Howardian Local Nature Reserve Close Window ~ Rigid Hornwort (Pond Weed) Ceratophyllum demersum ~ July North American native spread worldwide as a result of it's use as aquarium and pond plant. In North America, it occurs in the US and Canada, except Newfoundland. In Europe, it has been reported as far north as latitude 66 degrees in Norway. Howardian Local Nature Reserve Back to top