The best low cost marketing (to me) is working with your current client base. You already have these people in close contact so now capitalize on them to spread the word. You can do little things like give free tree's out on earth day, have little raffles and "good customer prizes" something to keep the client close and interested in your company. That to me is the easiest way.
You can also try stuff like Sean Adams said about giving out free coffee and donuts during election day. I'm still working on that one, I can't get a hold of anyone.
I'm going to attempt to use yard signs this year also. A friend is doing lawn care on the side and all he had was a cheap little stenciled sign in his front yard (his yard is a main road in our development so it see's alot of traffic) and he has more work then I do and he's never invested in anything but that sign. I don't live on a main road but I've thought about giving a little discount to customers that allow us to put a yard sign up for a couple weeks. You can get them dirt cheap, like $2 a piece if you buy 50 or 100 on certain websites.
To do cheap advertising you need time and imagination.
\"Every strike brings me closer to the next home run\"
What if you mapped out your area and choose certain high profile intersections where you could provide possibly free flower bed upkeep or something similar for a commercial/residential property in return for being able to leave a sign there.