Urban growth boundaries are where cities decide to make laws against new development occurring outside the boundary. Critics hate them because they're anti-... The claim is that they have all historically grown and thus are failures. But that is not their purpose. Their purpose is to force developers to reconsider reclaiming interior land rather than having rings of decay.
Peter Calthorpe explains this and new urban design succinctly in an interview with Scott London:
They had two parking lanes, so the on-street parking basically got used once a year during the Christmas party and the rest of the year the street looked like you could land an airplane on it. So we said, why don’t we "park" some trees in these stalls. Then you can park cars between the trees. The public works official said, "Well, you can’t do that, the cars will run into the trees." I said, "Well, why don’t they run into the parked cars, they are in the same spot?" And very quickly he said, "Because the cars have reflectors on the back." We finally got the trees approved by applying reflectors to the trunks, which satisfied him. But the idea of parking a tree in a street is kind of a metaphor for the whole thing.