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
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.
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.
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.
It seems to me you're pretty much making an assumption that the petroleum
components of the asphalt are going to migrate upwards through the soil of
the raised bed - but you're not quite explaining how you expect this to
happen. The components could leach, it is true - but then they would leach
downward, not upward. (The situation MIGHT be different if the entire raised
bed were built over an asphalt pad that was a sinkhole - ie, at the very
lowest level of land around - in which case there might be frequent flooding
of the raised bed, and the potential for petroleum to leach upward). My bed
is at the top of a hill. If the roots are not contacting the asphalt
therefore, they are not taking up petroleum byproducts from it. And the
roots of my vegetable plants are not contacting the asphalt. It is pretty
much a truism in any case, that most of the feeder roots for plants are in
the top few inches of soil - not at the bottom of tap roots, if they even
Also, the asphalt underneath the bed is the very end of a driveway parking
pad that was not used for many years, and will not be used in the future, so
I'm not concerned as to its condition over time- IF it is deteriorating.
However, I live in a dry, cool, low rainfall, low humidity climate, with low
activity of microorganisms, so most things that are subject to deterioration
over time, such as wood, take many many years to break down here, so I would
be surprised if the asphalt exhibited much damage, even after 4 -5 years.
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