How do I remove witchgrass from perennials?

This article may help you solve the problem. https://www.gardeningknowhow.com/plant-problems/weeds/witchgrass-weed-control.htm

get rid of the 'GREEN' --- cut at ground level with either shears or a weed eater if you can; repeat as needed at the first hint of 'green' ... removing the leaves/stems/anything above ground will eventually rob the roots of nutrients and the plant will die... ongoing process, but no chemicals -- 15 minutes a day is better than 1.45 minutes a week!

How to Get Rid of Witchgrass Witchgrass weed control in smaller locations may occur from pulling the plants but in open fields and less managed spaces, clean cultivation methods and chemical management are recommended. Use a hoe or pull small weeds where you are able.Ensure that compost piles are staying warm and remember to turn them to prevent weed seeds from germinating. Wash any garden tilling tools to prevent spreading the seed and, in bad cases, rinse off shoes and check pant legs before walking into fields. Witchgrass weeds can be controlled with most annual weed herbicides. Broadcast spray the mixture onto the afflicted area. Only spray when temperatures are 55 degrees Fahrenheit (12 C.) or above and when winds are calm. You can also try mowing the area before seed heads form. Witchgrass will die back when cold temperatures arrive. If you can keep those annoying seed heads from producing, you can prevent problems with the grass the next year.This annual weed is common in cropland but it is not controlled by atrizine, which is a common crop herbicide. When that chemical is used, all other weeds succumb but witchweed takes their place and quickly spreads to become a nuisance. Controlling witchgrass starts with cultural amendments and finishes with a chemical herbicide for complete management.