*At the plaza of the 24th Street BART station, there is an installation about the power of tree roots. Not intentional.

Plant a tree. Leave around it dirt for an area on each side of the truck about as long as an arm from elbow to fingertip. Beyond that, create a slab of concrete about three finger-widths deep and having, for decorative purposes, embedded rounded rocks.

Wait.

The tree will grow above and below the ground. The roots will lift the slab, slowly. The slab will start to crack, slowly.

A good book called "The Urban Garden" talks about the wise ways to plant plants, including trees, amidst the concrete. It forsees things like that and says how one might avoid them. It is very down on planting trees singly--if you're going to plant six trees in an area, plant them together. They get more dirt to work with, feel better.

The tough thing about reading "The Urban Garden" is that it was written in a white-hot rage. It is difficult when exposed to an idea for the first time to have the person telling you be very, very angry that you don't already know it. But that happens, and often the ideas so delivered are indeed obvious and rarely done so you can see why the person is mad.

"Streets are for people" is another book like that, obvious great ideas delivered in anger. What's excellent is that those ideas happen more now in the United States than they did when the book was written.

Good to have it be easy and interesting to hang out in public with other people. Watching concrete crack in a predictable way is probably not an ideal activity.