Hairy and hoary puccoon, despite their similarities, don't seem to like each other very much. You will rarely see them close to each other. One item mentions that hairy does not like limestone. That leads me to suspect the difference is limestone in the water. Hoary seems to like places with limestone dissolved in the water. Hairy likes sand, which is often limestone, but in well drained areas like beach dunes that have more drainage, and therefore less mineral in the water. Our area in and near Lenawee County has a lot of limestone glacial deposits where water has a high mineral content. Hairy puccoon was only collected here once, many years ago, along a railroad. Hoary is fairly common among the sandy and gravelly glacial hills. The puccoons are among the flowers with styles and stigmas of differing lengths, which helps prevent self fertilization.