A small plant typically found in the southern United States. Rowling saw hogwort at Kew Gardens circa 1992 and the name stuck in her mind.
From an interview with Rowling:
Do you put images from your dreams into your books?
Ideas come from all sorts of places and sometimes I don't realise where I got them from. A friend from London recently asked me if I remembered when we first saw Hogwarts. I had no idea what she was talking about until she recalled the day we went to Kew Gardens and saw those lilies that were called Hogwarts. I'd seen them seven years before and they'd bubbled around in my memory. When Hogwarts occurred to me as a name for the school, I had no idea where it came from. (SMH)
Croton capitatus, known as the hogwort or woolly croton, is an annual plant with erect, branched stems, densely covered with light brown, wooly hairs that give it a whitish appearance. It grows in dry, open areas, especially sandy and rocky soils. It is distributed across the southern United States, and elsewhere.