Water Fern, Kiwakiwa, Octopus Fern. The bright green fronds (up to 50cm long) of Blechnum fluviatile form a very distinctive, ground-hugging rosette shape, with the sterile fronds standing erect from the centre. It develops a short trunk as the plant ages, and forms a surrounding colony. This variety requires moist, shaded conditions but is very easy to grow. New Zealand native fern.