Moshi is famous for that, but his is usually just a couple of dried blobs with smears on the fleece where he sleeps. We use unscented baby wipes (Target brand in the purple package) — to rub the poop spots between your fingers, followed by a wipe with a wet washcloth. Anything left gets snipped out carefully. Moshi tries to clean himself and usually does a good job, but hardened blobs are sometimes too much for him to take carevof. A sanitary shave by the vet/vet tech is a good idea though, especially with longer butt fur. It doesn’t even need to be a “shave”, but more of a “bikini trim”. Marlee had this done for mats, and the fur in that area was left about a third of an inch long (now she gets brushed under there nightly).
It sounds like a veggie issue, and if that’s the case, it can be a few days of poopy butt. Is she eating alot more salad since you have your garden or just more variety? If she’s eating a larger volume, try cutting back a bit quantitywise. If the quantity is the same as always, you might eliminate anything that she hasn’t always had and try adding the items back one at a time (give the new item at least 3 or 4 days to see if poopy butt shows up before adding another new item back). Moshi’s veggie issues are quantity plus eating too fast so we have to give him a smaller salad and hand feed him to slow him down.