Here's a link that I posted recently on another thread...take a peek at the entire text contained within the original link.
Note that the author states that the patterns of the painted imagery on the barns have been around since at least 1900, but they're much older than that.
I find it interesting that many barns are made to look as if they are inhabited buildings. Remember, the most important building on a small farm is the barn; it's usually the first real structure to raised and it ordinarily took a community effort. Some appear to have church steeples. False windows are very common. Many embellishmensts are much more ornate than any found on the actual residences. This took a lot of time, material and effort hen there is lots of other work to be done.Sort of a "if they show up, maybe they'll think that's the house until daybreak and they'll spend all their time trying to break into a false window" mindset. lol
Circle phenomena is not unknown here; it happens on a regular basis (with a 'twist') in the South if you know where to look. Trouble is, in the deep South (esp. southern Appalachains), grain crops are not the predominant crops. Except for corn, of course. Grain crops are easier to 'work' with...a tauter canvas, if you will.
We called them "weavings". Muscadines and Scuppernogs would also have "weavers" visit them.
There does to seem to be some commonalities beyond the actual designs. Time of year, plant type, date of planting, age of the crop, phase of moon, proximity to ancient sites (known or otherwise) etc. all seem to have correlations.
Excellent point you've pondered on a fascinating subject, Deep1. Hope I helped a bit.