MICROGREEN GROWING · WATERING
How Much Water for Microgreens? Per Tray, Per Variety, Per Day.
Almost every guide answers this question with "keep it evenly moist." That instruction has probably cost more trays to mold than any other sentence in microgreens, because moist is not a measurement and two growers never mean the same thing by it. Here is the actual number, by variety and by grow day, and the drain check that confirms the number is right for your room.
The question behind the question
When a grower searches how much water for microgreens, they are almost never curious. They are standing in front of a tray that has gone wrong, or they are about to make the same decision for the fourth day running and they have realised they are guessing.
Ask yourself which one you are. If you have lost a tray to mold in the last month, did you water it the same amount every day of the grow? Most growers, when they answer that honestly, find they poured roughly the same volume on day one as on day six, and the tray on day one had no leaves and nothing to drink it.
That is the whole problem in one sentence. The plant's demand triples over the light phase. The pour does not.
Why "keep it moist" fails
Moist describes an appearance, and appearance is exactly what you cannot read in a microgreen tray. The top half inch of substrate looks damp long after the bottom has gone dry, and it looks damp again the moment it is waterlogged underneath. The surface tells you almost nothing about what the roots are sitting in.
The three practical failures of watering by look:
- It is not repeatable. Two identical trays get different water because you were in a hurry on Tuesday.
- It is not comparable. When a tray fails you cannot tell whether water was a factor, because you have no record of what you gave it.
- It is not teachable. The first time somebody else covers a weekend for you, "keep it moist" means whatever they think it means, and they will almost always overdo it.
A number fixes all three. That is the case for measuring, and it is the case for the app.
The measured amount, by variety
Every variety in the GLAP library carries a full water amount, which is the volume that tray takes on its last day of the light phase when the canopy is complete and demand is at maximum. It is stored in both fluid ounces and millilitres, along with the variety's watering frequency and the length of its light phase.
| Variety (10 by 20 tray) | Peak daily water | Light phase | Frequency | First light day |
|---|---|---|---|---|
| Sunflower (black oil) | 16 oz / 473 ml | 7 days | Once daily | 5.5 oz / 165 ml |
| Pea shoots (sweet pea) | 16 oz / 473 ml | 10 days | Once daily | 5 oz / 150 ml |
| Microgreen radish | 9 oz / 266 ml | 5 days | Twice daily | 3.5 oz / 105 ml |
| Broccoli | 8 oz / 237 ml | 6 days | Twice daily | 3 oz / 90 ml |
Two things to notice in that table, because they are where growers go wrong.
The first day is about a quarter of the last day. Not two thirds. Not "a bit less." Sunflower goes from 5.5 oz to 16 oz across seven days. If you pour 16 oz on the first day, roughly 10 of those ounces have nowhere to go and no plant to take them, and they sit in the bottom of the tray at room temperature. That is not watering. That is incubation.
Twice daily means split, not double. Broccoli on 8 oz a day taken twice is 4 oz in the morning and 4 oz in the late afternoon. Radish and broccoli want the split because their totals are small and a single dose sits too long. Sunflower and pea hold a much denser root mat and are fine on one pour.
How the daily amount ramps
The curve is simple enough to do on paper. The tray gets 25 percent of its full amount on the first light day and 100 percent on the last, rising in a straight line between the two.
In plain terms: daily amount = full amount multiplied by (0.25 plus 0.75 times the light day divided by the total light days).
Worked through for broccoli, which has the tidiest numbers of the four:
| Light day | Percent of full | Daily total | Per watering (twice daily) |
|---|---|---|---|
| Day 1 | 37.5% | 3 oz / 90 ml | 1.5 oz |
| Day 2 | 50% | 4 oz / 120 ml | 2 oz |
| Day 3 | 62.5% | 5 oz / 150 ml | 2.5 oz |
| Day 4 | 75% | 6 oz / 180 ml | 3 oz |
| Day 5 | 87.5% | 7 oz / 207 ml | 3.5 oz |
| Day 6 | 100% | 8 oz / 237 ml | 4 oz |
Day 1 here means the first day out of blackout, not the sow date. The blackout window is variety-specific and is counted separately, which matters because a variety with a 4 day blackout and a 6 day light phase is a 10 day tray, and the ramp only starts on day 5.
This is the part an app genuinely does better than a clipboard. You are not looking up a number, you are computing a different number for every tray on the rack every single day, and each tray is on a different day of a different variety's phase. Doing that by hand for twelve trays before your morning coffee is how growers end up back at "keep it moist."
The drain check: what it is and what it is for
Now the confirmation step. Read this part carefully, because it is easy to take the wrong lesson from it.
The drain check does not replace the measured amount. It verifies it. The measured amount is the accurate way to water and it is the thing that makes your trays repeatable. The drain check tells you whether that number suits your trays, your substrate depth and your grow room, which nobody can know for you from the outside.
The procedure
- Water the tray with the measured amount. Bottom water into the solid tray under the mesh tray, which is where the amounts above assume the water goes.
- Wait 15 to 20 minutes. This is the part people skip. The substrate takes that long to draw up what it can hold. Pour it off at two minutes and you will remove water the tray was about to use.
- Tip the tray and pour off every drop of standing water. Not most of it. All of it. Hold the tip until the stream stops.
- Let it drain naturally. No bouncing. No pressing. No shaking. Gravity only.
- Look at what came out.
What the runoff is telling you
Whatever pours out of that tray is water the substrate could not hold. If you had left it in the bottom, it would have sat against the root mat at room temperature for twenty-four hours. That is the water that causes mold on the surface, root rot underneath, and the fungal problems that make a tray smell sour three days before you can see anything wrong.
- A small amount, an ounce or so: the number is right. Carry on.
- A steady stream, several ounces: your amount is too high for this substrate depth and this room. Reduce by 15 to 20 percent and check again tomorrow.
- Nothing at all, three days running, with a canopy that looks tired by evening: the amount is likely too low. Increase by 10 percent and watch.
- Nothing at all but the tray looks excellent: leave it alone. A tray taking exactly what it is given is the target, not a problem.
Notice that every one of those outcomes is an adjustment to the number, not an abandonment of it. That is the whole point. You are calibrating a measurement, not replacing it with a feeling.
Why no bouncing and no pressing
Two mechanisms, both of which cost you yield.
Bouncing the tray to shake out the last drops breaks the root mat away from the substrate. In microgreens the root mat is the thing that holds the whole flat together at harvest, and a tray that has been bounced twice tends to lift in patches when you cut it.
Pressing is worse and it compounds. Compacting the medium closes the air spaces that roots need, which means the next watering drains more slowly than this one did, which means more standing water, which is the exact problem you were trying to solve. Every press makes the next drain check read worse.
If drips are still coming after twenty seconds of steady tipping, stop. That water is held in the substrate rather than standing free, and it is supposed to be there.
The drain check when you are away from the app
This is the honest use case for the technique and it is worth saying plainly. Phone dead. Helper covering a weekend. Somebody else's setup. A variety you have not grown before that is not in any library.
In all of those cases the drain check is a complete method on its own. Water a sensible amount, wait 15 to 20 minutes, pour off everything that has not been taken up, and you will not drown the tray. You lose the repeatability and the record, but you keep the tray. That is a fair trade for a weekend.
The two techniques are not competitors. Precision gives you a number you can repeat, compare across trays and hand to somebody else. Feel gives you the check that the number is right for your conditions. Growers who run both stop losing trays to water within about two cycles.
I ran the drain check on eight trays the first evening because I did not believe the amounts. Six poured off almost nothing. Two poured off about four ounces each, and both of those were in the deepest substrate I had mixed that week. I dropped those two by a fifth and they were fine. I had been blaming my humidity for a year.
Grower, Montgomery County PA
What moves the number for your room
The measured amounts are a starting point built from a working farm's records. These are the variables that shift them, in rough order of how much they matter.
- Temperature. A room at 76 degrees F evaporates and transpires far more than one at 62. Warm rooms need more water and are simultaneously more prone to mold, which is why the drain check matters most in a warm room.
- Airflow. A fan across the canopy dries leaves, prevents mold, and increases water demand. Add a fan and expect to add water.
- Substrate depth and type. A deeper fill holds more and drains slower. Coir, compost-based mixes and soilless mats all behave differently at the same depth.
- Humidity. A 30 percent winter grow room and a 70 percent summer basement are different problems with the same trays.
- Seed density. A denser canopy transpires more and shades the substrate more. Both effects are real and they partly cancel.
- Tray drainage. The hole pattern on your mesh trays changes how fast excess leaves. This is the variable growers never think of and it is the one the drain check exposes fastest.
Blackout, light phase, top and bottom
The amounts in this article are light-phase, bottom-water amounts. The other two situations behave differently.
During blackout, there are no leaves to wet and the seed surface must not dry out, so a light top mist has a real place. The tray also has no root mat yet, so bottom watering does little. Keep the volume small and the surface damp, not wet.
Once the tray is out of blackout, switch to the bottom. Water sitting on stems and cotyledons in a warm room is the most reliable way to establish mold there is, and the drier you keep the canopy the fewer problems you have. If you must top water an established tray, do it early in the day so the canopy dries before the room cools overnight.
Reading the tray: over and under
| Sign | Usually means | Do this |
|---|---|---|
| White fuzz at the stem base, spreading | Overwatering plus poor airflow | Run the drain check, reduce 20 percent, add a fan |
| Sour or swampy smell before anything is visible | Standing water in the bottom tray | Pour off after every watering without exception |
| Yellowing lower leaves on a well-lit tray | Roots sitting wet | Reduce and check drainage holes |
| Canopy wilting by late afternoon, recovers overnight | Under-watering at the current growth stage | Increase 10 percent, or split into two waterings |
| Substrate pulling away from the tray edge | Well under-watered | Increase and consider a shorter interval |
| Even canopy, no runoff, no smell | The number is right | Nothing. Log it and repeat it. |
One caution on that first row. White fuzz at the stem base is not always mold. Fine, evenly distributed white fluff on a young sunflower tray is usually root hairs, and it disappears after a watering. Mold is patchy, spreads outward from a point, and does not vanish when wet. If you cannot tell, photograph it and ask Glappy, the in-app assistant, which is available from the Solo tier at 10 messages a day and 30 a day on Grower.
How GLAP handles this daily
The mechanics, briefly, because this is the workflow the article has been describing.
Each variety carries its full water amount in ounces and millilitres, its watering frequency, its blackout days and its light phase length. From a tray's sow date GLAP works out which light day that tray is on today, applies the ramp, and shows you the amount to pour for that specific tray. The watering queue lists every tray due, with its number, so the morning round is a list rather than a calculation. Mark them watered in bulk, or skip one and the schedule adjusts rather than nagging you about a tray you deliberately left.
Watering reminders are on the Free tier, which is $0 with no card and covers 8 active trays with unlimited microgreen varieties and the full cheat sheet. That is enough to run the drain check against the app's numbers on your own rack before you consider paying for anything. Solo at $3.00 a month raises the tray count to 25 and adds basic analytics and CSV export. Grower at $12.99 a month covers 200 trays and adds Bluetooth temperature and humidity sensors, which is what tells you why the number drifted in July.
Get the per-tray amounts free →Frequently asked questions
How much water does a 10 by 20 tray of microgreens need per day?
It depends on the variety and on how far into the light phase the tray is. A full-size 10 by 20 tray at peak demand takes roughly 8 to 16 fluid ounces (about 237 to 473 ml) per day. Sunflower and pea shoots reach 16 oz (473 ml) at peak. Microgreen radish reaches 9 oz (266 ml). Broccoli reaches 8 oz (237 ml). Early in the light phase the tray takes about a quarter of that peak figure, and the amount ramps up as the canopy fills in. Watering the peak amount on day one is the single most common cause of mold.
How do I know if I am overwatering my microgreens?
Run the drain check. Water the tray with your measured amount, wait 15 to 20 minutes, then tip the tray and pour off every drop of standing water. Let it drain naturally. Whatever comes out is water the substrate could not hold, and it is the water that causes mold, root rot and fungus if it stays. A small amount is normal. A steady stream means your amount is too high for your substrate depth and your room, and you should reduce it. No runoff at all across two or three days usually means the amount is too low.
Does the drain check replace measuring the water?
No. The measured amount is the accurate method and the drain check is a confirmation on top of it. Measuring is what makes trays repeatable, comparable and teachable to a helper. The drain check tells you whether the measured number suits your particular trays, substrate depth and grow room, and it is how you work when you are away from the app. Use both. Precision sets the number, the drain check verifies it.
Should I bounce or press the tray to get the water out?
No. Tip the tray and let gravity do it. Bouncing dislodges seedlings and breaks the root mat away from the substrate. Pressing compacts the medium, which reduces the air spaces roots need and makes the next watering drain worse than this one did. If you are still getting drips after 20 seconds of tipping, that water is held by the substrate rather than standing free, and it is supposed to stay there.
Do I water microgreens from the top or the bottom?
Bottom water once the tray is out of blackout and has a root mat. Water sitting on leaves and stems in a warm room is how mold gets established, and the drier the canopy stays the fewer problems you have. Top misting has a place during germination and blackout, when there are no leaves to wet and the seed surface must not dry out. The measured daily amounts in this article are bottom-water amounts for the light phase.
How often should I water microgreens, once or twice a day?
Variety-specific. Sunflower and pea shoots do well on one watering a day because they carry a lot of water in a dense root mat. Microgreen radish and broccoli do better split across two waterings, typically morning and late afternoon, because they take less total volume and a single dose sits longer than they want. Split the daily total, do not double it. A twice-daily variety on 8 oz a day gets 4 oz in the morning and 4 oz in the afternoon.
Why does the water amount increase through the grow?
Because the plant gets bigger. A tray on the first day out of blackout has almost no leaf area and loses very little to transpiration, so most of the water you add just sits. By harvest the same tray is a full canopy moving water constantly. GLAP ramps the daily amount from about 25 percent of the variety's full figure on the first light day up to 100 percent on the last, which matches the demand curve instead of averaging across it.
What changes how much water my trays need?
Room temperature and humidity, airflow across the canopy, substrate type and depth, tray drainage hole pattern, seed density, and whether the tray sits under strong light. A basement at 62 degrees F with no fan needs noticeably less water than a spare room at 76 degrees F with a fan running. That is exactly why the drain check exists: the measured amount is the accurate starting point, and the runoff tells you how your own room shifts it.
How does GLAP tell me how much water to give each tray?
Every variety in the library carries a full water amount in both ounces and millilitres, a watering frequency, and the length of its light phase. From the sow date GLAP works out which light day each tray is on and computes that day's amount, then shows the queue tray by tray with the figure to pour. Watering reminders are included on the Free tier at $0, which covers 8 active trays and unlimited varieties. Mark trays watered in bulk, or skip one, and the schedule adjusts.
The bottom line
Water by number, then confirm with the drain. The number is what makes a tray repeatable and lets you hand the job to someone else without losing a crop. The drain check is what tells you the number fits your trays, your substrate and your room, and it is the method that still works when you are standing in a grow room with no phone.
Pour the measured amount. Wait 15 to 20 minutes. Tip out every drop. No bouncing, no pressing. Whatever comes out of that tray is the mold you did not get.