I saw a you tube video of a group which built raised beds, really just piles of compost and soil, on top of a paved parking lot, and grew great crops of sweet potatoes. For bushes, I would think a much greater depth of soil would be necessary, but the hugel beds might work well. I would probably start with one or two, maybe 6x20 or so, to test them out, before investing too much effort, time, materials, etc. I know Mel Bartholomew talks about making patio beds 6" deep for veggies, but berry bushes would need a good depth of soil, maybe several feet, so a tall hugel with large logs at the bottom might serve in that way, once the wood starts to break down.
I think it would also be valuable to grow a lot of green manure cover crops, N-fixers, Dynamic Accumulator plants, etc in the first couple of years to build up the soil. But a lot of the success possibilities would have to depend on rainfall or irrigation, planning the way or ways to get water to the beds to help break down the wood, and help the plants to get started; maybe putting wood chips on the paths would help to absorb water and not just have rain sheeting over the lot, and prevent possible overheating off the pavement, etc.
If a
project like this is carried out in a high-rainfall area, there could be enough natural rainfall for plants in a semi-wild area which has access to the groundwater, but still might not be enough, or at the right times, to keep raised beds on pavement, without roots sunk deep into the ground, watered enough for plant growth without added water from a hose or water-collection devices.
The bulldozed wood piles described do show a good example of how plants grow on fallen trees in the woods, but those areas are not on sealed ground. I have seen areas in western Washington where new trees grew on top of old, rotten trees. When the rotten tree was totally decomposed, the roots of the new tree were exposed, still reaching around and through what had been the old tree, sinking deeply into the rich, moist soil underneath that is partly composed of the old, rotten tree that had become soil, organic matter, mulch and humus. But, as those "tree nurseries" were in the woods, the old trees and younger ones were shaded, so were not exposed to the hot sun reflecting off a pavement, and had a rich, deep humus layer on the forest floor to absorb and hold moisture and nutrients for the young trees.
Lots to consider. Hope it works out well for you. Let us know your results. It is great to learn from one another.
Tracy, and others. I would be very surprised to think one could plant perennial anything, but especially shrubs, in the autumn, and get a harvest in the spring. Everything I have read suggests that one not expect or even allow fruit to form the first year, and not pick perennials the first year, so the perennials and shrubs etc are allowed and encouraged to form strong roots before being stressed by producing fruits. Most fruits that I know of take 2 to 3 years or more to really make a harvest-able crop. Unless perhaps, living in an area that grows year-round; I have never been in a situation like that.
Example: I put in a couple of Nanking Cherries 4 to 5 years ago, and got my first harvest last year. So far, I have not been successful in getting strawberries or raspberries to grow at all. I know I am in a colder climate, with a shorter growing season than many of you, but that has been my
experience everywhere I have lived and gardened.
I do think it is important to test our methods and crops, and specific types of gardens and plants for our region and climate, on a small scale, for family use, before trying large scale plantings of anything. I believe that is why
Bill Mollison emphasized in many of his writings to start with a nucleus, and get a small area, close to the house, or to the center of the design, under control, and then gradually expand from there after observation of what works and what doesn't, for that specific region, climate, and garden.
Also, my understanding of
permaculture is that we try to grow a variety of species, as many as will grow and thrive in our specific area, and not engage in monoculture farms of one or a few species. As Paul Wheaton states in some of his podcasts,
Polyculture 20 or 40 instead of polyculture 2 or 6. So, a large area devoted mostly to berries with maybe a clover groundcover, is not really permaculture, based on my reading of many books on the subject. As a teen I picked strawberries in a huge field of berries, on Vashon Island, Washington, and have seen many monoculture orchards of one kind of fruit or nut; My hubby picked blueberries in Maine in large monoculture fields. That is not permaculture. That is what permaculture is trying to get away from.
Perhaps Alley cropping, with rows of various trees on contour, and alternating rows of fruiting shrubs, with an understory of perennial herbs and ground covers, might also lead to a economically viable farm, more like
Mark Shepard's discussions of savannas and farm forestry, and could provide cash income from a variety of crops, so even if one crop had a bad year, others might still do well.
It is not cheap to buy large numbers of plants, and put in the work to grow them, soil prep or hauling in organic matter, etc, especially if they all die because not adapted to the climate or able to get enough water, etc to grow. So I think it is wise to spend some time experimenting first on a small scale.
djn