Distinguishing Polypodium Ferns

Dummies guide to Polypodium genus ferns in Washington/Oregon/British Columbia, where there are four common species that can be hard to distinguish, especially amorphum and hesperium.

P. glycyrrhiza (licorice fern) is usually on tree trunks in wet forest or on large wet rocks associated with luxuriant moss growth, and grows one frond at a time from a rhizome. P. amorphum and P. hesperium are often on rocks or rock faces in surprisingly dry looking spots with a fair amount of sun. Look for P. glycyrrhiza especially on big leaf maple trunks (Acer macrophyllum).

P. scouleri is strictly Pacific coastal and has thick leathery leaves, usually growing on tree trunks. Only near true ocean, hardly ever found on inland waterways such as the Puget Sound.

P. amorphum vs P. hesperium:

P. amorphum usually retains previous years fronds (P. hesperium does not).

P. amorphum sori contain specialized spore-like bags on a short stalk called sporangiasters, which look rather like bunches of grapes scattered among the sporangia (https://www.inaturalist.org/observations/103057091). P. hesperium does not (https://www.inaturalist.org/observations/103039917). Each sporangiaster cluster is about the size of the adjacent sporangia (sacks of spores) and, at the right stage, is translucent and jelly-like in appearance. This character requires an extreme closeup picture or a good hand lens to see and is only apparent at the right stage of spore development (late spring/summer?). Probably involved in keeping spores hydrated/conditioned during development.

P. amorphum range is mostly in west Cascades and Coastals. P. hespirium is mostly farther east, except around Mt. Rainier. P. amorphum general range is much farther north.

P. amorphum fronds look a bit more waxy and thick compared to thinner P. hesperium (??)

P. californicum is much farther south. P. calirhiza is farther south and is (roughly speaking) the southern form of the licorice fern P. glycyrrhiza. P. saximontanum is in the eastern Rockies and has scalloped leaf edges. P. sibericum is much farther north.

Posted on July 31, 2022 12:39 AM by jhorthos