Long ago, a friend gave my wife a rhubarb start and said "Just plant it anywhere in a sunny place and it'll grow like crazy". I planted it on the south-facing side of a building. After several years, it was still there, just about the same size, and not big enough to harvest more than a couple of stems each season. Then I moved it to a raised bed last summer, and this spring it's way bigger and we've taken lots of stems. Turns out, rhubarb will grow just about anywhere, except in a poorly drained clay soil.
And about all that "wild rhubarb" that Natalie saw at her son's soccer game; There's a plant called Burdock (google it) that looks very much like rhubarb, and it's commonly found on forest edges and over-grown, neglected areas. Later in the season, stalks of thistle-like flowers appear, that later turn into round, acorn-sized burrs. If you have a shaggy dog who runs loose very much, you've probably pulled burdock burrs out of his fur.
As far as treatment for varroa mites, I think MB is right, there's probably not enough oxalic acid in the leaves to do the job, and if there was, you'd have to do something like put a bunch of leaves in juicer and then spray the resulting stuff all over the bees and the inside of the hive. I'm sure it would be every bit as effective as putting dog-hair clippings around the lettuce patch to keep rabbits away.