Birth of a concrete garden

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Q: I just purchased a lovely 1927 home in the hills above East Oakland, Calif. The previous owner didn't want to have anything to do with anything green outside, so she had two concrete patios built in the backyard -- one even with the lower level of the house, which I will keep, and the second one, consisting of individual concrete slabs, a few steps below that, extending to the property line. I would love to have a real garden back there, not just containers, and have had various suggestions from friends and contractors about how to make the lower patio into a garden. Because the backyard is on a slope, it's been suggested that the lower patio is probably helping prevent erosion and that I may want to keep the concrete in place and build raised beds on it. Does this sound reasonable, and if so, would it also make sense to break one of the slabs up a bit if I want to plant a tree or two? Or, should I try to find someone to break up and haul away all that concrete and star...