The Kratky Method

Doctor Kratky realized you could grow plants in a non-circulating hydroponic system by simply allowing the plant to consume water and the roots to become exposed to air. Rather than oxygenating the water or spraying the roots, just let them stand in the humid air left behind as the water level drops. Start with enough nutrient-water for the plant to grow to maturity, or transplant/re-water before it runs out.


The Process in Short

  1. Pick pretty much any container you wish to hack. I prefer jars with 2 inch openings (standard wide-mouth jars).
  2. Suspend a growing medium in the top of the containers. If you're using standard wide-mouth jars, 2 inch net cups are ideal. Otherwise, hack something together that fits your containers of choice.
  3. Choose an appropriate growing medium. Something sterile and inert which won't rot. I prefer rock wool.
  4. pre-soak rock wool or moisten growing medium.
  5. Fill containers with plain, clean water so that the bottom of the growing medium is sitting in water.
  6. Plant some seeds in the rock wool or other growing medium.
  7. Once the seeds sprout, replace the plain water with nutrient water.
  8. Allow the water level to drop as the roots grow.
  9. Care for your plants, just a little. Don't let them dry out, give them LOTS of light, and don't sweat it.

The process broken down

We could go into endless detail, but most crops will thrive without much scrutiny. Read below for more details on each step above...

1. Workable containers

I like to use jars...

This can be done in anything from a Slurpee cup to a decorative vase. From a tin can to a 53 gallon whiskey barrel. Pick a container that your plant can grow to maturity in, or anticipate transplanting it.

2. Net Cups or hacked suspension

Net cups fit nicely in them.

Put net cups in the jars or hack another method to suspend your growing medium securely. The roots should be able to easily grow down into the water, while the stem should be able to hold itself and the rest of your growing plant up.

3. Growing Medium

Rock wool is simple.

Rock Wool is amazing stuff. You could use peat moss, clay pellets, course sand in cheese cloth, or just about anything else that will allow your seeds to sprout and roots to grow. Avoid woody materials, mulch, compost, or anything that rots. The medium should be fairly inert and sterile.

4. Pre-soaking

Rock wool gets wetter than most things.

While this isn't strictly necessary, it's helpful to promote healthy germination, especially for something fairly synthetic like rock wool, or anything that might have a strong PH or other funky attribute. Soak your rock wool or other growing medium in clean water, ideally without a wonky PH, for 5-20 minutes.

5. Just Add Water

No, really. While the sprouts are young they get all the nutrients they need from the seeds. Plus, the water should be touching the medium, which will lead to algae or moss growth if nutrients are available.

About 1/4 to 1/2 inch of the net cup should be submerged.

I like to start with the water a little less than 1/2 inch above the bottom edge of the rock wool. Sometimes I put clay pellets or gravel under the rock wool, so this might mean a couple inches above the net cup bottom. The water you use should be clean (bottled or filtered if your tap water is hard) and the PH should be fairly neutral, potentially optimized for your particular crop.

Of course, put the growing medium in there too...

6. Sow the Seeds

Garden variety seeds are fine... Got these from Fred Meyers in Oregon:

Plant the seeds directly in the rock wool. Very small seeds may be difficult to handle. I learned from Jeb Gardner's YouTube channel that you can dab them with a moist chop stick, dowel, pencil, or similar poking device to lift just 1 or 2 at a time then poke them into the rock wool.

Start each jar with several seeds. The rule is 2, but I usually plant more because seeds are cheap and I like to plant them.

Chances are they will all germinate, but you'll thin them out later. While they germinate keep the water level just touching the growing medium. Avoid letting the seeds or sprouts get dry.

7. Feed It and Leave It

When roots emerge and descend into the water, replace clean water with nutrient solution and keep the water level below the growing medium. Avoid moistening rock wool or other medium with nutrient rich water since this could lead to uninvited cohabitants (mainly algae). You can use bottled or purified water and optimize the PH for your particular plant based on charts on the internet.  However, most common greens and herbs will thrive with any clean water and standard nutrients.

Standard nutrients, you say? Masterblend, Calcium Nitrate, and Magnesium Sulfate... in a ratio of 2/2/1. For best results, dissolve in order: Calcium Nitrate, Masterblend, Magnesium Sulfate. The following chart provides an almost universally applicable quantities.

dissolve in order --> (all in grams except water)
Water (Galolons)Calcium NitrateMasterblendMagnesium Sulfate

These are nutrients from Amazon:


8. Let It Happen

As the water level drops, the roots will grow. As the plant grows, the water level drops. For most plants this happens in perfect step to allow the exposed roots to get plenty of oxygen while the submerged roots pick up plenty of water and nutrients. If the water level drops too fast, add a little to keep the roots wet. The Kratky Method is all about replacing complex tooling, pumps, etc... with a hands-off, cheap, simple system that just works.

The roots above the water stay healthy because the air in the jar is humid. They also get plenty of oxygen, so you don't have to pump air into the water (Deep Water Cultire) or spray the roots with water (Aeroponics) or any other fancy stuff. Growth rate is usually about 70-80% of what you would achieve with all that added complexity, which is plenty of growth.


9. Plants do need care

While the Kratky jar is one of the most care-free and hard-to-mess-up approaches to growing greens, they are still living plants and should be looked after. Make sure they have water. Check them regularly for pests. Don't let them outgrow the containers. Just look after the plants. If something looks wrong, it probably is. Since this is so easy to do and re-do, in most cases if the problem isn't easy to solve I suggest literally throwing the plant away. If you're starting a new jar every week or so, you won't run out of fresh stuff.

If the water level gets too low for the roots to reach, or it runs out entirely, add a bit (preferably with nutrients). If the roots get crowded, transplant to larger container and ensure the roots just barely reach the water. You can avoid this by anticipating the needs of the plant to maturity, but the stepping process can lead to greater longevity and health due to the way the roots grow.

Roots just after transplanting from 32oz to 64oz

As you can see, the plants roots started to group up and grow in circles at the bottom of the cone. This is because it was just transplanted out of a 32oz jar which had been re-watered several times with the water level hovering below 2 inches in the jar. Now, the plant will extend its roots into this new space, the water level can drop more, and the roots can get more oxygen. Had this plant just been started in the 64oz jar, the cone would have grown longer and more narrow, and the roots would just be coiling in the bottom of this large jar. So, stepping the plant up to a deeper (not wider) container can lead to a longer, healthier growing period than just starting it in a deeper container.

The Tail End

The above steps are more than you need to get started, but if you're still here and reading, there's more...

Like pictures of success:

Freshly replanted from 32oz to 64oz
A collection of plants all started in jars, though some were transplanted to a DWC tub (Deep Water Culture)
I let this cilantro mature and bolt in a 32oz jar...

And a little more about transplanting...

First, soak the roots in fresh water so they are strong, supple, and moist:

Soaking the roots before re-planting

After at least 20 minutes, put them in clean, larger jars:

Replanted in clean jars

Fill the larger jars with nutrient solution, just enough that the bottom 1/2 to 1 inch of roots are submerged

They will continue to grow in this new container for quite a while, and the roots will descend further. Watch closely in the first few days. Some plants drink water a lot faster than others, and may need to be topped off before the roots descend enough. As long as you keep the roots wet but not drowned, a plant can live like this for many months.


More can be had

If you have questions or would like the article to be refined please discuss below. This is literally the first article on the CoSourcery platform, which means it's rough. But, due to the very open nature of the platform I expect this to become one of the most refined articles on the site, since you could edit it yourself or suggest improvements below.


Thanks for reading!