I've been thinking about this for years and puttering away at it now and again in the real world. If you have access to an asphalt-covered lot, especially one that you don't actually have rights to, how much could you do and how cheaply to make it beautiful? I think that there are some non-obvious answers, especially if you have access to a vehicle and a couple of friends. Since moving next to an abandoned area of over five square blocks I've been doing some slowly more ambitious jiggerings of the dynamic and maybe one day I'll write about that here but for now let's consider one thought experiment.
Let's say you've got access to a double-sized lot that's covered with parking spaces and (pretty much) ignored by the owner. We'll assume a temperate climate for now. We'll assume that the temperature never goes below 45 degrees Farenheit in the time that you're working.
So let's say that in one day you go with friends and pick up a few hundred used bricks somewhere. Perhaps you found them on freecycle. This is your starting gun since it can take months to get that cheap load of bricks. That night or soon thereafter, you all go to Home Despot and buy 80 cinder blocks and various other supplies.
You bring them all to your chosen lot.
There you mix some concrete. While one person does that at least one other is cleaning up garbage. If you're being really slick, you can take any paper or otherwise biodegradable garbage and shred it to use as filler along with the soil in your project.
You pour a foot and a half wide line of concrete about an inch tall in a wide circle about 11 feet in diameter.
Along the inner line of this circle you put the cinder blocks so they'll be held to the asphalt by the hardened concrete. Around four inches outside that circle of cinder blocks you put a ring of bricks. Inside the ring of cinder blocks you fasten about five or six granite "Belgian blocks" with more dabs of concrete.
Five of six cinder blocks are "spot welded" to the asphalt with dabs of concrete, in other "random" locations in the lot. Each of these cinder blocks is bordered by one to three Belgian blocks placed a couple inches away but aligned with the cinder block so as to create protected nooks and crannies between the cinder block and the stone for plants to grow in.
In each corner of the lot you use the remaining cinder blocks and concrete to create one brick high boundary walls about a foot and a half from the actual lot line. Be extra sure that you clear any garbage from right around there since that low wall is going to attract trash like birds to breadcrumbs.
Then everybody goes home and leaves the concrete to cure.
The next time you can all get together you all go back and pour about an inch of rough gravel or sand into each cinder block, into the space between the cinder blocks and bricks, and for about a foot outside the bricks and inside the cinder blocks. Then onto patches on all the cinder blocks you spray "moss smoothies".
Now pour soil over the whole shebang. Every brick and cinder block, until they're barely visible. This will settle quite a bit.
Into this loose soil you shove hundreds of various beans obtained by the simple expedient of buying bags of beans in the food section at the supermarket. Each bean should be about two inches down and you should end up with about ten various beans per square foot. Why two inches? Isn't that deeper than gardening books suggest? Yep. These plants are going to be living in a tough environment and it's better for them to have protected roots and be harder to pull out than to grow "just right" like some Martha Stewart fancy project.
Into this soil you also put hundreds of drought-resistant "plugs" of sedum, alyssum, and other very tough plants that propagate themselves and can stand up to plenty of abuse. You can get flats of these plants really cheap, especially if you're willing to take "imperfect" ones, at any nursery.
Over ALL of this, you then spread a thin layer of fine gravel. Fine enough to be the type used in fish tanks. This will allow things to "rest up a bit" and get settled and keep soil from blowing away or be forced to the side too much by things like dog pee.
Before leaving that day, water this all plenty. Just be sure not to knock the soil and such too much into chaos while you water it.
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Think of what this can all look like within a month. Elegant, welcoming, and with the beginnnings of a local ecosystem. Among other things, some kind of plants or other will "volunteer" and start growing there without your having had to put them there. All those bean vines will be encouraging the soil to stay healthy and tying it all together, literally.
And now that the basics are in place anybody can even, if the mood strikes them, drop by any random day with a bag of soil and just pour it onto the ground wherever on the lot seems cool and just keep making the whole place closer to being an urban wilderness. It's a hell of a lot easier to spend five bucks whenever you feel like it and know that you'll add stuff to a place that's already alive than to have the weight of trying to make something live in a dead space. Same thing goes for now depaving some little five foot by five foot area on the lot now that you know it'll be part of a living environment, no matter how much or how little you have time to do.
The last step, just to add the final touch to bring people in, is to occasionally scatter inexpensive pretty things around the space. On the lots near me I scatter handfuls of semi-precious stones like cornelian, hematite, and clear quartz. Bought in bulk from the right stores, they're only about eight dollars a pound. Shells work well, too. Just be sure to not put them all out on one day or in one place or somebody will go out and just take them all. But if a few are scattered here and there you'll be amazed at the sense of magic it gives somebody to find something like a tiger's eye lurking under a bean vine.
Now let's itemize out our shopping list to see just how much this would all cost. I need to get to bed so I'll come back later and fill in the prices.
80 cinder blocks ($1.40 x 80) $120
200 bricks (used)
40 Belgian blocks ($2.60 x 40) $104
6 bags of cement
6 bags of sand for concrete
3 landscaper's flats of mixed plugs (3 x $50) $150
10 bags of beans ($1.20 X 10) $12
20 bags of soil ($2.30 x 20) $46
30 bags of rough gravel or sand
5 bags of fine gravel
semi-precious stones $40
transport costs $100
random tools,etc. $60
Anybody care to tell me that this will cost more than a large party?
There are dozens of variations on this. Not to mention less permanently attached means like this.