Edge trimmer/fence posts
When I install fence posts I keep this in mind and put a border of concrete or even tamped down gravel one or two inches around. In other words, if you set posts in concrete, pour it all the way to the top. I usually pour it just a little above grade and smooth it to grade away from the posts. If you look closely, you can see the concrete or if you prefer, pour it at grade. Better than having to gouge the posts, etc. As far as the gravel, I don't mean leave it so high that it will pick up with weedeaters. Tamping it down good will discourage most growth and trimming will be minimal.
In your case, you could just dig out a collar and fill with concrete or gravel.
That's just how I do it.
If it's worth doing, it's worth doing right. If you wouldn't put your name on it, it ain't done right!