My husband would like to install a raised bed about 8 by 10 over an existing asphalt driveway pad. We would like to grow tomatoes, cucumbers and peppers. Will it work? How deep will it have to be? Also, we need to move some asparagus. Will it live in a barrel? How about in the new raised bed? Thanks, Carolyn
You might as well remove the asphalt. The conditions the asphalt will be facing will mean it'll never be suitable as a driveway again. And, unless you poke holes in it, the raised bed over it will turn into a sopping-wet mud hole that'll kill anything you try to grow in it. I also wouldn't put food crops over asphalt, which is essentially sand and small aggregate held together with a petroleum product. It's not going to break down enough to provide suitable drainage for a few years (depending on how thick it is), but it will break down enough to make it no longer usable as a driveway within a year if it's a typical thickness for a residential driveway. And that's even if you can provide enough drainage out the sides.
Also, a driveway isn't going to be flat. It's going to drain one way or another. Will what you can drain out the sides be draining across the rest of the driveway, leaving behind a film of silt, or will the rest of the driveway be trying to wash under the sides of your bed, bringing in even more petroleum products?
If you were putting in a temporary bed of colorspots that would only be there for a few days, like for a weekend festival, you might get away with a bed built on an asphalt base. But I just don't see success for anything meant to be more permanent than a week or two, and certainly not for anything that's going to be used to grow edible crops.
You may be comfortable eating vegetables grown over petroleum products, but that doesn't make it a good idea. You're also going to have an interesting surprise when you remove the bed, and examine the condition of the asphalt. After 4 years of being covered by soil, that asphalt will be in a very different condition than the surrounding driveway.
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