Wild oniongrass is hard to kill with chemicals, but it can be done. Use a combination broadleaf herbicide product, such as Trimec, that contains two or three chemicals; ask for the ester formulation. You’ll need to make the application in early spring, when the oniongrass is actively growing. One precaution: Ester formulations are a little more potent than other formulations of the same product. You should apply ester formulations only during cool weather (spring and fall) and never use them close to ornamentals.