to see it in action!

---

Step 6 — Germination

Place your tray on a dark shelf. Check and water twice daily — morning and evening. Keep the medium consistently moist. Most crops will germinate in 2–4 days. You'll know they're ready to move to the next step when you see more yellow plant coming through than dark seed coatings, and the tray or weight is starting to lift from the pressure of the seedlings pushing up.

---

Step 7 — Blackout

Remove the weight and cover tray. Give your seedlings a mist, then take the green tray, flip it upside down to create a dome, and place it over the mesh tray. Put the whole setup back on a dark shelf for 1–2 days depending on the variety. The goal here is to give the stems a little stretch so they're taller and easier to harvest. Don't overdo it though — too much blackout leads to a lanky crop that topples over.

---

Step 8 — Move to light and begin bottom watering

 

Once your crop is the right height (a slight stretch is perfect — not flopped over), flip the green tray back underneath the mesh tray so it acts as a reservoir. Move your crop to light for the first time.

This is where the sprouting tray becomes a little different from working with 10x20 trays. There's often a noticeable gap between the bottom of the mesh tray and the base of the green tray. Because of this, you'll need to add significantly more water than you might expect when you first start bottom watering — usually around 2 to 2½ cups to start, compared to the ¼ cup you'd begin with in a standard 10x20 setup.

The way to check: add water, let the tray settle, then slightly lift the mesh tray and look for water droplets on the underside. If you see them, you're good. If you don't, add a bit more. You want the roots making contact with the water to prevent your microgreens from drying out. The bottom of the tray itself does not need to be sitting in water — just the roots.

Watering notes by medium:

• Soil: Bottom water with plain water only. Do not add fertilizer — the soil already has nutrients and adding more can stunt growth or cause tip burn.
• Coco Coir: You can boost growth by adding hydroponic nutrients when bottom watering. We use OceanSolution™ 2-0-3 mixed at 0.5 oz per gallon, then pH balance the water to 5.5–6.0.

Check your tray twice daily. You don't want the medium staying overly saturated — if it looks and feels very wet, skip one watering to let it dry out a bit. If plants are wilting or flopping, add more water. Watering really is the key skill with these trays, and once you get a feel for it they become very easy to work with.

Every 2–3 days, also check whether old water is still sitting in the green tray. If so, dump it and refill with fresh water — coco coir and soil retain moisture well, so stagnant water can build up faster than you'd expect and invite disease.

---

Step-by-Step: Growing with a Reusable Silicone Grow Medium

If you'd like a soil-free, reusable option, our Large Sprouting Tray Full Set comes with our silicone reusable grow medium and fits right into the white mesh tray, the medium can also be purchased separately for the sprouting trays. Here's how the process is a little different:

---

Step 1 — Set up the tray

Place the white mesh tray on top of the green solid tray. Lay the silicone medium flat inside the mesh tray. Give it a light mist with water before seeding — this helps the seeds land and stick instead of bouncing around.

---

Step 2 — Seed evenly

Measure your seed and distribute it as evenly as possible across the silicone surface. Mist again lightly after seeding to make contact with the seeds. If any water is pooling on top of the medium, wick it up gently with a cloth or paper towel — you don't want standing water during germination.

Seeds that work well on reusable mediums in sprouting trays include broccoli, kale, radish, sunflower, peas, cabbage, basil, kohlrabi, and most brassicas. Varieties that don't perform well on reusable mediums include Swiss chard, beets, and cilantro — stick to soil or coco coir for those. Arugula can be grown on reusable mediums but tends to have tangling roots and can be more challenging to clean afterward.

---

Step 3 — Cover and weight

Flip the green tray over the top and optionally add a 5–7 lb weight. Check your free seeding guide for the recommended weight vs. empty tray guidance per variety.

---

Step 4 — Germination

Same as soil/coco coir — dark shelf, mist twice daily, 2–4 days for most crops.

---

Step 5 — Blackout

Once germination is complete, mist the seedlings, flip the green tray upside down as a dome, and blackout for 24hours just like with soil or coco coir.

---

Step 6 — Move to light and bottom water

Remove the green tray on top and move your crop to light. The same watering gap applies here — start with 2 to 2½ cups and check for moisture and root contact under the mesh tray.

With reusable mediums, we highly suggest adding hydroponic nutrients when bottom watering. Unlike soil, a silicone grow medium is completely inert — meaning it contains no nutrients of its own. There's nothing in the medium to feed your plants, so the only way they're getting nutrition during the grow is through the water you give them and the grow lights. Skipping nutrients with a reusable medium means your crop is essentially growing on water alone, which can result in slower growth and a lighter harvest. We use OceanSolution™ 2-0-3 mixed at 0.5 oz per gallon, then pH balance the water to 5.5–6.0.

---

Step 7 — Harvest and clean

Harvest as normal. After harvest, clean the silicone medium by hand using a gentle free and clear soap like Dr. Bronner's or Seventh Generation. Never use brushes — just your hands. You can also boil the medium for 2–3 minutes to sanitize. Do not put it in the dishwasher. Once cleaned and dry, it's ready for the next grow.

---

Common Troubleshooting Tips

Crop falling over: Usually an underwatering issue with these trays due to the gap between the mesh and green tray. Check root contact with water first. Could also be overwatering or a pathogen — check the roots. Sliminess or localized die-off points to disease, not watering.

Overwatered medium: If the medium is visibly saturated and waterlogged, skip one watering entirely. Even after a full day off, the crop should still have enough moisture. For reusable mediums, if water is pooling onto the medium above, simply pour out a splash of water until its not happening any more.

Stagnant water smell: Dump the old water from the green tray every 2–3 days and replace with fresh. Don't let water sit.

Mold during germination: Usually caused by too much water at seeding, poor airflow, or overwatering during germination. Reduce spray amount, ensure you're on a dark shelf with reasonable airflow, and don't seal the tray so tight that no air gets in.

Seed hulls not coming off: Make sure you're using weight during germination for varieties that need it. The weight pressing down on the tray makes contact with the seeds and helps dislodge those hulls as the plants push up.

---

Final Thoughts

Sprouting trays have a learning curve — mainly around watering — but once you've got that dialed in, they're genuinely one of our favorite trays to use. They're affordable, compact, and they produce great results across all three grow medium types.

If you want to go deeper on growing microgreens from start to finish, check out our brand new Beginner's Guide to Growing Microgreens PDF — it's a great starting point if you're just getting into growing and want everything laid out in one place. We also have our Tray-Specific Seeding Guide which covers sprouting tray seeding rates for multiple crop varieties, our book Becoming a Microgreen Master, and our full Microgreen Masterclass course for growers who want to go all in.

CJ & Mandi, On The Grow

---

Partial Reference: How to Grow Microgreens in a Sprouting Tray
Updated: 3/18/2026
Published: October 29, 2021
All content is property of On The Grow®, LLC