One of the less appreciated sights of autumn is the change in ferns as winter approaches. While lacking the spectacular color of the autumn trees, ferns undergo color changes as well. The fronds of most ferns are deciduous. For an exception to this, click here. The fronds of most ferns die back in winter, and the fern persists as an underground rhizome until the next spring when the familiar fiddleheads re-emerge.
A common fern in the Pennypack Preserve is the Sensitive Fern (Onoclea sensibilis). Its common name does not reflect that its feelings are easily hurt, but rather that it is sensitive to cold. The first chilly days of fall cause the rhizome to cut off circulation to the frond which gradually dies back. You can see this in the Sensitive Ferns in the preserve now, in mid-October.
While I find ferns in general to be somewhat difficult to identify, Sensitive Ferns are quite easy. The structure of a fern frond consists of a mid-rib, with multiple pinnae sticking out from either side. In many ferns the pinnae are complete separated down to the mid-rib. See the picture of Christmas Fern here for an example. However, the fronds of Sensitive Fern are pinnatifid, meaning that the pinnae are divided, but not all the way to the mid-rib. A wing of tissue extends the length of the mid-rib as seen in the picture below taken in late summer.
This feature is less evident as the fronds die back and the tissue along the mid-rib withers and decomposes.
Like many ferns, there are two kinds of fronds. The ones pictured above are trophophylls (vegetative fronds) whose purpose is photosynthesis and energy production. In summer, sensitive ferns also produce sporophylls (fertile fronds) which produce the reproductive spores. These fronds are narrower and bead-like giving these ferns their other common name, bead ferns. Sensitive Ferns are wide-spread and common throughout the eastern half of the US. However, this is your last chance to see them, since the fronds will mostly die back in winter. Take notice when you are out enjoying the fall colors.