If a sidewalk garden is too much, consider weeds

Knotweed (Polygonum capitatum) is considered a weed in San Francisco because it’ll take over a garden.   I wouldn’t advise planting it in a yard but look how great it looks when it grows in cracks in the sidewalk:

It’s invasive in natural areas but it seems like it couldn’t spread far if it’s surrounded by concrete.  I’d hesitate to plant it but probably wouldn’t rip it out if I found a situation like this one.