"Crack garden" is probably giving you some interesting mental images. In reality, it refers to plant life that is squeezing up through cracks in our streets, sidewalks and parking lots against all odds. Some people are harnessing these tiny access points to soil in areas where they would otherwise not be able to garden. I suppose it's a form of guerilla gardening, but not quite the same.
I first learned the term from de LaB when I was reading about a workshop they were giving at Homegirl Cafe, a local non-profit restaurant. They were giving a brief tour through Los Angeles' Chinatown to show where produce is literally growing out of cracks in the pavement.
It's an interesting idea and I started looking for more information about it. I discovered that the rogue tomato plant growing along an improbable industrial fence line wasn't the extent of it. Parsley growing in the gutter of a busy metropolitan street, corn sprouting in back regions of a parking lot. Even one of the newer parks in Los Angeles is nicknamed The Cornfield because of the trains that use to traverse the space, dropping seeds that would later sprout.
Tokyo Green Space is finding inspiration in the creative use of small urban spaces, integral to the Japanese psyche, as pictured above.
(Images: Tokyo Green Space)

Stanley Console by ...
Too cool! I've actually gotten vegetable and herb "volunteers" this year in several of my sidewalk cracks at home: http://farm6.static.flickr.com/5051/5471610712_f5d874b26c_b.jpg
I love this idea! It would do wonders for NYC
I love the idea of growing beauty in any available earth, but I wouldn't plant produce intended for human consumption in some of these places. You don't know what's in the soil.
I've been wondering about this, too. Not only because you don't know what's in the soil, which is very true, especially in industrial type areas, but is it a good idea to plant food where there's so much traffic, exhaust, etc? I've read that if you are going to eat the fruit (like tomatoes) and not the leaves (like lettuce) that it's not so bad. Still, it doesn't SEEM like it would be a good idea. Anyone know for sure?
I would agree with the worry- just thinking about what a horrible winter we've had here in New England with the snow, I know the roadsides have been just drenched in salt and road chemicals. (Not to mention the unique and special "stew" of city trash.) I'd think that flowers and decoratives are nice, but I'll keep my veggies in soil.
Of course, if it comes to a question of veggies from road salt infested dirt versus NO veggies at all, what is then the most food-chain responsible answer? If this gives an inner city dweller SOME way to get real vegetables, is it worth the chemical exposure? I can't answer that.
I don't know about this. I love the idea of seeing beautiful plants in our city - but first we need to make sure they are not invasives.
Second - I know this sounds crazy - but plants can experience distress and I think an empty lot would be better (call the city reps and ask them for more urban parks). I'm not saying plants have any kind of cognition - but if we're intentionally doing this, we should at least study what plants naturally grow in cracks and then plant accordingly so the plant will survive.
Sorry, I did not mean to sound like a naysayer. In fact I love this idea, it just takes some planning. Cities desperately need plants and trees to filter pollutants from air and water, and to give residents a mood boost. ;)
Cities need more green spaces. Maybe next green revolution is a revolution.
Perfect!