The informal name for
cattails, which I always think are
bullrushes but they're not, it's just that you find them growing together in the
swamp. They're called dickweeds because there's a big spike with a brown hot dog shaped tassel on the end, which looks kind of like a
dick when you're 14 years old.
In the animated
Walt Disney version of
The Legend of Sleepy Hollow,
Ichabod Crane hears the wind
drumming some dickweed against a log and he thinks it's horse hoofs. So when he sees it isn't, then he relaxes, but soon ...