UPDATE: I was contacted by a reader who shared an excellent tip. 3″ net cups fit perfectly in a mason jar. Instead of having to cut a hole in the lid, simply remove the lid, place the net cup on top of the jar, and tighten the ring down. Why didn’t I think of that???
Kratky mason jars are great for new hydroponic growers. They’re cheap, durable, and best of all easy to make. This method makes it easy to dip your toes in the water and see if hydroponic gardening is right for you.
Plus, if you’re artistic you can paint some really cool jars that all your friends will be jealous of!
DIY Kratky Mason Jar
The Kratky method, a non-circulating method of hydroponic gardening, gets its namesake from Dr. Bernard Kratky the original publisher of the method. Non-circulating hydroponic systems don’t require pumps, or aerators, or electricity.
In a Kratky system, only the tips of the roots touch the water. As the plant root system grows, the water in the reservoir is depleted. This leaves an air gap in the jar, and in response to this, the plant roots will develop “oxygen roots” to take advantage of this air. The goal of the system is to run out of water at the same time the plant is ready to harvest.
Kratky mason jar hydroponics requires very few materials. The biggest challenge is cutting a hole in the jar lid that the net cup will fit in. I use 3” net cups and rockwool cubes. The net cups in the photos are cheap cups I bought in bulk on Amazon.
I prefer to use a quart jar because it holds twice as much solution as a pint jar. Sadly, I didn’t have any quart jars around the house when I started working on this article. All the steps are the same regardless of what size jar you’re using. The only difference will be in the nutrient solution.
- Mason jar with lid
- 3” net cup
- 1½” or 2″ rockwool or starter cube
- Nutrient Mix
General Hydroponics Flora Series is my favorite pre-mixed liquid concentrate for Kratky hydroponics. The simplicity of using a liquid concentrate is ideal for small systems like this—no need to break out the gram scale and get sciency. Simply measure the amount of concentrate you need and add it to water.
To make things easier, I mix at least 1 gallon of nutrient solution. That way you can easily follow the dosing recommendations on the nutrient label. Graduated cylinders make mixing nutrient solutions more precise, but they’re not required. Instead, a regular teaspoon measuring cup will work just fine.
Assemble Your Kratky Mason Jar
Step 1 – Toss the Lid
If you have a 3″ net cup, you’re in luck! Toss the mason jar lid, and replace it with the net cup. Make sure you save the metal ring, you’re gonna need it to hold the net cup in place. Step 1 complete.
I’d like to give a big shoutout to the reader who contacted and let me know 3″ net cups fit perfectly in place of the jar lid. Previously this step involved cutting a hole in the lid to fit a 1 1/2″ net cup. That was a major pain in the butt! Thanks to him, we now have the easy way!!
Step 2 – Fit the Net Cup
If you cut the hole properly, your net cup will slide into place with the lip holding securely to the top of the lid. Be careful, the cut edges of the lid are sharp.
Step 3 – Mark Fill Line
The jar should only be filled high enough with nutrient solution to allow the bottom of the rockwool or starter plug to wick water. If the jar is completely filled with water, the seed or plant will die from oversaturation.
Step 4 – Paint the Jar
Direct sunlight on the nutrient solution inside the jar promotes the growth of bacteria and mildew. To combat this, paint your jar with an opaque (non-transparent) color, or get creative and decorate it to your liking! If you don’t want to paint, you can also wrap the jar in aluminum foil or tape.
I like to paint the jar after I’ve marked the fill line. That way I can still see the fill line from inside the jar as I pour the nutrient solution in.
Step 5 – Mix Nutrient Solution
Mix your preferred nutrient solution, and pH balance the solution. I use pH UP and pH DOWN from General Hydroponics. Once the solution is balanced, fill the jar to the fill line.
Step 6 – Soak Rockwool Starter Plug
If you’re using rockwool starter plugs, soak the plug in water adjusted to a pH of 5.5 for 15 to 20 seconds, or longer if you prefer. Rockwool is somewhat alkaline naturally, with a pH of approximately 7.8. Soaking in pH adjusted water will reduce this alkalinity, and make it more suitable for plant growth.
Step 7 – Place Seed in Starter Plug
If you plan to grow from seed, place 2-3 seeds in the starter plug. Place a small piece of rockwool fiber over the seed hole to block light. Don’t push the fiber down hard, it’s intent is simply to shade the seeds while they germinate. Once the seeds germinate, you may need to trim back excess seedlings.
Step 8 – If Germinating, Cover With Plastic Wrap
If you’re growing from seed, take a piece of plastic wrap or a plastic sandwich bag and loosely cover the top of the mason jar. This will help create a “greenhouse effect” and build humidity that helps encourage germination. Alternatively, you can use the paper towel seed germination method and then transplant seedlings into the rockwool.
What Plants Grow in Kratky Mason Jars?
Common Kratky Mason Jar Plants:
When selecting plants to grow in your Kratky jars, it’s important to think about how long the plants take to mature. Leafy greens like lettuce, kale, and spinach are great. Something like a tomato plant can grow in a non-circulating system but would need a much larger container.
To optimize the growth of the plant growing in the Kratky mason jar, place it under a grow light. If you don’t have a grow light, place the mason jar on the sunniest window ledge in your house. Kratky mason jars will work outdoors as well.
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