I worked on the tree circles in the National Grove of State Trees today. I sprayed roundup on weeds; my original intention was to spot spray the perennial weeds inside the (relatively) clean circles, but I soon realized that wiregrass had been growing into the mulch from the surrounding turf. It is very happy when it can grow in a bare sunny area and less happy when it has to compete with cool season grasses that are almost able to overwhelm it in fall, winter, and spring. I knew that I had to stop it, so I sprayed the perimeters of the circles and that should keep them for another year.
What I like to think of as the "corona effect" is an interesting, and bad, phenomenon that takes place with tree circles that have at some time not been properly maintained. I would guess that this includes most tree circles; certainly it includes all those in the Grove! The bare mulched areas are inviting places for seed germination and a variety of weeds take advantage of this. If they are ever allowed to go to seed themselves, when the weeds in the tree circle proper are sprayed, a corona of weeds is left in the foot or two or three of turf immediately surrounding the circles. Untreated, these weeds become a source of seed that can continuously reinfect the circles themselves.Dandelions, oxalis, violets, plantain are all common in this location. Often they don't move farther out into established turf. Killing the entire area and reseeding is obviously a possible solution, but I prefer less invasive techniques, so whenever I am spraying the circles, I make a point of killing the largest of these weeds any any that form large enough groupings to spray without hitting a lot of turf. This is important: When you spray tree circles with preemergent herbicide be certain that you spray these corona areas as well. You won't hurt the turf; it spreads by tillering not by seed. You will prevent germination of existing weed seed. That's a good thing.