A shin-high carpet of ferns covers the ground in the shaded understory of a redwood forest. The leaves are shaped like a narrow triangle and have a shaggy reddish hair along the lowest parts of their stems. This is coastal wood fern (Dryopteris arguta), a common sight throughout Marin.
One of the things you need to look at to identify ferns is the number of times each leaf is divided. Each stalk rising from the ground is a leaf; the smaller leaflets branching off the stalk are pinnae. If the leaf branches only once, it is once-pinnate. The leaves of coastal wood fern are 2-pinnate, meaning the smaller leaf also branches off the smaller central stalk (see the photo). The other species of Dryopteris in the area looks similar but can’t be confused with this one because its leaves are 3-pinnate.
Another common lookalike for coastal wood fern is lady fern; but instead of being roughly triangular, each lady fern leaf tapers so that the leaf outline is diamond-shaped. In other words, the lowest wood fern pinnae are among the longest. In lady fern, the pinnae that are nearest the ground are quite small (click here for a photo).
Other names for this plant are shield fern and California wood fern.