I put in 5 beds 16ft by 4 ft, and two 4 by 4. Planks are two inches wide, and 12 inches tall. If you work with sixteen foot planks, they half to eight feet and by four to four feet, just to state the obvious. Screw them together at the corners, and hammer in stakes outside them to hold the planks from moving apart. Soil to the top of the raised bed will settle down about two inches over two years, and you can top up with compost.

Dig out the ground underneath the raised bed to about a spade depth, and then you will have about 18 inches in which to grow the plants. Make your own mix, I use gravel, vermiculite, John Innes 3 and Multi-purpose compost, but you can get good tips from "Square Foot Gardening" by Mel Barthelomew.

When the beds are 4 feet across you can reach right across them to work them, which makes it a lot easier for you. It is also a good size and shape for using a directional sprinkler for watering. Put down weed blocking material between the raised beds and put on top of that two inches of gravel or chippings. Chippings are better, gravel just provides an ideal environment for weed seeds to fall down and germinate.

>> Dig out the ground underneath the raised bed to about a spade depth, and then you will have about 18 inches in which to grow the plants.

I strongly agree (as long as your drainage is good enough to keep the below-grade part from flooding during a heavy rain). I have to dig the 'floor" with a little slope towards a slit trench for drainage.

Giving a raised bed a below-grade foundation of loosened or improved soil deepens the roots zone and encourages the deeper soil to become more organic and better soil.

As compost in the top 18" breaks down, I think and that some of the 'drippings' soak into the subsoil and attract worms. Over time, roots will also dig down and gradually improve the sub-soil to give you an even deeper root zone.

I like to mix medium and large bark chunks into the under-layer, on the theory that they will add some drainage and water retention, but mainly because they will break down slowly, like over three or more years. They release a little organic matter, and also create small voids or air spaces deep under the bed as they decompose. I hope that helps the deeper soil levels to drain and stay aerated.

As clay and silt settle out of the the top 18", these voids give them a place to emigrate to, without plugging and clogging the soil. And if the soil above the voids settles or subsides into spots where bark nuggets used to be, I imagine that that movement creates mini-voids throughout the soil column, like ongoing deep turning with really tiny spades.

Speculation or wishful thinking? Either way, I don't think it hurts to mix some long-lasting soil amendments into the layers beneath a raised bed.

Sounds like sort of a mini-hugelkultur, Rick.

Exactly. If I had more wood and more space, that's what I'd do. I started one little brushpile, intending to bury it once I had enough wood, but I only had a little and then it disappeared.

Maybe the guy I brought in to help weed and prune thought it was trash and removed it while I wasn't watching.

Kind of like the barber that cut the sideburns i was trying to grow, when I was a teenager. HE knew they were just unshaven stubble, while I was trying to cultivate them into something more than fuzz.

When I complained, he said "WHAT sideburns?!?"

Thanks Rick, I started using bark chippings in the mix this year, to about 25% of the bed mix. I let them compost down for a year. I am told they take nitrogen until they decompose right down and release it again, but I think the beds are rich enough to take it, since I also use about 25% homegrown compost.

I find the watering brings the bark to the surface and think the winter heave that bro he the stones up will do the same. It's a tad unsightly so I rake the surface bark chippings to keep it tidy, it's only a very minor observation.

Everything is growing exceptionally well this year.

>> I am told they take nitrogen until they decompose right down and release it again,

I've also read that, along with the claim that bark is not as bad as wood of the same size (at hogging nitrogen while it decomposes in the soil). Bark breaks down a good bit slower, and bark has a little more N to start with than wood does.

Or so I've read.


>> I find the watering brings the bark to the surface

I think you must have much lighter, more friable soil than I do! Even after I've amended and added as much compost as I can make or am willing to buy, it's still fairly clayey. Anything trying to claw its way to the surface through that stuff needs an auger.

>> the winter heave that bro he the stones up will do the same.

Good point. We do have some frosts in PNW Zone 8, and some frost cycling, but not very deep or long. For me, it would actually be a benefit if the bark came back to the surface each spring: then there would be at least SOME some mulch even before I got around to refreshing it.

My beds are pretty messy, anyway, until the new growth fills in.

I had one bed where I had about an inch of coarse bark mulch on top (nuggets from 3/4" up to 1.5 inches). I was in a hurry at one point, and turned that coarse bark under. Next spring, not much had come back to the surface and indeed, there seemed much less buried than I had started with.

Either the layer was thinner than I remembered, or the soil was so low in organics (certainly true) that the soil organisms saw the bark and said "we don't care if it's only bark, we are HUNGRY" and tore it to shreds and chewed it up fast.