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Build manual · Kratky still water · Herbs

Kratky Herb Jars: three 1 L windowsill jars that beat a supermarket pot by week six

A supermarket herb pot costs more than the whole setup described here and dies in ten days. Three opaque mason jars, a bag of clay pebbles and one bottle of nutrient will produce cut mint, Thai basil and parsley from a windowsill for two months per charge, with no pump and no noise.

By The Grow DeskPublished 15 Sept 2026Difficulty Beginner
System specification60 day cycle
Reservoir
3.5 Lworking volume
Planting sites
3sites
Pump
nonepassive
Air stones
0not required
EC target
1.1 mS/cmtarget
pH window
5.8 - 6.2pH
Light at canopy
180 PPFDat leaf
Photoperiod
15 hper day
Cycle length
60 daysplant to harvest

Top-up routine — A 3.5 L jar of three herbs needs a 200 mL top-up roughly every 5 days; mark the starting line with tape so you never overfill.

Bill of materials7 line items
Bill of materials
PartSpecificationQtySubstitute
Glass mason jars1 L wide mouth, painted or sleeved to block light3Recycled 750 mL sauce jars, wrapped in two layers of black gaffer tape
Net pots50 mm rim, black3A 60 mm plastic cup with slits cut in the side works if the rim still rests on the jar mouth
LECA4-8 mm fine grade for herbs0.6 LPerlite alone floats; mix it 50/50 with LECA to stop the pot tipping
Herb plugsMint, Thai basil, parsley, 3 weeks old1 eachSupermarket herb pots can be divided and rinsed free of soil, but expect a 10-day stall
Single-part herb nutrientSoft-water formula100 mLAny two-part at half the labelled strength for herbs in a small reservoir
pH test drops6.0-7.6 range indicator1A pen is better, but drops are accurate enough on a 3.5 L volume
Clip-on LED10 W, 4000 K, flexible neck1A south window at 15 cm gives roughly the same 180 PPFD for six hours a day
Assembly, in order6 steps

  1. Make the jar opaque before it holds anything

    Wrap the sides in tape or slip the jar into a black sock, leaving the mouth clear. A 1 L clear jar under 180 PPFD grows a visible green algal ring along the water line within six days, and once that film establishes it strips nitrogen continuously.

  2. Mark the fill line, then never exceed it

    Fill with 700 mL of nutrient solution and wrap a strip of tape at that height. Small reservoirs are where overfilling kills plants fastest: in 3.5 L of total volume, an extra 100 mL raises the water line enough to drown the root crown of a parsley seedling.

  3. Charge at 1.1 mS/cm, weaker than you think

    Soft herbs like mint and parsley are light feeders. At 1.1 mS/cm they grow steadily for two months; at 1.6 the leaf tips burn and mint stops pushing new runners. Mix to half the bottle's recommendation and check with a meter before planting.

  4. Plant only one species per jar

    Mint is aggressive and will out-compete parsley for light and nutrients within three weeks. One jar per species also lets you set different refill intervals, since parsley transpires roughly 40 % slower than mint at the same light level.

  5. Rotate a quarter turn every two days

    A clip-on light from one side produces a plant that leans hard toward it and develops a bare back. A quarter turn every other day keeps the stem upright and roughly doubles the usable leaf area on a windowsill setup.

  6. Replace the whole jar at week eight

    On 700 mL of solution there is not enough buffering capacity to nurse a crop past about eight weeks. Rather than trying to rebalance a depleted jar, start a fresh one and compost the old root ball. The turnover is fast enough that you can run two jars out of phase and never be without mint.

Week-by-week maintenanceWeek 1-2 · Week 3-4 · Week 5-6 · Week 7-8

Week 1-2

EC 1.1 · pH 6.0

  • Check roots touch water by day 5
  • Rotate a quarter turn every 2 days

Week 3-4

EC 1.1 · pH 6.0

  • Top up 200 mL when the line drops 2 cm
  • Pinch mint tips to keep it bushy

Week 5-6

EC 1.0 · pH 6.1

  • First real harvest: outer parsley stems, mint tips
  • Never take more than a third at once

Week 7-8

EC 0.9 · pH 6.1

  • Flavour drops as EC falls; harvest hard now
  • Start replacement jars at week 8 so nothing is idle

Small reservoirs change the rules

On 700 mL of working volume there is very little room for error. A 1.1 mS/cm charge falls to 0.9 within three weeks, pH moves faster than in a large bucket, and a missed top-up of two days can drop the water line below the root tips. The advantage is that everything is visible: you can lift the net pot and inspect the entire root system in a second.

One species per jar is the rule that saves the crop

Mint spreads by runners and will physically overgrow a parsley plant sharing the same jar, shading it out within three weeks. Separation also lets you match refill intervals to the plant: mint in a warm window can pull 60 mL a day, while parsley in the same spot uses closer to 35 mL.

Why supermarket pots always die

A shop-bought basil or mint pot is grown in a peat plug at high density and sold root-bound in a 9 cm pot. Once home it dries out within hours, and watering it heavily causes the peat to slump and the roots to rot. Splitting that pot, rinsing the roots free of peat and moving each division into LECA under a Kratky regime resets the plant onto a system that does not depend on you guessing moisture.

Light is the limiting factor on a windowsill

A north-facing windowsill in winter delivers under 30 PPFD, which is not enough for basil to do more than survive. A 10 W clip-on LED at 15 cm brings the leaf surface to roughly 180 PPFD for the 15 hours a day the plant needs, and that is the difference between a plant that stalls at 12 cm and one that reaches harvestable size in five weeks.

Tubing diagram: Kratky Herb Jars: three 1 L windowsill jars that beat a supermarket pot by week six
Tubing diagram: Kratky Herb Jars: three 1 L windowsill jars that beat a supermarket pot by week six

Key points

  • Opaque jars only; a clear jar grows algae in under a week
  • 700 mL working volume, marked with tape, is the refill ceiling
  • Plant one species per jar so refill intervals can differ
  • Half-strength nutrient at 1.1 mS/cm suits soft herbs best
  • Retire the jar at eight weeks and start a fresh one instead of rebalancing

Sources

Please noteThe EC, pH and light values given are general references for home setups and will differ with your water, nutrient brand and cultivar. Always follow the product label. This is informational content, not professional horticultural advice.

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