Carolyn LeCrone wrote:
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?
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.
--
Warren H.
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Disclaimer: My views reflect those of myself, and not my
employer, my friends, nor (as she often tells me) my wife.
Any resemblance to the views of anybody living or dead is
coincidental. No animals were hurt in the writing of this
response -- unless you count my dog who desperately wants
to go outside now.
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