Kratky jar instructions

Materials needed

  • Jar: one of the following
    • Wide-mouth mason jar (included or not depending on kit)
    • Peanut butter jar (included in peanut butter jar kits)
    • Other food-safe jar with an opening between 3″ and 3.25″
  • Jar cover: one of the following
    • Sleeve (included depending on kit)
    • Paper bag large enough to reach the top of the jar
    • Aluminum foil
    • Thick, opaque paper large enough to fully wrap around the jar
    • Paint (spray paint or brush-on)
  • Net pot: included in your kit
  • Adapter ring: only needed for peanut butter jar kits, included with those kits
  • Clay pebbles: included in your kit
  • Nutrients: included if you chose that add-on
  • Gallon jug: for mixing nutrient solution
  • Spray bottle: for misting the pebbles until the seedlings develop roots
  • Rock wool cube: optional, included if you chose that add-on
  • Seeds

Quick notes

  • For a gallon jug for nutrient solution, I recommend either buying a jug of distilled or spring water from the store, or repurposing a milk jug rather than recycling it.  To fully sterilize the milk jug, thoroughly wash with soap and water, then sterilize with bleach water.  Let it sit open on the counter for several days to fully dry and let all the bleach evaporate off.
  • While any food-safe jar of the right size will work, the larger the jar’s capacity, the less you’ll have to fill it.  And glass is preferable over plastic.

Painting jars (if not using a removable cover)

  1. Put the lid on for an easier way to handle it without smudging or messing up the paint.
  2. If you want an easy way to check the water level, use some masking tape to make a vertical stripe down the whole jar.
  3. For spray paint, place the jar lid-down on some newspaper or other painting surface and use several thin, even coats to make the jar fully opaque.
  4. For brush-on paint, keep applying coats of paint until the jar is fully opaque.
  5. If you used masking tape, carefully peel away the tape to create your viewing window.

Setting up your Kratky jar

  1. Carefully remove the lid from your kit so that the clay pebbles don’t spill everywhere.  Remove the net pot, put any stray clay pebbles back in the net pot, and rinse thoroughly under running tap water.
  2. If you are using a rock wool cube, soak it for 30 minutes in distilled water (boiled tap water works in a pinch but distilled is best).  Pull it out, shake out some of the water (don’t squeeze!), and soak for another 45 minutes.  This helps to even out the pH of the rock wool to limit variances that might affect your seeds.  Remove enough of the pebbles so that when you put the rock wool cube into the net pot, the top of the cube is level with the top of the pot.  Put some of the remaining clay pellets in around the rock wool cube to hold it in place.
  3. For peanut butter jar kits, put the plastic ring included with your kit around the net pot like a collar.  This helps the net pot fit more securely in the slightly-larger top of the peanut butter jar.  Place the net pot back into the top of your jar.
  4. Mix up your nutrient solution.  If you have your own nutrients, mix according to the packaging directions.  If you chose the add-on to have nutrients included in your kit, mix the included nutrient powder with 1 gallon of water.
  5. Place your jar in a sunny window or under a grow light, and put on the sleeve, bag, paper, or any other jar cover to block sunlight from getting into the jar.
  6. Slowly pour nutrient water through the pebbles.  Don’t pour too fast since the pebbles float and this can make a mess.  Fill until the water level comes up to about one finger’s width below the top of the jar.
  7. Add your seeds.
  8. Once or twice a day depending on humidity and sunlight levels, use the spray bottle to lightly mist the top of the pebbles.  Do this until the seedlings start taking up water from the jar and the water level begins to drop.

Maintaining your Kratky jar

  1. Don’t refill your jar until the water level drops to half of the jar’s total capacity, then keep it topped up to that level.  Don’t fill past halfway since it will drown the roots.
  2. Once a month, fully empty the jar, rinse it out, then refill to the halfway point.  As plants take up water, they leave behind salts and other minerals that build up over time, and this removes them just like changing the water in a fish tank.
  3. If algae develops, remove the net cup and plant, rinse the roots gently and carefully, and discard the water in the jar.  Refill, and make sure the jar cover is firmly in place and blocking out as much sunlight as possible.  Change the water fully every 2 weeks until the algae goes away.

Reusing your Kratky jar

If you plan to reuse your jar for something other than countertop hydroponics, thoroughly wash with hot soap and water, then sterilize by fully submerging in boiling water for ten minutes.

To reuse your jar for more countertop hydroponics:

  1. Remove the plant(s) currently in the net pot and the clay pebbles over a large bowl, and carefully tease all the clay pebbles out of the roots.
  2. Thoroughly rinse the clay pebbles; sterilize by boiling in water for ten minutes if desired.
  3. Wash the jar with hot soap and water, and thoroughly rinse to remove any soap residue.  Sterilize by fully submerging in boiling water for ten minutes if you suspect any sort of infestation or infection.
  4. Go through the steps to set up your Kratky jar again.