Lettuce Not: Kratky Hydroponics DIY

Growing food in the windowsill has millions trying it, but not everything you read out there in blogland should be taken to heart… like growing herbs in tea cups. Cute, but not very productive. Recycling empty food containers is hot too, and I’ve seen the coffee can trick shown off on a number of sites, so naturally I had to experiment. At least this container is big enough to allow for some root growth. And so began my Kratky hydroponics DIY lettuce grow using the sun for light. That was in late March.

As you can see in the photo above there was no lovely large heads of lettuce to harvest, though the plants did grow very nicely for a while. If you haven’t read the first post on this grow, it looked very promising.

What Went Wrong?

Let’s start off with the fact that the Kratky method does work when done properly. I don’t think it’s particularly suited to this kind of setup though, unless you have nothing at all on your schedule but keeping an eye on your letti. Someone who works away from home will probably have crop failure faster than I did. I may be home all day, but I work long hours, and all manner of wilt can be happening in the windowsill before you remember to check on things… it’s not visible from the office.

The curly leaf lettuces never grew roots that could extend into the reservoir. Lots of top growth, but unless the solution level was kept up to the bottom of the net cup constantly, droop set in. Do you have time to check this situation 8 times a day? One was leggy, and one was not for reasons unknown, and the leggy one had less root growth than the other one even though it was twice the size! To the new gardener this might not be puzzling, because they’re just so ecstatic that they’re actually growing food. Stick with it awhile. Experience is a great teacher.

Don't Try This Kratky Hydroponics DIY MethodDo you have any idea what happens to the sunshine passing through window glass? It’s magnified… which is why glasshouses at fancy garden centers have shade cover on the roofs in summer to stop the sun from burning everything growing inside of it. Thermal pane windows might also increase this effect because there’s two layers of glass the light travels through. The southern exposure you need for efficient light in winter becomes brutal as the sun shifts position in spring. Now not only is the solution getting warmer, but the thin leaves of lettuce that’s fully hydrated aren’t designed to withstand those kinds of exposure.

So, in the end the best lettuce of the bunch was burned by the sun. The damage was on the leaf tips closest to the glass. And at that point I gave up, which is exactly what all the new indoor gardeners out there will do when your grow is such a pain to keep up with, and an exercise in futility in the end. The problem isn’t that you can’t grow stuff. Your thumb isn’t brown. You’re just approaching the project backwards when you attempt to grow in containers that are too small or poorly suited to the job. Recycling is great – when it works.

Now, if I had drilled holes in the bottom of these coffee cans and potted the lettuce starts like I would have for outdoor growing on the patio/driveway they would have fared far better. It would have been so much easier to keep them from wilting, but they still would have burned from the intense May sun blasting through that glass. Since lettuce has a compact root system compared to what you get above the soil, they wouldn’t have had root bound issues that some of those trendy growing ideas will produce.

Kratky Method Done Right

The reason that the Kratky method works is the right way to grow food with it is to plant it in a raft. It’s supposed to float on top of the nutrient solution, not be suspended above it. This hydroponics method is not suited to coffee cans. How would your grow go if the styrofoam is sinking down inside this little tub as the reservoir level drops? You either have to yank the float out by grabbing the plant by it’s leaves, or dump the whole thing out. Not practical for maintenance, and certainly a loss of sunshine for your plant, along with wear and tear on it’s leaves as it matures.

Things would have gone better if there was a wick, but that’s capillary action, not Kratky hydroponics. If your going to try this growing method, then get a tub so you can raise the raft without abusing your lettuces! No, it’s not cute, it’s not repurposing (unless you have one laying around), and you probably won’t find one in your trash can, but it works. A dark tub too with walls that aren’t transparent, so the sun can’t cause algae to grow in your solution reservoir.

It’s okay to get a jump on warm weather by starting such a grow out in a southern window, but once you hit mid-May… take your poor crop outside. That is if you want to eat that salad, because if you don’t you’ll morn the loss. Of course, if you had it growing under lights, this wouldn’t be an issue at all.

One last tip for you newbies out there. If you’re going to pot lettuces in a lightweight plastic coffee can and grow them outdoors, be prepared to anchor them somehow. Potting mix is light. A full grown, or nearly so, lettuce plant is heavy, and when the wind hits this setup it will knock over your pot. One way to avoid that would be to put an inch or two of gravel in the bottom, or a fist-sized rock.


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  • Nick says:

    The Kratky method is distinct from DWC (aerated deep water culture) in that it specifically does NOT float on the surface, but rather remains above the solution, with the roots in partial contact. You are describing DWC at the end of the article.

    The Kratky method hinges on the idea that the airspace or void below the plants allows most of the root bundle to grow in a humid, pseudo-aeroponic environment, while the submerged roots wick up the nutrient solution.

    A more likely explanation for the problems you experienced is insufficient initial contact with the nutrient solution – i.e. no direct contact.

    Obviously you can’t rule out nutrient issues but frazzled, stunted root growth suggests it was simply an issue of water availability. Next time submerge the net cup by, say a third, and see what a difference it makes!

    It’s likely your leaf-burn was due to lack of nutrients although I don’t know how hot your windowsill is.

    • Tammy says:

      Hi Nick,

      Actually, there is a floating raft setup using Kratky’s technique without aeration. He describes it in his lecture notes:

      Another modification of this method is a float-support system in long rectangular raceway tanks. Lettuce is planted or transplanted into net pots placed in a sheet of extruded polystyrene. The cover initially floats on the nutrient solution, and then, comes to rest on 2 parallel plastic pipes (10 cm diam) resting on the tank floor as the nutrient solution level recedes due to plant growth.

      I’ve also seen photos of his floating raft method somewhere (university in Hawaii perhaps). You’ll find the lecture transcript HERE.

      The suggestion that it was the type of lettuce itself is also a possibility to explain why one pair of plants developed roots past the net cup while the other did not.

      I haven’t tried this again after such dismal results, but if I do again, I’ll certainly try submerging the lower third of the pot in the solution until longer roots develop. But that window is in full sun from dawn to a couple hours before sundown, and as the weather warms it gets pretty hot.

  • katyadog1 says:


    No. Your lettuce failing to grow roots was what went wrong, I just don’t know why it happened. Nutrient imbalance, perhaps?

    Many, many people use kratky with great success. And when I mean kratky, I mean what you did here; Coffee can with nutrients placed such that the water level is at the bottom of the pot initially. As the seedling grows, the roots go into the water and the water level drops. The roots lengthen and grow, and “follow” the water down into the reservoir. By the time the plant is finished growing, the nutrients are nearly exhausted.

    I used the kratky method in the summer of 2014, to grow basil. I did exactly what you did here. I’d post a photo of the plant and its roots, but unfortunately I don’t know how. Remember: the roots must grow into the solution. Once they do, the plant will take off, as it has enough food to grow the roots wherever it needs to.

    Your reference of “commercial raft growing” is called Deep Water Culture. The water is aerated by electric pumps, and so the lettuce roots can be fully immersed with no problem as they have enough oxygen dissolved into the water. The rafts are floated on the surface. If those air pumps in those facilities shut down for a week, all those plants would die because of the lack of air.


    See the illustration on the second page; you don’t need to move the nutrients to the roots, the roots are supposed to grow towards the nutrients and follow them as they decrease. Your plants not doing that was the primary problem here. The sun burning the lettuce was unfortunate, but as you said it is avoidable.

    I encourage you to try this again! You may find that it works the second time around. I’d also be interested to know what nutrients you use, I think that may have had a bearing on the roots not growing.

    • Tammy says:

      Hi Katya,

      Thanks for your encouragement to not toss in the towel after the 1st attempt. Perhaps I should! After all, it is the dead of winter again.

      I used GroTek’s SoloTek GROW nutrients, which should be fine for fast growing greens. How they developed tops without any roots is mystifying, as is why one type of lettuce did grow roots and the other did not when they were all using the same bath of nutrient solution.

      And there is a raft setup for Kratky method, but I forgot all about it using sunken pipes to keep the raft from floating on the nutes as they drop. Oxygen is a definite must.

    • katyadog1 says:

      Interesting. I hadn’t thought of using pipes as pontoons to keep the raft above the nutrients for oxygen. I may try that.

  • nik M. Hanif says:

    Hello. Are you sure the correct way of doing Kratky’s is by floating the planters? Because from the paper itself, he mentioned that the pots need to be suspended above the solution and therefore letting the roots grow towards the solution.

    Source: http://www.ctahr.hawaii.edu/hawaii/downloads/A_Suspended_Pot_Non-circulating_Hydroponic_method.pdf

    • Tammy says:

      Hi Nik,

      In this trial the fixed height suspended pot allows the nutrient level to drop too far below the root zone too quickly. If growing the lettuce was your job, then it would be easier to insure that nutrient levels do not fall too low, but this isn’t the case when recycled coffee cans are used for hydroponics – this is a DIY approach. One that is not very efficient for the hobby grower a.k.a. home gardener, though it is super cheap. Floating raft uses a tank, but the foam raft’s thickness keeps the substrate suspended above the nutrient at the same distance even if it drops suddenly… as long as the tanks sides do not narrow as the level drops as is the case with many tote tubs. If you look at how commercial lettuce growers raise their crops, you’ll see that the raft is preferred, and the issues experienced here certainly point out why. It’s highly unreliable – plant by plant root development varies. You check your levels for days, and nothing changes. Skip one by accident, and you’ve got wilt, which is damaging to leaf lettuce’s leave structure.

      The important things to note from this trial are that it’s a lot easier to succeed at growing food with a system that moves the nute solution to the roots, or in the case of a raft, keeps them at the right level in relation to the pot at all times. And secondly, using sunlight through a window is not what it’s cracked up to be, which is why greenhouses are covered in polyfilm today, not glass. The delicate leaves of lettuce fried when the sun shifted to it’s spring location in the sky.


Tammy Clayton

Contributing Writer at Garden Culture Magazine

Tammy has been immersed in the world of plants and growing since her first job as an assistant weeder at the tender age of 8. Heavily influenced by a former life as a landscape designer and nursery owner, she swears good looking plants follow her home.