Hi,
My wife and I are interested in using up an old but sturdy concrete pad in the back of our yard for a raised bed. The pad is about 10' x 14', and we'd like to build the beds about 3' high, in a U-shape so that it's possible to easily reach all areas of the bed. I haven't found anything in the archives of this newsgroup that really discussed raised beds that were not built on top of soil, and I'm a pretty new gardener, so I have a few questions:
- Is 3' of soil adequate for this kind of bed? We want more than 2' since the plants will have no chance to get into the soil beneath the bottom of the bed.
- Should I make special plans for drainage? I was already thinking of some gravel to fill in a dip in the center of the pad and to make a nice path in the center, is it also worthwhile to have a couple inches of gravel in the bottom of the bed boxes too? Hopefully I don't have to knock some kind of holes through this pad itself.
- I've considered both timber and block construction. The problem with block is that I'm uncomfortable building it three feet high without some kind of reinforcement, and I'm a little uncertain about the best way to anchor, say, rebar into the existing pad. I'm not sure if I can put in support posts outside the layout of the pad, because of adjacent structures and the way the foundation of the pad flares out below grade. Has anybody done this?
- I had also considered 4x4 timber. I don't want to use the treated wood, and cedar's a little expensive around here (Chicago area). How long would regular untreated pine 4x4 last? What would be a good exterior coating to help extend that life? (I already plan for a liner on the interior against the wood, although maybe not at the bottom depending on the drainage situation.)
- I've seen a lot of people recommend 2x lumber, but I wonder if it can support a 3' high bed without bowing out a lot (especially at the bottom). I had been thinking about using 2x4s laid flat, so that the 4" thickness through the wall is still there. This would save some money vs. 4x4 lumber but this would be double the drilling and cutting compared to 4x4s. Also, it may look very bizarre.
Thanks for any advice that comes to mind, and sorry if some of these are sort of off the deep end :)
Kevin Finn snipped-for-privacy@speakeasy.net