I remember reading a thread about it like 3 years ago...it was from someone I think around the St. Louis, MO area. Do a search and you might find it (no, not who you are thinking), and don't hold me to it though.
I have never tried it. And I don't think I've seen it anywhere around Portland (the biggest city in OR - near where I live) anywhere in the 8 years I've been in business. I doubt even the big LCOs and Landscapers around here have the $$ to afford what they charge for the billboards in the big city.
BUT, I am seeing them pop up like crazy all over Salem, OR - which is where I grew up - about 50 miles from here. Smaller town, less per capita income, much smaller population, advertising costs a lot lower, etc. So that's probably why. I go back there and visit family every few weeks. And every time I've been in Salem for the past several weeks I've seen at least one or two billboards for LCOs. Not Landscapers - just LCOs. Just billboards for maintenance companies. It's weird. I've seen two different companies advertising on billboards there in the past month.
I can't imagine that's the best form of advertising or the best ROI. But I guess it must be working a little.