Portland thing?

Not sure if this is just a new trend around the country? Since I have relocated here I have noticed these informational boxes in certain neighborhoods. Usually put up on tree lawns, you know that skinny strip of city owned land  before the road. Most people here take them over and plant gardens there. Here is one box that I passed on the way to catch my bus to the community garden the other day. I also took a photo of the intriguing note left inside. Notice the yarn bombing around the post.