Well, I pretty much have to disagree with everything Warren wrote below,
since I have had a raised bed for vegetables over an old asphalt driveway
for 4 years, growing radishes, lettuces, chard, sugar peas, tomatoes,
collard greens, parsnips and chinese cabbage. My raised bed is about 12-14
inches high and about 5 by 10. I used old concrete/aggregate that was sawed
up from an old patio into neat rectangles as the sides of the bed. One side
of it is bounded by an old loose stone retaining wall. I filled the bed with
various bags of potting soil, top soil, dirt from other parts of my garden,
chicken manure, steer manure, etc. I add a bag or two of manure every year
to the top of it. My driveway is at the top of a gentle slope. Drainage has
never been any kind of an issue. (Well, the blocks that make up the sides
are not tight together. Soil doesn't leak through in any quantity, but I
suppose if I overwatered, the excess would come through the openings between
the blocks). When I pull up plants at the end of the season, they have not
put down roots into the asphalt. The average root depth for most of the
vegetable crops is about 8-10 inches. A neighbor's maple tree hangs over the
bed from the north side of it, so it rarely gets rained on. I have to use a
sprinkler on it, but that is not unusual in my climate.
"Warren" wrote in message
...
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.

==========
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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