The Super Simplified Science Behind Sub-Irrigation, as I understand it (with diagrams! exciting!):


The basic idea is to water the plants from the roots up. It’s incredibly efficient because there is no water lost, and the plants only drink as much as they need. However, you can’t just plop a plant in a puddle because the roots would rot, and it would also make a big mess. So the planter needs a resevoir for the water that is separated from the roots. This barrier needs to have holes in it to provide aeration to the roots. But then how does the water get to the plant? You need a wicking chamber – something to wick the water up into the soil. You fill the chamber with your potting mix and the water rises up though capillary action. You then cover your barrier (which is holding up the plant and potting mix) and wicking chamber with landscaping fabric to prevent the roots from growing down into the water reservoir.
If you’re interested in further reading about this, I learned a lot from reading Inside Urban Green. I also studied the Earth Box and the Earthtainer (free detailed instructions for building a sub-irrigating planter out of a plastic storage bin).

Tutorial
What you need:
Planter – completely sealed (no hole in the bottom), must be something you can drill a hole through (plastic works well). I used this one.
Plastic canvas – available at craft stores (or Amazon), used for beginner’s needlepoint
Plastic egg crate light panel – 2′ x 4′ sheet available at Home Depot for about $10 (or this pack of 10 on Amazon if you’re making a ton of planters). Looks like it’s for giant needlepoint (see scale in the next photo), but it’s actually used for lighting in drop ceilings. Don’t know why it’s called egg crate.
Zip ties – I used 4″ ones I already had, from the dollar store
PVC pipe – from Home Depot
Utility knife
Pliers
Scissors
Hand saw
Note: As there is a saw, a knife, and jagged bits of plastic involved, I urge you to be careful and use eye protection. Also, you should read this DIY safety info (from my dad) with some things to keep in mind.

Here’s a piece of the plastic “egg crate” to show you the scale (below). Use pliers to cut it by holding it firmly where you want the break and wiggling it until it cracks. It breaks easily, but I was serious about the eye protection. Don’t worry about the jagged edges, because they help to wedge the barrier in place.

This is how my barrier ended up looking (below). Yours will depend on the size of you planter. Mine sits in my planter about 4.5″ from the ground. I had to keep breaking off bits until it fit. If you’re making more than one planter, you can copy the shape once you know it fits. You’ll need to break open a space for the wicking chamber, and the fill pipe.

Now to build the wicking chamber. You want the chamber to be touching the bottom of the planter and the barrier, so the height is determined by how big your water reservoir is. Cut a strip this wide (mine was 4.5″ but I had to trim it down a bit for a good fit). You’ll also need a square for the bottom of the chamber (see below). My wicking chamber turned out to be about the size of a can of coke zero. The decision wasn’t very scientific. In fact, if anyone here is an avid gardener and would like to chime in regarding the size or planter to chamber size ratio, please comment. My planters seem to be working just fine so far.

Use zip ties to secure the plastic canvas into a cylinder (below).

You can cut off the ends of the zip ties. Now to start securing the bottom on (below).

Zip tie the bottom on (I used four ties).

Now you can trim the ends of the zip ties and the bottom, being careful not to cut into the squares the zip ties are secured to (below).

Tie it onto the barrier over the hole, in four places (below).

Seen from the top:

(Below) You’ll need to cut at least four supports to help hold the barrier up out of the water. You can add more supports if your container is bigger. The size, again, is determined by the size of your water reservoir. You’ll want one flat edge (from the edge of the 2′x4′ panel) to rest on the bottom of the planter evenly. I found that the jagged bits on the top helped to secure it to the barrier, so don’t worry about them.

(Below) Secure them to the bottom of the barrier. If you fiddle with the zip tie to have the head resting on a flat edge, you can get it nice and tight.

Here’s all four supports tied to the barrier (below).

(Below) Cut your pipe to the height you want (a bit longer than your planter height) and cut the bottom on an angle. If it’s not cut on an angle it could sit right on the bottom and not let water in. I used a 3′ length of pipe and only had to make one diagonal cut (for two planters).

Here it is all ready to go:

(Below) You also need to cut a small overflow hole in the side of your planter. It should be a bit below the barrier so that there is .5 – 1″ space between the water level and the barrier. You can use a small utility knife to make the hole if you are using a plastic planter. Just twist it around and around.

Just wedge the barrier in place in the planter. You want all four supports and the wicking chamber to be touching the bottom. You need to tap it in a bit, and the jagged edges cut into the sides making it more secure. Once it’s in, you can slide the pipe in place with the diagonal cut on the bottom. Here’s what it looks like inside:

(Below) Here’s what it looks like from the side. Yeah, not much. The hole is on the right side. You can put it anywhere as long as it is either visible or easily accessible. You need to know when water starts trickling out so you know the reservoir is full. I hold my hand below the hole while I’m filling so I can feel it.

My reservoir holds about 7L of water. Two of my planters have tomatoes which do need to be watered fairly often, especially if it has been hot. Probably every three days or so. The herbs I have, on the other hand, can go quite a bit longer. If you keep track of how much water you put in the first time, you can water every day for the first few days to get a sense of how fast the reservoir is emptying.
I fully intended on documenting the planting process, but it occurred to me while I was covered in dirt that I didn’t want to sacrifice my camera. It may be for the best, because I just started gardening myself. I am quite confident in the actual, physical planter, but not as much in my planting. Allow me to direct you over here, where you can find detailed planting instructions (all they ask is you make a donation to Feeding America).
If you have any suggestions or questions, let me know in the comments section. Thanks for reading!