Tabletop Hydroponics: What I’ve Learned About the Quirks of Kratky Jars

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Since I outgrew taking electrical outlets apart by age ten, I’ve been playing with other things. Right now, it’s Kratky jars, and they’re a bit more challenging than I expected. Contrary to what I’ve read on the Internet, they are not set it and forget it projects. They, too, have their quirks. Please allow me to explain. 

Quirky Kratky jars.

I made my Kratkys from 1/2 gallon Kerr jars. Those accept a 3” net pot very readily, and the jar ring works well to secure the pot. It’s important that roots have some air, so they don’t rot, especially in these. I had a nice micro-tomato in my other unit, so I transplanted it to see how it would grow in the jar. And it grew very well until one day, I noticed that the nutrient solution in the jar was very low.

The tomato had set fruit but was a long way from ripening, so I added more solution. The tomato wilted and died within a few days. Upon doing more in-depth research, I learned that this is actually a quirk of the Kratky method. 

Lesson:

When growing something Kratky style, it’s important to grow something that will be mature and harvestable by the time the nutrient solution is used up. Adding more is NOT an option, at least not in the traditional way. 

Solution:

Take the plant completely out of the unit. Clean the jar with some bleach, refill it with the nutrient solution, and replace the plant. I have no explanation for why this works, but it does. The micro-tomato I’m currently working with didn’t die and is now setting fruit. 

Extra note regarding pollination: be sure to shake your tomatoes or otherwise provide for pollination. The insects are dying back for the cold winter, so they can’t help me. I hate houseflies and Asian beetles anyway and usually feed them to my carnivorous plants. 

(What do you do with the food you grow? Why not learn how to can it with our free QUICKSTART Guide.)

Jiffy pellets in Kratky jars are tricky.

Another thing I’ve been playing with is germinating seeds in Jiffy pellets within the net pot. I’ve been less than successful with this, and I’m not sure why. I did have some salad greens sprout and grow. Then I made the mistake of pulling the Jiffy pellet out of the net pot to see how far the roots were growing. I put it right back into the solution, and the plant died.

I’ve also reseeded some of the jars with the exact same varieties I planted in the smaller IDOO unit at the same time. The IDOO seeds have sprouted, and these have not. The Jiffy pellet being too wet may be an issue, but really, since I germinate seeds in wet paper towels with no problems, I can’t imagine why.

Some of the videos I’ve watched use net pots and sponges similar to those I already have to fit a smaller reservoir. The other possibility is that my seeds are falling too far down in the pellet, similar to planting too deeply. I tried germinating in Jiffy pellets on a heat mat, then transferred the entire thing to the jar. This didn’t work either, so I tried germinating in the sponges that came with my IDOO units.

The seeds germinated and started to grow but then damped off and died.

I may try the paper towel and then pot method. But it’s so odd that seeds will germinate and grow when those sponges are in the tabletop units but not in the jar. Possibilities include not enough heat or not enough water circulation in the jar. The LED lights in the tabletop units provide lights in red, blue, and white spectra. The pump circulates water. Those are the only differences I can think of right now.

The other solution I’m considering is to germinate the plants in the tabletop unit, then transplant them into the Kratky when they’re old enough. As I noted above, this worked well for micro-tomatoes. Perhaps it’ll work with greens. 

So far, my micro-tomatoes have been my biggest success using this method. I have two right now, both Minibel, and both are setting fruit. Neither are as big as the Minibel in my larger tabletop unit, but they are nearly 2’ tall with some branching. When combined with the salad greens grown in the tabletop units, I have a nice winter salad or a hummus sandwich. 

There is a matter of the electricity costs since even a Kratky requires lights up here.

The winter sun is simply too weak, and the space next to a window is too cold. In fact, my windows will have lovely frost patterns during the coldest part of winter! This is much too cold for anything to grow. Therefore, grow lights are required. The tabletop units have simple aquarium pumps as well. With the caveat that this information may be a bit out of date with inflation, check out Aerogarden’s calculations on energy usage by their units.

As we can see, tabletop hydro really doesn’t use that much electricity per unit. Of course, having several will add up. They do give the formula they used, which interested parties can use as well.

Remember: Kratky jars only need light, though a heat mat in colder climates could be beneficial as well. And it seems to me quite a bit cheaper than the grocery store! Produce needs to be shipped in during the winter months, which requires fuel, trucks, and workers. Greens have been expensive in summer when they grow on the farm under natural conditions. I don’t see them being any cheaper during the winter.

Therefore, the energy used to produce them at home will be a net cost savings. And it is possible to save the seed, including from greens. Not only that, but the cut-and-come-again principles can be applied here. Grow an indeterminant tomato. It’ll keep producing as long as the indoor weather is right. The greens and medicinal herbs I’m growing in my tabletop units are producing very well. 

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So that’s my experience so far with my Kratky jars!

I’m finding that I learn a great deal more by gardening than by watching videos. Yes, research is helpful, but nothing teaches like doing! I’ve learned how to replace the nutrients in my reservoir without killing the plant and a few different ways that germinating seeds does NOT work. Garden and learn! And don’t let failure stop you. Let failure TEACH you. Then you’ll be unstoppable! 

Have you tried using Kratky jars? What have you learned? Are you considering adding this method to your home food production? Let’s talk about tabletop hydroponics in the comments.

About Amy Allen

Amy Allen is a professional bookworm and student of Life, the Universe, and Everything. She’s also a Master Gardener with a BS in biology, and has been growing food on her small urban lot since 2010.

Tabletop Hydroponics:  What I\'ve Learned About the Quirks of Kratky Jars
Amy Allen

Amy Allen

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    • Google the term. You’ll find plenty on it, including videos from Kratky himself. They’re also discussed in our growing food at home course along with many other of the basic points of gardening. Courses are found under the Courses and Books tab above.

  • Thanks for the info. I am wondering how you made the hole in the jar lid to fit the net pot. Also, do you cover the jars so there is no light on the roots? I use a tabletop (Aerogarden) unit and have been reseeding the cages and using rockwool for the grow medium. Mainly greens and herbs. I did grow a cutting from a determinate tomato (from my garden), rooted it in the cage, in a kratky jar. I covered the jar in tinfoil to keep light out. Once the roots got to the bottom of the jar I put the whole cage into a pot of soil. I didn’t think this size tomato plant would work in a jar. I am currently starting some more greens in the Aerogarden and will try moving them to a kratky jar soon. The grow lights I use are LED so they do not use much electricity at all! Contact me directly to discuss/share any other info, thanks.

    • Yes, I also use tin foil to cover the jar or else mold will form in the medium. The Mason jar comes with a two part lid composed of the lid and the jar ring. I simply discard the lid for use with other jars and keep the ring. And I agree: a regular sized tomato isn’t a good choice here. That’s why I’m using micro-tomatoes, aka dwarf tomatoes. Interested parties can find good prices at Bunny Hop Seeds Dot Com.

      Sharing info here helps everyone 🙂

  • Adding nutrient solution just killed the plant, but removing the plant, bleaching the jar, and adding fresh nutrient solution worked. This suggests some one-celled organism was consuming your nutrients. Adding more nutrient solution allowed that pest to overwhelm your plant. Removing the old solution and bleaching the jar removed most of those organisms. Then the plant got most of the fresh nutrient solution.

    • If so there was certainly no sign of it. Not impossible but I have no evidence to confirm or deny. The plant was quite healthy, until it wasn’t.

  • An old friend of mine has a terrace full of spices. She says she hasn´t bought any condiment in like 2 years. Keep going. And those reading should start doing it too. I´m not a fear monger, but I do prefer to go to my terrace with a scissor instead of going to a shop…

    • Oh yeah! Having my own food, greens, and medicinal herbs nearby is very comforting. No supply chain issues either!

    • One of the nice things about most spices is you can easily make more plants. I have ginger, turmeric, sage, rosemary and a few others within arm’s reach of my computer station. Cuttings for most of them is quite easy. Root crops like ginger require a bit of the harvested root replanted.

      I often give small starts to friends as gifts.

    • I only have 2 Kratky jars right now. Since I used things I had around the house from other projects, materials were free. They’ve just begun setting tomatoes so I haven’t harvested anything in the 2-3 months I’ve been working with them. To be fair these were a side project during regular gardening season. The tabletop units were purchased in February and July of this year, and those have been fairly productive. I was harvesting greens from the larger unit every 2-3 days, enough for a sandwich or small salad. I’ve harvested one batch of herbs & dried them so far, and this weekend will be another harvest. Enough to dry and put up, perhaps a 1 quart bag. I use the same Master Blend nutrient mix for all and buy in bulk so it’s pretty cost effective. I’ve also purchased the off brand units rather than the name brand so that helps costs. A good DIY’er could make one, in fact. Replacement sponges for the IDOO units were also bought in bulk, so about $0.20 each and they’re reusable if I’m careful. I haven’t calculated electricity but I believe it’s negligible; one grow light for the Kratkys and two small tabletop units. I believe it’s much cheaper than inflation, as I point out. And no supply chain issues!

  • I tried Kratky in larger totes on my deck this year with mixed results. All of my plants were started in potting soil/vermiculite/perlite mix and then the seedling had it’s roots washed and was transplanted into the Kratky net pot. There are plenty of YouTube videos showing how to do this.

    I had success with yellow onions and leeks, but the leaves make the net pot top heavy and I had to continuously reset my net pots. I had initial success with Napa cabbage and celery, but we had heat and drought and my plants were literally cooked. (The Napa cabbage in the dirt garden cooked, too.)

    I did have to add more nutrient solution and had no issues. I only brought the level back to half the original fill level so that I kept ‘air roots’ alive and didn’t drown the plants. Occasionally I only added water because I felt the outdoor heat caused a lot of evaporation and I didn’t want to concentrate the nutrients.

    I did develop a healthy crop of algae, even though my totes were dark and the lids were more or less opaque. In doing some reading, I plan to add hydrogen peroxide to the nutrient solution to inhibit/prohibit algae growth. As it breaks down, the oxygen will be added to the nutrients and help the roots.

    With indoor jars, I would watch for algae to develop and then add some very dilute peroxide to the jar diluted with just water and no nutrients.

    Just my two cents.

    • Thanks for sharing your experiences! Everyone benefits from that. I do add peroxide to my jars and have learned that once algae forms, the only solution is to wash the jars out before adding any nutrients. It’s interesting that you were able to add nutrients without removing the plant. I’ll keep that in mind.

      • How does washing the jar help when there is algae in the root system??? If it was me (and it may well be me before Winter is over), I’d add 1 tsp of peroxide to 1 qt of water to soak my plant roots in while I’m cleaning the jar. Then add nutrients with peroxide added to the Kratky jar, and then return the plant to the jar.

        • I don’t think there was algae. From my research, as I pointed out in my article, this really is a bizarre quirk of the method. However, a few drops of peroxide in the water can’t hurt! I do that with my tabletop units.

  • Thank you for this article.

    All the examples for Kratky growing that I had seem previously had shown the needs for some rather large growing areas, and I have room for only window sill growing. You show me that this is possible even in areas like what I have.

    One upgrade that I can think of—get the equivalent of a bird cage waterer, install it such that the tip of its tube reaches down about 1.5 inches, fill it with grow media. That will allow the liquid surface to go down enough to give roots some air, but keep the roots from drying out by maintaining a sufficient liquid level. Do you think that would be a workable idea?

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