I am not sure what type of weed you are battling. Perhaps if you sent a photo I could ID the culprit. It it is a grassy weed it may be hard to control with Roundup. You may need to wait until winter, remove all the flowers but not the trees and cover the entire circle with cardboard and at least six inches of mulch. If you block all sunlight from the area for at least a year you may be able to starve out the roots of that weed. The other idea is to bring a sample to your local nursery or Master Gardener to get a name of the intruder. Then you can find an herbicide that may have it listed on the label. It does sound like this weed has infested the soil with invasive roots so you do need to be aggressive if you want a chance of growing flowers in the area. Don't give up, Keep growing, Marianne Binetti