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Bottom watering for trays and microgreens

Water from below to wet the mix without soaking the leaves. Then let the tray drain.

By Ryan Moalemi. Updated November 9, 2026.

Bottom watering means the mix drinks through holes in the tray, from a shallow pool underneath, instead of from a stream poured over the leaves. Extension guides recommend it for microgreens and for seedlings because wet foliage in a dense tray is how disease gets a start, and because a gentle soak does not wash seed to the corners. You still have to pour the leftover water off. A tray left in a puddle overnight is not bottom watering. It is a swamp.

Photo coming soon. A nursery tray with drainage holes nested in a solid tray holding a shallow layer of water, on a kitchen counter.
Photo coming soon. Planned photo: a drained tray nested in a solid tray with a shallow layer of water.

Why the leaves should stay dry

Penn State Extension says sub-irrigation keeps microgreen leaves and stems dry, which matters for disease. At home, set the tray in shallow water and take it out once the mix is wet. Francesco Di Gioia says to wet mix from below before sowing, then keep watering from below so water and leaves meet as little as possible, and to let trays drain so they do not mold.

Maryland says to mist for the first few days, then water from the bottom or the top, moist but not saturated. Iowa State says water from below after germination so foliage stays dry, and that extra water leads to root rots or damping-off. Illinois says water lightly from the bottom and do not let the mix dry out. Mississippi State says bottom watering works on a kitchen sill, and that a hard flood can wash seed off a mat.

The same idea shows up in seed-starting handouts that are not about microgreens. The Sonoma County master gardener page says you can set containers in an inch of warm water until they are saturated, then drain them, and that they should not sit in water constantly or seed rots. Seneca County's Cornell Cooperative Extension handout says about an inch of water for no more than an hour. Those time limits are the part home growers skip. They are the part that keeps the method honest.

The two-tray setup

You want one tray or pot with holes, and one solid tray or tub beneath it. University of Maryland Extension describes a commercial pair: a channeled tray with holes, nested in a tray without holes that holds water. Di Gioia shows the same idea with ordinary trays and says the holes are what make bottom watering possible and what let excess water leave. Without holes, "bottom watering" is just pouring water beside the plants and hoping. Recycled food containers work if you punch holes and if the plastic is clean and food-safe, which is Di Gioia's condition.

Utah State University Extension writes the coir version in numbered steps for a 1020 flat. Pre-moisten coir until a squeezed handful is wet and not dripping. Put about 2 quarts of coir in the tray that has holes. Put about 2 cups of water in the tray that has no holes. Set the coir tray on top. After about 30 minutes, pour out any water still in the bottom tray. That 30 minute cap is a concrete version of "do not leave it soaking." Other media, the page says, follow a similar pattern.

Rocks in the bottom of a pot are not this method. UC Marin Master Gardeners say drainage holes are essential so roots do not sit in stagnant water, and the UC Agriculture and Natural Resources container handout says not to add rocks in the bottom of a pot. Rocks do not create drainage. A hole does. The handout allows a bit of netting over holes if you need to keep mix from falling out. The water still has to be able to leave.

Steps for a microgreen tray

  1. Fill the drained tray with moist soilless mix about 1 to 1.5 inches deep, which is Maryland's depth, or with the coir amount Utah State uses. Level it. If the mix is hydrophobic and sheds water, wet it in a bowl first so the first bottom soak can move through it.
  2. Set the drained tray in the solid tray. Add a shallow layer of water to the solid tray, on the order of the 2 cups Utah State uses for a 1020, or about an inch in the seedling handouts. Do not fill it to the brim of the mix.
  3. Wait until the surface darkens and feels moist. For the Utah State coir setup, check at about 30 minutes and discard the rest. For seedlings, Brewer's Cornell handout says no more than an hour. If the surface is still dry in the middle, the mix may be too deep or packed too hard. Lift one corner and see whether water reached it. Do not keep adding water for the rest of the day.
  4. Remove the growing tray and let it drain. Empty the solid tray so the next hour is not another soak. Di Gioia and the Sánchez article both end the step by getting the extra water out.
  5. Sow on that moist surface, press the seed down, and mist if the top dries during germination. Maryland's mist is for the first days, before roots are holding the seed. A heavy pour from a cup at that stage moves seed into piles, which ruins the density you measured. A spray bottle is the right tool until roots grab.
  6. After the crop is up and uncovered, return to bottom watering when the tray feels light. Frequency depends on the room, the mix, and the size of the tray. The published target is moist, not saturated. A fan on low, which Maryland, Utah State, and Penn State all mention, dries the leaf surface even when the mix is damp.

Microgreen trays and transplant pots share the motion and not the schedule. A microgreen tray is shallow and dense, so it can go from moist to sour quickly if it sits in water. A transplant cell pack is deeper and thinner, and the Cornell hour limit is there so the mix does not stay airless. If you are doing both on one shelf, use two solid trays and label which crop is which. The sowing rates are in the density guide and the seed-starting guide.

When the tray is too wet

Penn State Extension's class notes say damping-off, possibly from Rhizoctonia or Botrytis, was the disease they saw, and that it likes warm, humid conditions. Iowa State names root rots and damping-off as the result of overwatering. You will see seed that germinates and then collapses at the mix line, or a fuzzy growth on a wet mat, or a sour smell. This page cannot identify the organism in your tray. The response the guides support is practical: empty the reservoir, increase airflow, skip the next watering until the tray lightens, and sow the following crop in fresh mix. Penn State prefers new media each cycle to limit disease. Maryland allows a half-and-half refresh after old roots dry. If you just lost a tray to rot, use new mix.

Maryland allows top watering after the misting days. On a dense tray it wets the leaves and can splash mix onto them, so a fine spray in the morning is the less risky version. On a sill, empty the solid tray in the sink. Mississippi State points to bottom watering there specifically. More on cold glass is in the windowsill guide.

FAQ

How long should a tray sit in water?

Until the mix is moist, then out. Utah State pours off the leftover after about 30 minutes in its coir example. A Cornell seed-starting handout says no more than an hour in about an inch of water. The Sonoma County handout says not to leave containers sitting in water. Pick the short end if the mix is already dark.

Can I bottom water a tray with no holes?

No. Water has to enter from below through holes, and extra water has to be able to leave. Di Gioia and the Cornell handouts both require drainage. A solid tray is the reservoir underneath, not the tray you sow in. Punch holes in a food container if that is what you have, and set it inside a solid one.

Should I put gravel under the mix?

No. The UC container handout and UC Marin Master Gardeners say rocks in the bottom of a pot do not replace drainage holes. They can hold water against the mix. Use a hole, and a tray under it to catch what comes out.

Is misting the same as bottom watering?

No. Misting wets the surface where new seed is sitting. Maryland uses it for the first few days. Bottom watering is for once the mix can wick, and for later growth, so the leaves stay drier. Most trays need both, in that order.

Sources

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