You're on the right track, I'm building something very similar to this (calling it a raised wicking bed table), only with some important changes that I think will make it much better solution. It's based on the classic self irrigating planter idea but modified for a large raised bed. Here's some issues I've had with raised beds:

-Refreshing soil is a pain, it can be very time consuming and physically difficult, especially in large/deep beds.

-If there's a gap between the bed and the soil underneath, you're essentially growing in a big container, meaning you're not getting the benefits of raised bed growing and you'll have to completely refresh the dirt every season (peat mixes break down in 6-9 months in my experience and turn to concrete).

Here's my design:

-I have a waterproof bed 2-6" deep and filled with pumice. Use a pondliner to make the bed waterproof. The pumice (could be perlite, diatomaceous earth or just fabric capillary cloth) acts as a wick to draw water up into the containers above.

-Then, you place your containers on top of the bed nestled into the pumice. Since the bed is roughly 24" high, your veggies are at table height. You can also move the containers around much more easily, refresh the soil easily, and choose containers to fit the plants' needs. Plus, you're saving money on soil since you're only using the soil needed for the specific plant pot.

-The waterproof bed has a float valve inside (protected from the pumice) connected to a rain barrel or hose.

-The bed is great for starting seeds.

-Also, instead of plastic pots, I use fabric Root Pouches that provide better root pruning and are far less expensive.

Here's an example from the internet someone else made on a small scale:

Let me start by saying thank you to Jay-Part-Shade for responding to my questions.

I have seen the problem you describe with using peat as part of my soil mixture, but this is integral to the Square-Foot method. What would you suggest instead of peat?

The design you are suggesting was similar to my original plan that involved using expanded clay balls. This proved to be very expensive and after thinking it over I rejected the design because the clay balls where essentially being used as both a wick and as physical support for the soil above. This is not practical for a large container and wasteful of the space that could be better used to hold water.

My current thought is to put down metal plates with small holes drilled in them and put one layer of cinder blocks above them to hold the soil. I planned on using twine or cloth ropes to draw the water up into the soil zone.

The issues I anticipated include stagnant water attracting mold and low oxygen content water leading to root rot.

I'm still not clear why this technique is not universal. Is it just a question of cost? It definitely costs more to make a waterproof container than to make a simple planter, but I suspect there is another reason.

What do you think?
-Mell

Hey Mell,

Good thoughts. Couple of points:

-To clarify, my design has all the plants growing in pots (1-7 gal), not in the actual raised bed itself. The bed is just a large reservoir of water with a support medium. If you look up SIPs (sub irrigation planters) or Eathboxes or earthtainers, you'll see this method at work, albeit in the form of a bucket in a bucket (or tote with a reservoir in the bottom).

-You can also see this system concept at work in something called the "rain gutter irrigation planter" (link below) - it's just bags or buckets with wicks sitting on top of a rain gutter filled with water. That's perfectly fine and tons of people do this. I'm proposing something similar, just in the form of a raised bed that looks nice and not like a paint mixing station.

-So, yes, lots of people use this method, just not in a nice wooden raised bed. I like my garden to look nice, so I'm sanding and staining my raised bed. If pure performance was all that mattered, I'd probably go with the bucket version.

-I'm not sure how people with true wicking beds replace their soil, but it must be a horrible pain the ass. People who grow in earthboxes or bucket SIPs can easily dump their soil out each season.

-Peat is fine to put in the pots, just not in the wicking bed itself. The wicking bed should only have inert media that doesn't break down. I tailor my soil for the plants I grow in -- 5-1-1 mix for longer term trees or perennials. Normal peat-based potting soil for the rest of my veggies. With veggies, you want to get rid of the soil anyway since it's filled with diseases (tomatoes especially). Once I get my system up and running I'm going to experiment with different soil mixes, but you can go to the Earthbox website and see a ton of recommendations for soil mixes. I like Gardener and Bloome Blue Ribbon, but lots of different brands work.

-Yes, clay balls are very expensive, but pumice, lava rock, etc. or gravel by the yard is cheap. Home Depot sells 1/3 yard of gravel for $35. I'm not concerned about how much water the reservoir holds since it's controlled by a $15 float valve. I'm guessing a layer of red bricks could also work as a support and wick, but haven't tested it yet.

-Twine and rope will break down or get saturated with salts in one season. The system will work in the beginning, but will cause all kinds of problems in future years.

-These systems don't lead to root rot or stagnant water for a number of reasons. But, again, check out the link below for a lot more info on the basic principle. Moss and algae only really grow if water is exposed to sunlight. In this system, the water level is 1" below the rock level and never exposed to air. So no mosquitoes either.

-Cost wise, my design is pretty cheap: ply for the bottom, 4x4 or cinder blocks for support, 2x4s all around to form the bed (you could even go shallower if you use a float valve). Pond liners are only $50-70 depending on size. No metal plates or anything else. $70 for the gravel support. Or go crushed lava rock and it's even cheaper and will act as the wick to the plant pots above. In total, my 12'x3' is about $150 or so for materials, not including additional fancy options like trim or stain.

-Best part, this system doesn't break down. You can easily pull the planters off the bed for harvest time, refresh the dirt, etc. I'm building this system for my disabled mother. She can manage 1-3 gallon pots, but digging in dirt is a no-go. Her system will have two beds: one low bed for tomatoes, corn, etc. and a table-height one for salad, herbs and everything that doesn't get huge.

-Finally, here's a picture of a commercial "capillary bed" working in the exact same principle I'm talking about. The commercial version uses a plastic or stainless steel table with a layer of black wicking cloth that's always saturated with water from a reservoir.

Here is a link that might be useful: Rain Gutter Grow System

This post was edited by Jay-Part-Shade on Sun, Aug 17, 14 at 20:15

Here's a video of someone's system very similar to what I'm proposing, using the same fabric bags I use. He forges the legs and grows directly in water:

Here is a link that might be useful: Grow Bed video