How to prevent mold on microgreens. The daily walk, then the between-cycle clean. Fix it from only $3 a month.

Step 1. Run the mist test before you compost anything

White fuzz on day 3 is usually root hairs, not a colony. Do not decide with your eyes. Do this instead.

1. Set the nozzle to its finest pattern.
2. Stand about 12 inches back and give the suspect patch one light pass. You want a fog sitting on it, not a soaking.
3. Watch the water for five seconds. Root hairs are hydrophilic, so the droplets soak straight in, the fibers pack together and the fuzz lies flat and seems to vanish. A colony repels water, so the droplets bead and sit on top and the fuzz underneath is unchanged.
4. Put airflow on the tray and walk away for 30 to 60 minutes.
5. Come back and look once more. Gone, or reduced to a soft halo at the stem line: root hairs, carry on growing. Still standing, still bright white, maybe a little wider: go to step 8.

Do it before coffee. Thirty seconds, one squeeze of the trigger, and it is the most useful half minute in the room.

Step 2. Read the humidity at leaf-tip height, not on the wall

A meter screwed to the wall reads the room. The seedlings live in a wetter pocket a few inches above the medium, and that pocket is the one that decides. Lay the hygrometer probe on the shelf beside the tray, level with the tops of the greens, and give it ten minutes to settle before you trust the number.

• 40 to 60 percent through the lit hours.
• 50 to 60 percent once the cover has come off.
• Over 70 percent held for a day or more: act today, not at the weekend.
• Under 30 percent and the medium dries too fast, which gives you patchy uneven germination instead.
• 65 to 72 F is the temperature to hold alongside it.

Read it twice, once in the morning and once mid-afternoon, and write both down. One reading tells you nothing about a room that swings. If you sit over 70 persistently, open a door, move the rack off an exterior wall, or run a small dehumidifier beside the single rack that is causing it.

Note down: morning reading, afternoon reading, shelf, temperature, date.

Step 3. Position the fan, then judge it by the leaf edges

Stand the fan 4 to 6 feet back from the rack, on the lowest speed that produces visible leaf movement. Aim it across the shelves, parallel to the fronts of the trays, so the air sweeps along the gap between the shelf above and the greens below. Do not point it into the trays. Switch oscillation on if the fan has it.

Judge the setting from the far end of the rack, not from the fan. Leaf edges should tremble. The greens should stay standing. If anything is leaning away from the fan, move it a foot further back or drop it a speed. One small fan per rack of eight trays is enough.

Leave it running continuously once the cover is off. A fan on a timer gives you eight quiet hours a day for a colony to establish in, and the whole point of moving air is that it never stops moving.

Check the dead end: the shelf furthest from the fan, at the back, is where stagnant air collects. If the leaves there are still, the fan is in the wrong place.

Step 4. Weigh the seed so the mat can dry

Sow too thickly and you build a wet mat that cannot dry between waterings, and no amount of cleaning will rescue it. Tare the cup on the scale, then pour. Do not scoop by feel.

• Broccoli, kale, radish, cabbage, mustard, arugula: 20 to 25 grams dry per 10 by 20 tray.
• Sunflower: 80 to 120 grams, soaked first. Big seed, low count.
• Pea shoots: 200 to 250 grams, soaked first.
• Amaranth and other very fine seed: start 25 percent under whatever the packet suggests, because it closes in tighter than anything else.
• Cilantro: sow it thin. It germinates over 8 to 12 days, so the medium sits damp far longer than for any other crop on the rack.

Scatter in two passes, north to south then east to west, then rock the tray gently so the seed settles into contact. Going 25 percent over the weight above raises the pressure on every other step in this list.

Step 5. Bottom water, and measure what you pour

Watering over the top after the seed has rooted wets the exact surface you are working to keep dry. Once roots are down, everything goes in underneath.

1. Lift the tray by one corner first. Light means it is ready for water. Heavy means leave it another day.
2. Pour 1 to 2 cups into the solid bottom tray, reading the mark on the jug, never over the greens.
3. Wait 15 minutes while the medium wicks it up through the drain holes.
4. Decant whatever has not been taken up. Standing water past half an hour keeps the root mat saturated and the surface damp all day.
5. Water in the morning, so the surface has the whole lit period to dry down before the dark hours.

The dry-down test: the surface should feel cool and barely damp to the back of a finger, never slick. Slick at midday means you poured too much, or you poured it too late in the day.

Step 6. Stack tray, not a sealed dome, and lift it on the right day

Invert an empty 10 by 20 on top and weight it with a brick or a second tray. Leave the corners proud so air still travels around the edge. A sealed dome or a snap-on lid holds that space near 95 to 100 percent, which is the one microclimate that produces a colony reliably.

Then lift it on schedule rather than by eye:

• Broccoli, kale, radish, cabbage, mustard, arugula and salad mixes: day 3.
• Sunflower and pea shoots: day 4 to 5, because the seed is large and comes up unevenly.
• Basil: day 2. Tiny seed, slow start, the first crop to damp off if you leave it under.

The seed leaves will look pale yellow when the cover comes off. They green up within about six hours under light. Leaving a brassica covered until day 5 buys you stem you did not want and a damp, still pocket for spores to land in.

Do it now: write the lift date on masking tape, stick it on the end of the tray where you will see it, and set the same date as an alarm.

Step 7. The daily walk, thirty seconds a tray

Same route, same order, same time. You are looking for the start of something, not the middle of it.

• Corners and edges first. That is where the air stalls and where a colony almost always begins.
• Back of a finger on the surface. Cool and barely damp is right. Slick is not.
• Nose over the tray. Fresh and green is right. Musty, earthy, or like a wet basement means part the canopy and look at the stem line.
• Sunflower, day 4: rub the stuck hulls off with a gloved hand. A hull traps a bead of water against the tops for days.
• Anything dense: part it with a finger and check the medium underneath, because the surface can look perfect while the floor of the tray is soaked.
• Under the shelf: glance at the bottom tray for water you forgot to decant.

Step 8. The hour you find a spot

Speed decides whether the rest of the tray is worth keeping, because a colony roughly doubles every 12 to 24 hours in conditions it likes. Confirm it with the mist test in step 1 first, then do this within the hour.

1. Slide the tray to the end of the shelf nearest the fan, where the air is strongest.
2. Cut a 3-inch margin all the way around the visible spot with sanitized scissors. If the spot is bigger across than a quarter, stop here and bag the tray instead.
3. Lift out the cut greens and the medium underneath them, straight into a bag you can close. Not into an open bin standing in the grow room.
4. Swab the scissor blades in the dilution tub before they touch another tray.
5. Mist the surrounding area once with the 1 part peroxide to 10 parts water mix. One pass. Heavier than that stresses the seedlings.
6. Wipe the shelf rail under the tray with the microfiber cloth damped in the same dilution.
7. Look again in 12 hours. Clean at 12 and 24 hours and the tray usually finishes fine.

Two or more spots, visible fuzz lifting off in the air, or a musty smell: bag the whole tray without shaking it and carry it out of the room. If several trays go at once it is not the trays, it is the room. Stop sowing for 24 hours, fix the reading in step 2 and the fan in step 3 first, and hunt for a new water source such as a leak, a dehumidifier switched off, or air conditioning that quit.

Step 9. The between-cycle clean, in this order

Most pressure carries over from the last tray. About 3 minutes per tray, done the same way every time.

1. Pop the root mat out over a bin and take it out of the grow room straight away, not at the end of the session.
2. Rinse the tray with plain water first. Peroxide spent on loose medium does nothing. Get the debris off before the chemistry goes on.
3. Mix the dilution fresh: 1 part food-grade 3 percent hydrogen peroxide to 10 parts water in the labeled bottle. Make it up weekly, not monthly.
4. Spray the whole inside surface plus the underside rim and the stacking lip. Wait 60 seconds by the clock.
5. Scrub with the stiff brush. Corners, the ribs in the base, and every drain hole. That is where residue hides.
6. Rinse with clean water. Tap water is fine. Leave nothing behind.
7. Air dry standing on edge in the wire rack. Never stack a wet tray on a wet tray.

If you use bleach instead, dilute 1 part plain unscented bleach to 30 parts water and rinse harder, because bleach residue depresses germination. Peroxide rinses cleaner and is the one to use in a certified operation.