Once your hardy marginal plants begin to brown, usually after the first hard frost, prune back excess foliage and discard any decaying material, so it does not compromise the water quality over the winter months.  Potted bog plants such as Pickerel, Thalia Dealbata, and Japanese Variegated Iris should be placed at a sufficient depth to avoid freezing the crown of the plant. Most of the other hardy bog plants can be left as they are and will return the following spring.