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Build manual · Kratky still water · Microgreens

Kratky Microgreen Tray: sunflower and radish in 12 days on 8 mm of solution

Microgreens are the fastest thing you can grow hydroponically and the easiest to ruin with kindness. The whole cycle is twelve days, the reservoir is 8 mm deep, and almost every failure comes from too much water, too much heat during blackout, or too much light at the end.

By The Grow DeskPublished 28 Sept 2026Difficulty Beginner
System specification12 day cycle
Reservoir
2 Lworking volume
Planting sites
0sites
Pump
nonepassive
Air stones
0not required
EC target
0.6 mS/cmtarget
pH window
6.0 - 6.5pH
Light at canopy
200 PPFDat leaf
Photoperiod
12 hper day
Cycle length
12 daysplant to harvest

Top-up routine — Keep 8 mm of solution in the lower tray; check every morning and add 100-150 mL of 0.6 mS/cm solution when the mat is no longer touching liquid.

Bill of materials7 line items
Bill of materials
PartSpecificationQtySubstitute
Two trays25 x 40 cm, one with drainage slots nested inside one solid2A standard 10 x 20 nursery flat inside a solid bakery tray
Growing matHemp or jute fibre, 8 mm thick, cut to tray size1Unbleached paper towel at 4 layers works once but tears when harvested
Microgreen seedSunflower, radish or pea, untreated35 gBroccoli and kale seed at 12 g per tray gives a slower but denser canopy
Weight for blackoutSecond tray filled with 2 kg of dry sand1Any flat board with a couple of books; the goal is firm seed-to-mat contact
Single-part nutrientGeneral purpose, diluted to 0.6 mS/cm100 mLMicrogreens live on seed reserves; plain tap water works for a 12-day cycle if your water is under 0.3 mS/cm
Spray bottle1 L, fine mist1A watering can with a rose head is too coarse and dislodges the seed
LED bar20 W, 4000 K, 60 cm1A bright east window for the last four days is often enough in summer
Assembly, in order6 steps

  1. Soak the mat, then drain it flat

    Submerge the fibre mat for two minutes and let it drip until it stops. The mat should be damp, not dripping into the lower tray. A mat that is still releasing water will keep the seed wet for 48 hours and that is where damping-off starts.

  2. Sow dense, then press

    Spread 35 g of sunflower seed so the mat is covered with almost no gaps, then press with the weighted tray for 12 hours. Seed-to-mat contact is the single factor that decides whether the roots anchor; seed that stays loose washes into a corner in the first watering and never recovers.

  3. Blackout for four days, at 20-22 C

    Keep the weighted tray on and put the whole stack somewhere dark at 20-22 C. In four days you should see 4-5 cm of pale stem. Warmer than 24 C and the stems stretch thin and fall over at harvest; cooler than 18 C and germination takes seven days and the mat may mould.

  4. Reveal the tray and drop the water to 8 mm

    Uncover on day 4 and pour 8 mm of 0.6 mS/cm solution into the lower tray, letting the mat wick it up. From here the requirement is simple: the mat never dries and the seed heads never sit in liquid. Anything more than 8 mm and the stems rot at the base within a day.

  5. Run 12 hours of light for six days

    Twelve hours on, twelve off for the final six days is enough for full colour and flavour development on radish and broccoli. More light does not make them bigger; it makes them tougher and pushes flowering. Sunflower at 200 PPFD finishes at 10-12 cm in this window.

  6. Harvest with scissors, above the mat

    Cut 5-8 mm above the fibre mat in one pass, holding the canopy with your other hand. Do not wash the cut greens; water drives the shelf life from eight days down to three. Refrigerate in a paper-lined box and eat within a week.

Week-by-week maintenanceDay 0 · Day 1-4 · Day 4 · Day 5-9 · Day 10-12

Day 0

EC - · pH -

  • Soak mat, sow 35 g, press 12 h

Day 1-4

EC - · pH -

  • Blackout under the weighted tray at 20-22 C
  • Do not water; the mat holds enough

Day 4

EC 0.6 · pH 6.2

  • Uncover, add 8 mm of solution to the lower tray
  • Start 12 h light cycle

Day 5-9

EC 0.6 · pH 6.2

  • Top up 100-150 mL when the mat stops touching liquid
  • Turn the tray a quarter turn daily

Day 10-12

EC 0.6 · pH 6.3

  • Harvest in one pass, 5-8 mm above the mat
  • Do not rinse; refrigerate dry

The seed does the work, not the nutrient

A sunflower seed carries enough stored energy to produce a 10 cm stem with no external nutrition at all. That is why a 0.6 mS/cm solution — barely above most tap water — is enough for a twelve-day cycle. Pushing to 1.5 in the belief that more food means faster growth produces tip burn on the cotyledons and a noticeable bitterness.

Eight millimetres is the whole water budget

Wicking mats move water by capillary action, so the moisture front rises from the lower tray into the mat. Eight millimetres keeps the mat damp to the surface without the standing layer reaching the seed. At 15 mm the stems sit in liquid, oxygen disappears from the base and damping-off takes the tray within 24 hours. At 3 mm the mat dries at the edges and those seedlings stop growing.

Blackout temperature decides stem quality

The four days under the weighted tray are etiolation, not growth: the seedling stretches to reach light it cannot find. At 20-22 C it produces a sturdy 4-5 cm stem. At 26 C the same seed produces 8 cm of thin, pale tissue that collapses the moment it is uncovered. This is the single most common reason home trays come out floppy.

Twelve days is a hard deadline

After about day 12 the cotyledons have exhausted the seed reserve and the plant either needs real nutrition or starts to decline. Leaving a tray to day 16 in the hope of a bigger harvest gives yellowing leaves, a tougher stem and the first true leaves pushing through, which changes both flavour and texture. Sow a new tray every four days instead of stretching an old one.

Tubing diagram: Kratky Microgreen Tray: sunflower and radish in 12 days on 8 mm of solution
Tubing diagram: Kratky Microgreen Tray: sunflower and radish in 12 days on 8 mm of solution

Key points

  • 0.6 mS/cm is enough; the seed supplies the rest
  • 8 mm in the lower tray, no more, or the stems rot
  • Blackout at 20-22 C for 4 days for sturdy stems
  • Press the seed for 12 hours so the roots anchor to the mat
  • Never rinse harvested greens; it costs five days of shelf life

Sources

Please noteNutrient strengths and temperature windows are general references for home microgreen production. Follow any seed supplier's food-safety guidance. This article is informational and is not professional food-safety or horticultural advice.

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