I also agree this is state specific but I would evict the tenants, no matter if they clean the yard or not. I would "attempt" to see if they'll do it under threat of eviction... I wouldn't mention that they're being evicted anyway.
the municipalities where I have my properties would have cited me months ago... perhaps this is a rural location where they aren't as strict.
I remember when I started reading this site there was some negative talk about collecting rent ... something along the lines of "make the tenant take responsibility". I collect rent and do part of the yard work. By doing so, they can get by with a little slack but it doesn't last long.
In my town they are driven by complaints. That may be the way it is there. No one does anything unless a neighbor or someone complains to city council represenative or inspector, then they look into it.