drainage layer / PWT question

Winston Salem, NC(Zone 7b)

Hi. I'm working on a tiered bed I'm concerned about it draining properly... I'm asking in this forum thinking it is basically a big container.
The soil beneath the beds is very hard -almost impermeable- red clay. Actually, I'm digging them out now with a pick-axe. What was IN the beds from the previous owners was also mostly just clay, but less solid, with about 2" of topsoil. I know the clay bottom will just hold water like a pot without a hole. Lots of places advise a gravel drainage layer for raised beds...but I know in containers, people now mostly advise against it. I've been reading the threads about the perched water table, etc.

So, would it be best to have the good soil right in contact with the clay, and try to drain the beds out the side (at a point above the clay layer?) I'm thinking of sloping the bottom to one side.
I'm feeling stuck and unsure about what I'm doing; any advice would be great.

Thumbnail by monkapotamus
Vieques, PR

You want to let the water out every way you can.

Shape the clay "floor" of each tier so that it's highest at the center of the back "wall", and slopes a.) down from the center to both sides, and b.) down from the back to the front.

Put a row of base stones along the sides and front of each tier, with about 1/2 inch between stones, so that the top surfaces of those stones are tilted slightly inward. The spaces provide some drainage, the top surface will support the rest of the stones, and the slight inward tilt will keep the rising wall of stones leaning in on themselves and not wanting to topple outward.

Set the next row of stones on the base row, leaving spaces between the stones again, but making sure the spaces are above middle of the lower stones, and NOT positioned above the base row spaces. In the more visible front, you don't need to leave spaces if they would look better tightly spaced.

Try to "interlace" the stones at the corners for strength and a better look.

Higher rows of stones can be tight against each other, as the first and second tier openings will provide drainage where it is needed.

This message was edited Apr 30, 2010 3:22 PM

Bay City, MI(Zone 6a)

I'm not getting a full understanding of what the two sketches represent - just the second sketch, actually.

Let's consider only at the top sketch for now. If I was making the beds, I would excavate a trench with a 1/4" fall/ft for each level, running parallel with the timbers (or the length of the bed). In the trench, I would place a perforated pipe and cover it with fabric designed for that application to keep soil out of the drain pipe.

In your sketch, the drains would look like Os, with the top of the O roughly level with the clay as shown. This will drain your beds of excess water.

Technically, you're not dealing with a perched water table. You're dealing with a layer of soil that is practically speaking, impermeable, because of how slowly water percolates through the clay - sort of like building a raised bed on concrete.

The reason I opt for the drain in the trench is this: Even though the soil is now hard clay, it won't be that way forever. Soil fauna is going to move the organic matter in the beds down into the clay, a little at a time. This will also eventually create that bathtub effect, just not as fast as if you excavated a hole and filled it with topsoil and OM. The drain you put in will be effective for years to come, while relying on the fall of the clay bottom (of the beds) will result in a bathtub effect after a few years.


Winston Salem, NC(Zone 7b)

Wow. Yippie. Egads! Thank you both for the quick and detailed reply!

The second sketch is the front view. Since it was a quick sketch, I had just copied and pasted and shown the timbers on both sides, but actually, on the left it is a poured concrete wall, with the driveway on the other side of it, so that is why I thought I should slope the bottom only to the right instead of off to both sides. The bottom does slope down toward the lower tier. I can cut or drill through the existing timbers to make a route for the water.. . the foot thick concrete side would be not so easy.

The beds are not huge- 5.5' X 2' wide, and the top of the tallest tier is just over 3'. The timbers and some sickly plants were already in place from the prev. owners. I'm digging out the contents of them now so that a plant could actually survive in there. So far, i've only dug out the center one, since it was completely empty, and I dug about 1' deep.

Would I want it set up like a french drain, with the pipe surrounded by gravel and fabric, or would just the perf pipe with the 'sock' around it, surrounded by the good soil suffice? The beds are shallow enough that the drain alone is going to eat up a lot of room. Do you think it might work to run the pipe along base the front without a gravel surround, and a french drain only on the wood side?

Pic is of what I'm thinking I might do- i tried to be more accurate about the drawing this time

Thumbnail by monkapotamus

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