Since Giant Foxtail is an annual grass that dies out over the winter, discouraging seed germination in spring might be more effective than trying to remove a current generation of the weed. When working with stormwater or bioretention areas, you may want to consult with your local Soil Conservation District or your regional Watershed Restoration Specialist, to verify which weed management/suppression approaches would be recommended to avoid any negative impacts to the habitat and stormwater retention pond functionality, and to see if they have any particular suggestions moving forward. (Which native plant species to try growing based on site conditions, for example, if prior choices failed for some reason like lack of enough water or some other environmental factor.)

Miri

You're welcome.