just did a couple tests (no pictures, they were just quick tests.) For long grass (I'm assuming you have the like inch or two long plastic kind?) there are 2 options. First, and quickest: run it with a small amount of pressure across the edge of a pair of scissors (like curling a ribbon). You can do this with any hard edge (I did it on the back of an X-Acto knife), but it's hard to get control over the curve of the grass, and too much pressure and it'll get actually curly.
Second option, and it works pretty well, is grip some in some locking tweezers (or use your hands if the heat doesn't bother you) use a hair dryer, heat on, on low. The low blows the grass somewhat sideways, and since the material is plastic, the heat helps mitigate the elasticity. After every couple seconds, switch the hair dryer off and see how it looks. You have a lot of control over how bent the grasses look this way.
Let me know how it turns out (as I said, I literally just did a test 10 minutes ago)