Here’s an example of something I did in school. Use breaks in mono-cultures to drop shadows and show height. Fields of grass reveal their stalks when they are blown sideways by the wind, and the wind can create eddies of swirling patterns which can be represented in plan. Within a larger monotonous expanse, you can “stipple” with simple plants symbols, and/or splashes of color to represent flowering plants. I also find color blending useful in creating variety.
You probably shouldn’t follow my example on the trees, though… I wasn’t extremely happy with how they came out. Which makes a great segue to another tip: practice and experiment with different methods.