Means that you use a tree or a bush as an umbrella. Basically because you're too damn mean to buy one. You can also make it a little more realistic by non-watering it, so it would become yellow.
Alejandro: Hey Sha, forecast says it's going to be really HOT this summer. Are you buying an umbrella for the yard?
Shayan: God NO! I've just kept my tree with no water during all the spring so now it is yellow dry Eco-friendly Umbrella and I can use it to sit in the shade!