Scientific name: Onoclea sensibilis
Common name: Sensitive Fern
(Information for this species page was compiled in part by Amber Bucinski for Biology 220W at Penn State New Kensington in Spring 2002)
Sensitive fern was given its common name by early European settlers of North America who noted that the fern's
fronds quickly died back after the first frost of autumn. This fern is also typically very intolerant of
drought and direct sunlight, so the adjective "sensitive" is most applicable and appropriate. Sensitive fern is a perennial, deciduous plant that can actually become
invasive in sites or habitats that satisfy its narrow tolerance ranges for shade and moisture.
Sensitive fern fronds arise directly from its root (the rhizome). The rhizome is a thick (up to one half inch), brown structure found just beneath the soil surface. Numerous adventitious roots extend from the rhizome and their mass and distribution can make the soil surface around the ferns lumpy and irregular.
Sensitive fern produces two distinct types of fronds: sterile (or
vegetative) fronds (pictured above and at right) and fertile (or reproductive)
fronds. The sterile fronds can be up to three or four feet in length and are typically light green, tough and leathery, with a broad, almost triangular overall shape. These fronds arch outward as they grow forming a broad, encasing plant volume. These sterile fronds have a regular, alternating system of up to twelve pair of straight leaflets
(pinnae). The sterile leaflets have prominent venous networks and a sparse covering of white hairs on their undersurfaces.
Fertile fronds, on the other hand, are shorter, held more up-right, and are extensively covered with numerous, small leaflets. These fertile leaflets are each composed of many bead-like sub-structures called pinnules that house the spore producing
sporangia. Pinnules become dark brown when mature and generate an extremely distinctive appearance for this fern species. It should be noted that sensitive fern can display a wide range of frond shapes and sizes. This morphological variability is thought to be an expression of a significant range of genetic diversity within the species.
Sensitive fern propagates itself vegetatively via its spreading rhizomes. In the spring (typically May) pale red, new leaves arise from the
rhizomes (see image at left). The dense, colonial masses of sensitive fern reflect the potential and vigor of this rhizomal, vegetative propagation strategy. Sexual reproduction is also seen in sensitive fern, and the species is capable of dispersing itself via its tiny, wind-blown spores. Spores are produced in the sporangia of the fertile leaves in July and August. A given plant can produce millions to billions of spores during its lifetime! Very few of these spores, though, will land in spots suitable for growth and development. Spores that do develop will form the gametophyte life stage of the fern within which sperm and ova will be generated and fertilization will take place. The sporophyte life stage formed by the fusion of the sperm and ovum can then grow into a new, independent fern plant.
Sensitive fern is native to eastern North America and eastern Asia. It has also been introduced via human activity to western Europe. It is often cultivated in shade gardens and in the wet soils surrounding ponds and streams. In its natural range, it is most often found in very wet habitats such as swamps, marshes, bogs, and in the wet woods along streams and rivers. It is relatively uncommon in upland or even moderately dry forest ecosystems. Full shading, as mentioned previously, is essential for this species' survival. Under optimal conditions of full shade and high soil moisture it quickly spreads to form expansive, homogeneous colonies.
Sensitive fern like most other fern species contains numerous, toxic chemicals in its tissues that are excellent defenses against both invertebrate and vertebrate herbivory. Deer seldom browse sensitive fern
and may actually work to accelerate the fern's growth and spread by their browse removal of
less toxic, under-story plant competitors. The toxic and sometimes fatal effect of sensitive fern on livestock and horses has been extensively documented. Avoidance of sensitive fern by grazers and care not to allow sensitive fern fronds to be incorporated into hay is essential.