Kratky Windowsill Microgreens Tray

At a Glance

How This System Works

Microgreens are harvested just 7–14 days after germination, when the first true leaves begin to emerge. This Kratky-style tray system uses two standard 10 x 20-inch nursery trays — one with drainage holes holding a thin grow mat, and one solid tray underneath acting as a shallow reservoir.

The grow mat wicks nutrient solution up from the bottom tray to the seeds and young roots. Because microgreens are harvested so quickly, they need only a shallow pool of dilute nutrients. There are no pumps, no air stones, and no electricity needed for water movement. Simply fill the bottom tray, set the growing tray on top, scatter seeds, and harvest in one to two weeks.

Why Kratky for microgreens? Microgreens have tiny root systems that thrive in a moist mat. The passive wicking action of the grow mat keeps the roots evenly moist without drowning them. This is the fastest, cheapest hydroponic project you can build.

Materials List

Our philosophy: Use what you already have. Hydroponics does not require store-bought equipment. People around the world grow food this way using recycled containers, scraps of fabric, and seeds saved from last season's harvest. The links below are for convenience if you prefer to purchase, but we encourage you to improvise with what is available to you.

This page contains affiliate links. If you purchase through them, we may earn a small commission at no extra cost to you. Learn more

Containers

Use what you have: Stack two shallow containers. The bottom one holds water (a baking sheet, takeout tray, or even a shallow plastic lid). The top one needs small holes for drainage (poke holes in a second takeout container, a foil baking pan, or a plastic produce clamshell). Microgreens have tiny roots, so you do not need anything deep.

If you want a proper tray set: 10 x 20-inch nursery tray sets are the standard and last for many growing cycles.

Growing Supplies

Use what you have: Lay a few layers of paper towel, a thin kitchen sponge, or a scrap of burlap in the top tray as your grow mat. Seeds just need something moist to grip onto while they sprout. An old cotton t-shirt cut to size also works surprisingly well.

If you prefer a dedicated grow mat: Rapid Rooter plugs (50-pack) or jute/hemp grow mats cut to fit your tray give a clean, consistent surface.

Nutrients: Many growers skip nutrients entirely for microgreens and use plain water. The seeds contain enough energy for the short 7 to 14 day growth cycle. If you want slightly larger, more colorful greens, a quarter-strength dose of General Hydroponics Flora Series (3-part kit) makes a noticeable difference.

Monitoring

Use what you have: For microgreens, precise pH control is less critical than for other crops. If you have pH test strips from a pet store or leftover pool supplies, those are plenty accurate. Just aim for the 5.5 to 6.5 range.

If you want a meter: Digital pH and TDS meter kit is useful if you grow other hydroponic crops too, but it is not essential for microgreens alone.

Additional Materials (no affiliate link)

• Hydroponic grow mat or coconut coir mat (10 x 20 inches) — one per crop cycle
• Microgreen seeds (radish, sunflower, broccoli, pea, etc.) — 1–2 oz per tray depending on seed size
• Spray bottle for misting during germination
• A second solid tray or piece of cardboard for the blackout dome

Tools

No power tools required:

• Scissors for cutting grow mat and harvesting
• Measuring cup or syringe for nutrients

Build Instructions

Step 1: Prepare the Bottom Tray

Take the solid (no-holes) nursery tray and set it on your windowsill or countertop. This is your reservoir. Ensure the surface is level so the nutrient solution distributes evenly.

Step 2: Prepare the Growing Tray

Place the perforated nursery tray insert inside the solid tray. It should sit about 1/2 inch above the bottom, allowing solution to pool underneath.

Step 3: Cut and Place the Grow Mat

• Cut your hydroponic grow mat (jute, hemp, or coir) to fit snugly inside the perforated tray — approximately 10 x 20 inches
• Soak the mat in plain pH-adjusted water (pH 5.5–6.5) for 5 minutes
• Lay the damp mat flat in the perforated tray

Step 4: Mix the Nutrient Solution

Microgreens need very dilute nutrients:

• Use clean, room-temperature water
• Mix nutrients to quarter strength (microgreens are extremely light feeders)
• Target pH: 5.5–6.5
• Target EC: 0.3–0.6 mS/cm

You will need about 2–3 cups (500–700 ml) of solution for the initial fill.

Step 5: Fill the Reservoir

Pour the dilute nutrient solution into the bottom solid tray until it reaches the underside of the grow mat — approximately 1/4 to 1/2 inch deep. The mat should wick the moisture upward without the seeds sitting in standing water.

Step 6: Sow the Seeds

• Scatter seeds evenly and densely across the entire surface of the damp grow mat
• For small seeds (broccoli, radish, mustard): 1–1.5 oz per tray, no pre-soaking needed
• For large seeds (sunflower, pea): 2–3 oz per tray, pre-soak in water for 8–12 hours before sowing
• Mist the seeds lightly with a spray bottle after scattering

Step 7: Blackout Phase

• Cover the seeded tray with another solid tray flipped upside down, or use a piece of cardboard
• Keep seeds in the dark for 2–4 days at room temperature (65–75 degrees F)
• Mist once daily to keep the mat moist during blackout
• Seeds will germinate and push upward against the cover

Step 8: Uncover and Grow

• Once sprouts are 1–2 inches tall and showing pale yellow leaves, remove the blackout cover
• Place the tray on a sunny windowsill (south-facing is best) or under a small grow light
• The sprouts will green up within 24–48 hours as they begin photosynthesizing
• Check the reservoir daily and top off with plain pH-adjusted water if the bottom tray dries out

Nutrient Guide

Microgreens have minimal nutrient demands because they are harvested so young.

Note: Many growers use plain water with no nutrients at all for microgreens. Adding a quarter-strength nutrient solution is optional but produces slightly larger, more vibrant greens.

Crop Suggestions

Estimated Cost

Tips & Troubleshooting

General tips:

• Harvest microgreens by cutting just above the mat with sharp scissors or a clean knife
• Harvest in the morning for the crispest texture
• Most microgreens do not regrow after cutting, except pea shoots which often give a second harvest
• Start a new tray every 5–7 days for a continuous supply
• Store harvested microgreens in a sealed container with a dry paper towel — they last 5–7 days in the fridge

Maintenance Schedule

Daily (during growth cycle)

• Check that the grow mat is moist — mist or add water to the bottom tray as needed
• During blackout phase, mist once and re-cover

At Harvest (Day 7–14)

• Cut all microgreens just above the mat surface
• Discard the used grow mat and root mass (compost if possible)
• Rinse both trays with mild soap and water
• Lay a fresh grow mat and start a new cycle immediately

Monthly

• Deep-clean trays with a dilute hydrogen peroxide rinse (1 tablespoon per quart of water)
• Inspect trays for cracks or warping and replace if needed
• Restock seeds and grow mats as supply runs low

Frequently Asked Questions

Do microgreens need nutrients or just water?

Many growers successfully grow microgreens with plain water alone because the seed itself contains enough stored energy to fuel growth through the short harvest window of 7 to 14 days. Adding a quarter-strength hydroponic nutrient solution is optional but can produce slightly larger, more vibrant greens with deeper color, especially for slower-growing varieties like sunflower and pea shoots.

How do you tell the difference between mold and root hairs on microgreens?

White root hairs are commonly mistaken for mold and are completely normal on microgreen seedlings, especially radish, broccoli, and sunflower. Root hairs appear as fine, fuzzy filaments clustered tightly around individual roots and disappear when misted with water, while actual mold looks like cobwebby patches that spread across the mat surface, may have a musty smell, and does not disappear when sprayed.

How many times can you harvest microgreens from one tray?

Most microgreens produce only a single harvest because cutting them at the stem removes the growing point and the plant cannot regrow. The one notable exception is pea shoots, which will often regrow for a second and sometimes third cut if harvested above the lowest leaf node. For all other varieties, start a fresh tray with new seeds and a clean grow mat after each harvest.

What is the fastest microgreen to grow at home?

Radish microgreens are the fastest commonly grown variety, typically ready to harvest in just 6 to 8 days from sowing. They germinate within 24 to 48 hours, grow vigorously with minimal care, and produce crisp stems with a mild peppery flavor that works well in salads, sandwiches, and as a garnish.