As noted yesterday, Koreans like to take care of their trees, especially if they're in a city. Here's another method they use: tree IVs. A fair amount of the trees on campus have these bottles in them, feeding them tasty tree food. You can also buy little bottles to put into house plants, etc. I guess you can buy "plant food" in America, but I've never seen a tree hooked up to an IV.
--If you look closely at the previous picture, you can see some of these bottles.