Sensitive Fern family (Onocleaceae)
Description: This native perennial fern forms a rosette of arching leaves about 3-5' tall. The compound leaves are pinnate-pinnatifid and dimorphic; the sterile leaves are much larger than the fertile leaves in the center of the rosette. The blade of each sterile leaf is up to 4' long, 10" across, and oblanceolate in shape, consisting of 20-40 pairs of leaflets. This blade tapers abruptly toward its tip, while toward its base the leaflets become very small (less than 1" long). Each pinnatifid leaflet is linear-lanceolate in shape and up to 5" long; the larger leaflets have 15-25 lobes. These lobes are oblong-lanceolate, rather short, and smooth along their margins (sometimes with a slight indication of teeth). The sterile leaf blades are medium green and hairless on their upper surfaces, while their lower surfaces are light green and hairless. The venation on the undersides of the lobes is simple-pinnate; the lateral veins are not forked. The central stalk (or rachis) of the sterile blade is light green and glabrous (rarely slightly pubescent); it is channeled on the upper side. The petiole of the sterile blade is up to 1' long, light green to brown, and mostly glabrous; however, young petioles have chaffy scales that are pale orange-brown.
The fertile leaves are up to 2' long, 5" across, and oblanceolate or oblong; they soon become dark brown. Each fertile blade has 10-25 pairs of leaflets that are ascending and contracted; the lobes of these leaflets have a bead-like shape. The sori (spore-bearing structures) are located on the undersides of these bead-like lobes. The petioles of the fertile blades are dark brown and rather stout at the base. The fertile leaves are produced during mid- to late summer; immature or weak plants often fail to produce them. The spores aren't released from the fertile leaves until the early spring; they are distributed by the wind. The sterile leaves are deciduous and die down during the winter. The root system consists of a stout vertical rootstock with a dense mass of fibrous roots; long rhizomes occasionally develop from the rootstock, forming vegetative clones of the mother plant.
Cultivation: The preference is light shade or dappled sunlight, wet to moist conditions, and soil that is mucky or sandy. Average moisture conditions are tolerated if this fern is watered during dry spells. The leaves are delicate and easily damaged. In more heavily shaded areas, Ostrich Fern often fails to produce fertile leaves.
Range & Habitat: Ostrich Fern occurs in widely scattered locations in northern and western Illinois, where it is rare (see Distribution Map); elsewhere in the state, it is absent from natural areas. Habitats include moist rich woodlands, swamps, and soggy thickets in both sandy and non-sandy areas. Ostrich Fern is circumboreal in its distribution, occurring in parts of North America, Eurasia, and East Asia. The North American variety is identified as var. pensylvanica. This fern is often cultivated in gardens and along the foundations of houses and other buildings.
Faunal Associations: The caterpillars of the Ostrich Fern Borer Moth (Papaipema sp.) bore through the stalks and/or root system. This rare moth is found in the New England area of the United States, where the Ostrich fern is more common, although it has not been found in Illinois to my knowledge. Because this fern has large leaves and often forms vegetative colonies, it can provide substantial cover to various kinds of wildlife where it is locally common.
Photographic Location: On the grounds of Japan House, which is located at the Arboretum, University of Illinois, in Urbana.
Comments: This a lovely fern with tall arching leaves. It is relatively easy to identify because of the oblanceolate shape of its large leaves, which have very short leaflets (pinnae) toward the base of the blades. Very few species of ferns have such short leaflets. One of them, Thelypteris noveboracensis (New York Fern), has elliptic leaves that taper very gradually toward both their bottoms and their tips. The leaves of Ostrich Fern, in contrast, taper abruptly toward their tips. Another distinctive characteristic is the simple-pinnate venation on the undersides of the leaflet lobes, where the lateral veins are undivided and straight. Other similar ferns (e.g., Osmunda spp.), often have forked lateral veins on the undersides of their leaflet lobes. The small fertile leaves in the center of the rosette have a rather peculiar appearance, although they are often hidden by the larger sterile leaves.