Friday, May 8, 2009

Ruminations on sub-irrigated containers

Now that I've made a few sub-irrigated containers (SIC) and planted a few plants in them, I have a few opinions:

Sub-irrigated containers or Self watering pots are called so because you fill their reservoir with water and the plants get the water from this reservoir as they need it. Actually, water moves from the reservoir up through the soil to the plant's roots although it is not uncommon for the plant to shoot the roots all way down into the reservoir.

I don't like making and using SICs from 18 gallon totes.

For one thing, they are harder to make, at least when compared to the SICs made from 5 gallon buckets.
For another, the totes that I can get from Target, Dollar Store, etc, are not UltraViolet light stabilized. The SIC I made from a tote last year is already getting a little stiff and brittle this year.
They are also harder to move once you fill them and put plants in them.

No, for me, 5 and 4 gallon buckets are the way to go. I finally found a restaurant where they told me to come any time and get 5 gallon buckets. Score! You will be shocked to learn that they throw them away if no one gets them!

  • It is easier to make a SIC with two 5 gallon buckets.
  • SICs made with 5 gallon buckets can be moved relatively effortlessly
  • They last longer, season to season.
  • They are big enough for just about any plant.
  • You can get 5 gallon buckets free from restaurants.
With 5 gallon buckets, you can standardize the making of your SICs. I bought one of those drill bits that are round (2.5 inches wide) that are used to cut holes into drywall and stuff and used it to make the big hole at the bottom of the 5 gallon buckets.
**NOTE** Make sure you buy a guiding bit that goes with the hole-making bit. You will need the guiding bit to attach the hole-making bit to the drill and to stabilize the hole-making bit when drilling the hole

For the 1 inch hole, I used a 1 inch "paddle" bit. I don't know the proper name of these bits but when you see them you will know why I call them paddle bits.

For the air holes, I used a plain 1/4 inch drill bit.

It takes two 5 gallon buckets to make a SIC.

Take one of the 5 gallon buckets and drill holes at the bottom as indicated in the graphic. This is your TOP bucket:
Stack the TOP bucket into the second bucket. 5 gallon buckets conveniently fit into each other to leave a space approximately 3.5 inches deep at the bottom. This space serves as the water reservoir.

I drill two holes in opposites sides of the BOTTOM bucket, about 1/2 inch from where the bottom of the TOP bucket would be. These holes are needed to allow air into the reservoir. Air is essential for the proper functioning of this system. Air comes through these holes and it gets sucked in by the soil.

Next, I take a small plastic cup and cut four evenly spaced slits around its sides and two small slits at the bottom. This cup acts as the wick by which the water from the reservoir gets delivered to the rest of the container. We have a bunch of small plastic cups we get from restaurants with our boy's drinks.
Originally, I drilled a bunch of holes into this cup and then I secured it to the bucket with plastic ties. This a lot of work and it took the joy out of making a SIC. I got the idea of cutting the slits into the cup and just letting the soil weigh it down from the Homegrown Evolution blog or maybe it was the Green Roof Growers blog. I apologize for not remembering!

Once you have cut the slits, drop the cup into the 2.5 inch hole . It will protrude a little and that's ok. If you want the cup to sit deeper, make the hole in the TOP bucket larger than 2.5 inches. The thing to avoid, is making the hole the same size as the mouth of the cup because if you do, the cup may go all the way through and move in the reservoir, messing up the wick idea.

Finally, put the fill tube into the 1 inch hole and make sure it goes all the way to the bottom of the reservoir. Cut a notch in the end of the tube that goes into the reservoir to allow for better flow of water when you are filling the reservoir.
Before I go on I want to talk about the fill tube some more. I have mentioned in this blog that I don't recommend using PVC pipe as a fill tube. PVC is easy to work with but I've read discussions on the web regarding the possible leaching of harmful chemicals into the soil of the container which then are taken up by the plant, ending up in your plate. It sounds plausible to me but I have no proof whatsoever that it is true. Still, I don't want to take unnecessary risks, especially when there are alternatives. I chose Aluminum as the material for the fill tube only to find out that the same thing may happen with Aluminum ions getting into the plant and into my food. Again, why take a risk. The nice thing about PVC and Aluminum pipes is that they come in big sizes, like 2 inch diameters, which make putting the water in the reservoir easier. Alas, being the paranoid type, I chose to use an old watering hose to make my fill tubes; thus the 1 inch hole for them in my SIC's.
I hope that because the 5 gallon buckets I use were made for food storage, they are safe for growing edible plants in them.

So now that the cup and the fill tube are in their respective holes, I pour the soil in. It is not recommended to use yard soil. The soil that works the best is the stuff you can buy at your Mega hardware store like Lowe's. This type of soil mix allows for the air and water to move freely from the reservoir to the plant. I fully intend to use less and less store-bought soil and use more and more of my home-made compost for these containers
When adding the soil to the container, make sure that the cup (wick) gets soil in it. I fill the cup first and pack the soil firmly with my hand. Then I add some more soil and pack that firmly as well. Then I add some water to make sure it is wet. I repeat this process a couple of more times. The soil has to be wet all the way down to the cup for this to work. If the bottom soil is dry or if you allow it to become dry, water will not travel up the soil mix to the plant. I have no direct experience with this since I haven't allowed the soil to dry but this is the warning I got from the people who did this before me and I don't want to test it.

Some people add dry fertilizer to the very top of the soil after the plant is in. Magically, the fertilizer travels down the soil mix. There is a very erudite and complete explanation as to why this is so in the GardenWeb forums. I don't use dry fertilizer in my containers because the soil I bought comes pre-fertilized. Eventually, when I no longer buy soil, I may consider feeding the plants or not.

The last things to do are to put your plant in and fill the reservoir. Since I use 1 inch wide tubing, I found it easier to fill the reservoir using a funnel. I keep filling until water comes out of the air holes.

I found out last year that I only had to re-fill the reservoir about once a week even in the hottest part of the Summer. I saw tomatoes growing abundantly in sub-irrigated containers in Phoenix Arizona during their Summer, which is brutal!

Also, there are other websites that show you how to make a SIC. I have seen some very fancy SICs with built-in tomato cages and all manner of improvements. I stick to my simple SICs. Once you make a couple of them, it gets very easy to make them. Especially if you standardize your procedure and make them all at once.

Hopefully I will get to take some pictures this weekend to show you how my tomatoes and peppers are doing in these containers.


  1. very interesting. This year, I'm opting to use slow watering techniques with tomatoes. I half buried 1 gallon milk jugs with tiny holes. Filled with captured rain water. Should slowly water the plants. We shall see.

  2. A short while ago I wrote about how I used to bring buckets home from the factory where I worked and plant things in them - in no way near as sophisticated a way as you do though.


  3. Melissa - I've tried something similar on occasions. I don't know whether you have problems with slugs where you live but I've found they make homes under the half-buried waster-containers.


  4. Oh this is just fabulous! I've been wondering how you make those - I'm going to bookmark this for whenever I might need it in the future!

  5. Esther,
    I never heard about this particular problem with slugs. Interesting.

    Melisa, post about your progress with the milk jug. I am interested to see how it works for your.

    Gosh, there are now multiple resources on how to build one of these containers. For example, most people cover the container with plastic (I've used garden fabric) to slow down evaporation and keep weeds out. I won't cover mine until the Summer when evaporation is an issue.