BENCH PROCEDURE · 20 MINUTES FOR A RACK OF TRAYS
How to fill and pre-moisten a microgreen tray. Ten steps at the bench. Fix it from only $3 a month.
Screen, hydrate, squeeze, fill, level, firm, nest, mist, sow, seat. Work a whole batch of trays in one go rather than one at a time, because the medium wants to sit at a single moisture across the lot and because your arms will thank you. Every number below is for a standard 10 by 20.
Want this procedure as a page-turning ebook you can read cover to cover? We made one.
Read the free Best Soil for Microgreens flipbook ebook →- Perforated 10 by 20 trays, and one solid tray with no holes for every tray you intend to fill.
- A screen. Quarter inch hardware cloth stapled to a timber frame is ideal. A stiff kitchen colander is fine at four trays a week.
- Two buckets. One for the medium, one for water.
- A one quart scoop or a marked jug, so the fill is a measurement rather than a guess.
- A straight batten longer than the tray is wide. A paint stirrer, a timber offcut or a rigid plastic ruler all work.
- One spare empty tray kept clean and used only for pressing.
- A hand fork, for fluffing a hydrated block.
- A spray bottle with the nozzle set to fine mist, not jet.
- Scissors, if you are working with fiber mats.
- Nitrile gloves, unless you are relaxed about coir under your fingernails for two days.
Step 1. Screen the medium before it goes near a tray
Bagged medium settles hard in the sack. What pours out is fine crumb mixed with knuckle-sized clods, bark chips and the occasional twig. Tip a bucketful onto the bench, break the clods between your palms, and pass the lot through the screen. Whatever refuses to go through goes back on the heap, not into a tray.
What you want on the far side of the screen is an even crumb roughly the size of a peppercorn, loose enough that it pours instead of tumbling out in lumps.
Why it matters: a clod sitting under the surface is a void. Seed that lands over a void germinates late and stays short, and it shows up in the canopy four days later as a dip you can no longer do anything about.
Step 2. Hydrate the block, if you bought compressed coir
Coir usually arrives as a compressed brick or a bale rather than as anything you can pour. Stand the brick in a bucket, pour warm water over it and walk away. Warm water breaks the compression far quicker than cold does. The wrapper prints the water volume that particular block wants, and the wrapper is right more often than the internet is, so go by the wrapper.
Give it twenty to thirty minutes, then break the remaining slab open in the middle with your hands. The core stays dry long after the outside has swelled, and a block fluffed only on top gives you a tray with a dry brick buried in the middle of it. Run the hand fork through the whole bucket until the texture is loose and uniform from top to bottom.
Buy buffered: unbuffered coir carries salts that hold germination back, and there is no step at this bench that repairs it afterwards.
Step 3. Run the squeeze test on the whole batch
Close a fistful of medium in one hand, squeeze once and firmly, then open your hand and read what you are holding.
- Water runs between your fingers as you squeeze: too wet. Add dry medium a scoop at a time and turn the bucket over.
- The ball collapses the instant you open your hand: too dry. Add water one cup at a time, forking between cups so it does not sit in a puddle at the bottom.
- The ball holds its shape and one or two drops appear at the base of your fist: correct. Stop there.
Do this in the bucket, on the whole batch, before a single tray is filled. Correcting moisture inside a filled tray means digging it back out again, and it never comes back level.
Step 4. Fill to depth and leave yourself a rim
Scoop into the tray until you have used one and a half to two quarts of pre-moistened medium. That puts the surface at roughly one inch deep in a standard 10 by 20.
Leave about half an inch of clear rim standing above the surface. That gap is freeboard, and it is what lets water pool on the top for a second and soak in, instead of running straight over the lip and carrying seed with it.
Do not fill to the brim: an overfilled tray sheds medium over the edge every time you water it, and the blackout tray on top then sits directly on the seed instead of clearing it by half an inch.
Step 5. Level with a straight edge, in one pass
Lay the batten across the rim at one end of the tray and drag it to the other end in a single steady pass, letting the surplus fall off the far edge into the bucket. Drop a scoop into any dip the batten reveals and drag again.
Then push your fingertips into all four corners. Corners are where the fill comes up short, because a scoop poured into the middle spreads outward and stops. A corner that sits a quarter inch low carries a canopy a quarter inch short on the day you cut.
Level to the rim, not to your eye: sight along the tray from the end, at bench height, and you will find the slope you could not see from above.
Step 6. Firm the surface. Do not compact it.
Set your clean spare tray on top and press once with the flat of both palms. Keep your body weight off it. What you want is a surface that holds a seed still, not a slab.
Compaction is the classic error here and it is easy to commit with a brick or a paving slab as a press. Medium squeezed tight sheds water sideways rather than taking it downward, so the surface dries by lunchtime while the base stays sodden, and the roots stall somewhere in between.
Fiber mats never get firmed at all. Skip this step entirely for them.
Step 7. Nest into a solid bottom tray and check the holes
Every filled tray sits inside a second tray that has no drainage holes in it. Water goes into the bottom one, the medium draws it upward through the perforations, and the surface stays drier than the base. That is the entire reason for the pair.
Check the perforations in the top tray are actually open before you nest it. New trays occasionally ship with the punched tabs still hanging on, and a blocked tray waterlogs from underneath while the surface looks perfectly healthy.
Keep one spare solid tray clean: it is what you invert over the sown tray as the blackout cover, and a gritty one drags across the surface every time you lift it.
Step 8. Wet the surface before the seed goes on, never after
Mist the levelled surface until it darkens evenly across the whole tray, then sow onto damp medium.
Sowing onto a dry surface and watering afterwards is the most common way to end up with seed piled in drifts along one edge and bare ground along the other. Even a fine spray shifts dry seed sitting on dry crumb. Wet first and the seed stays where you put it.
The nozzle matters: set it to a fine mist. Anything approaching a jet excavates a channel across the tray, and you will be levelling it again.
Step 9. Seat the seed into the surface
Once the seed is spread, mist it again lightly, then set the clean spare tray on top, press once evenly, and lift it straight up rather than sliding it sideways.
Seed wants contact with the surface, not burial in it. A seed perched on the crumb with air underneath dries out and comes up late. A seed seated into the crumb has moisture on every side of it from the first hour.
Rock the tray gently side to side afterwards to settle anything still sitting proud, then nest it, cover it and put it on the rack.
Step 10. Do it differently for a hemp or jute mat
A fiber mat replaces steps 1, 2, 5 and 6 outright. There is nothing to screen, nothing to level and nothing to firm. What is left is four things done in a different order.
- Trim to the inside of the tray, not the outside. A mat cut oversize wedges against the walls, buckles in the middle and lifts the canopy off centre. Cut it a hair small and let it sit flat.
- Press it into the corners by hand before any water touches it. A dry mat still moves. A wet one has already decided where it is going.
- Soak it from the bottom tray until the color is uniformly dark, then pour off whatever the mat did not take. A mat holds a small fraction of what an inch of loose medium holds, so standing water does nothing except sit there breeding trouble.
- Sow straight onto the wet mat. There is no separate pre-wetting step, because the mat is already saturated. Seat the seed with the spare tray exactly as in step 9.
- Check it twice a day for the first three days. Mats dry from the edges inward, and an edge that dries out is an edge that never germinates.
Fill depth and moisture, by medium
Every row is for one standard 10 by 20 tray. The volumes are of medium already pre-moistened, not of dry medium out of the sack.
| Medium | Depth | Volume per tray | Squeeze test target | Wetting before sowing |
|---|---|---|---|---|
| Compost based peat-free mix | 1 inch | 1.5 to 2 quarts | Holds shape, 1 to 2 drops | Mist until the surface darkens |
| Loose coconut coir | 1 inch | 1.5 to 2 quarts | Holds shape, springs back when poked | Light mist, coir already carries water |
| Hydrated coir block | 1 inch | 1.5 to 2 quarts once fluffed | Fork the core loose first, then squeeze | Light mist |
| Hemp mat | Mat thickness | One mat, trimmed inside the rim | Not applicable, soak and drain | None, sow onto the wet mat |
| Jute mat | Mat thickness | One mat, trimmed inside the rim | Not applicable, soak and drain | None, sow onto the wet mat |
What went wrong at fill time
Almost everything on this list is invisible on the day you fill and obvious four days later. Read the symptom, then look back at which step produced it.
| What you see later | What happened at the bench | What to do |
|---|---|---|
| Canopy noticeably taller at one end of the tray | The fill sloped and the batten was never dragged, or it was dragged one way only. | Nothing saves this tray. On the next fill, drag from both ends and sight along the rim at bench height. |
| Round bare patches with nothing growing in them | Unscreened clods sat under the surface and left voids beneath the seed. | Screen every bucketful. If you are mid-run, mist those patches twice daily and expect them to finish short. |
| Seed drifted into a ridge along one edge | The surface was dry when the seed went on, and the first watering moved it. | Redistribute now with a dry fingertip if the seed has not rooted. Next time, mist before sowing. |
| Water sheets over the rim every time you pour | Overfilled, so there is no freeboard left to hold a pour. | Scrape a quarter inch off the top with the batten and re-level, then water from the bottom tray only. |
| Surface bone dry by midday, base still sodden | Compacted. Something heavy went on top in step 6. | Bottom water only and stop misting. Next fill, press once with an empty tray and no body weight. |
| Medium shrunk away from the tray walls, a gap all round | Filled too dry, then wetted in the tray, so it drew inward as it took up water. | Push it back out with your fingers and top up the corners. Next time, hit the squeeze test in the bucket. |
| Mat curled up at two edges and lifted the seed | Trimmed to the outside dimensions, so it wedged and buckled. | Lift, trim a few millimeters off each edge, lay it flat and re-seat the seed with the spare tray. |
| Roots crawling across the surface instead of down into it | Seed sat on top of the crumb and was never seated. | Press once with the clean spare tray now, even mid-germination. It is late but it is better than nothing. |
Write down four numbers per fill
It takes fifteen seconds and it is the difference between a repeatable fill and a lucky one.
- Which medium, and which bag or bale it came out of.
- Volume actually scooped in, taken from the jug rather than from the recipe.
- Squeeze test result: correct, or corrected wet, or corrected dry.
- Whether you firmed with the spare tray, and whether you pre-wet before sowing.
Two trays filled a two weeks apart will differ on at least one of those four, and that is usually the whole explanation for why one came up even and the other did not. Logging a sowing in GLAP keeps the four beside the harvest weight they eventually produced, and the $0 tier carries 8 active trays at once with the whole variety library open, which is more than enough to run a fill comparison.
Trying to decide which medium to buy rather than how to fill it? See what each medium costs per tray once yield, harvest labor and disposal are counted → That page works through input cost, freight and which sales channel pays for which option. This page is the bench work once the bag is in front of you.
Questions growers ask at the bench
How deep should I fill a 10 by 20 tray?
Roughly one inch of pre-moistened medium, levelled to the rim, with about half an inch of clear rim left standing above the surface. That half inch is freeboard. It holds a pour of water on the top for a second so it soaks in rather than sheeting over the lip and carrying seed away with it, and it keeps the blackout tray clear of the seed instead of resting on it.
How much growing medium does one tray take?
One and a half to two quarts of pre-moistened medium per standard 10 by 20 tray. Measure it with a marked jug or a one quart scoop rather than judging it by eye, because eyeballing drifts by a quarter of a tray over a batch and every tray then finishes at a slightly different height. Fiber mats are one mat per tray, trimmed to the inside dimensions.
How do I know the medium is at the right moisture?
Close a fistful in one hand, squeeze once firmly, then open your hand. If water runs between your fingers as you squeeze it is too wet, so add dry medium a scoop at a time. If the ball collapses the moment you open your hand it is too dry, so add water a cup at a time and fork it through between cups. If the ball holds its shape and one or two drops appear at the base of your fist, stop. Do this in the bucket on the whole batch, before anything is filled.
Do I have to sift the medium before filling?
Yes, and it is the step most people skip. Bagged medium compacts in the sack and comes out as fine crumb mixed with clods, bark chips and twigs. Pass it through a quarter inch screen and aim for an even peppercorn-sized crumb. A clod under the surface is a void, and seed sown over a void germinates late and finishes short. It surfaces four days later as a dip in the canopy that cannot be corrected.
Should I tamp the medium down before sowing?
Firm it, do not compact it. Set a clean empty tray on top and press once with the flat of both palms, keeping your body weight off it. You want a surface that holds a seed still. A brick, a paving slab or a knee on the tray compacts the medium, and compacted medium sheds water sideways instead of downward, so the surface is dry by lunchtime while the base stays sodden. Fiber mats are never firmed at all.
Do I wet the surface before I sow or after?
Before, always. Mist the levelled surface until it darkens evenly, then sow onto damp medium, then mist once more lightly and seat the seed with a spare tray. Sowing onto a dry surface and watering afterwards is how seed ends up ridged along one edge with bare ground opposite, because even a fine spray shifts dry seed on dry crumb. Keep the nozzle on fine mist. A jet will cut a channel straight across the tray.
Do I prepare a hemp or jute mat the same way?
No. There is nothing to screen, level or firm. Cut the mat to the inside dimensions of the tray rather than the outside, because an oversize mat wedges against the walls and buckles in the middle. Press it into the corners while it is still dry, soak it from the bottom tray until the color is uniformly dark, pour off what it did not take, then sow straight onto it with no separate pre-wetting. Check it twice a day for the first three days, since mats dry from the edges inward.
Why does the tray need drainage holes and a solid bottom tray?
The perforated tray holds the medium and the solid tray underneath holds the water. You pour into the bottom one and the medium draws it upward, which keeps the surface drier than the base and puts the moisture where the roots are rather than where mold starts. Check the perforations are genuinely open before you nest a new tray, because punched tabs sometimes ship still attached and a blocked tray waterlogs from underneath while the surface looks fine.
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