The rhizomes of sensitive fern branch and creep, forming colonies. The leaves are in two forms: sterile (vegetative) leaves, which are fernlike but do not produce spores, and fertile leaves, which are shorter, with beadlike leaflets producing spores.
The vegetative leaves are oval to triangular overall, with a stalk that is longer than the blade. The leaves are bright green and are deeply lobed over most of the blade, but the lowest lobes, which taper at both ends, are so separated at the base as to be true leaflets. The lobes toward the leaf tip are connected with wings along the main stalk and taper little at their tips. The edges of the lobes/leaflets range from entire (smooth) to wavy to deeply lobed (near the base) but are not sharply toothed.
The fertile leaves are shorter, with the blades crowded, twice feather-compound. The leaflets (pinnae) are linear, ½ to 2 inches long, with beadlike lobes (pinnules). The pinnules fold tightly around the spore-producing structures and turn hard and dark brown when mature (they overwinter). The fertile leaves mature June–November, but spore release is March–April.
The vegetative leaves of sensitive fern usually wither quickly upon the first frost, leaving the upright, beadlike, spore-bearing spikes to be noticeable.
Habitat and Conservation
Like other ferns, sensitive fern has a two-part life cycle. The part we usually notice is the sporophyte generation, of typically fernlike plants that produce spores. Then, upon germination, the spores become gametophytes, which are very small, green, flat, heart-or kidney-shaped plants that produce eggs and sperm. The sperm require water in order to swim to the eggs, which explains why ferns typically occur in wet, cool, or shaded places. The fertilized eggs develop to become new sporophyte plants, and the cycle is complete.
Sensitive fern is slightly unusual, since its fertile leaves mature in summer and fall but retain the spores in their beadlike containers until the following spring, releasing them in March or April.
Vegetative reproduction is common, too, as the rhizomes grow along the ground, forking and branching, creating colonies of this species where it occurs.
Sensitive fern is named for its sensitivity to frost, as the vegetative leaves wither quickly when freezing temperatures arrive in fall. The alternate common name, bead fern, comes from the beadlike, upright fertile fronds. Once the sterile fronds die back, these become the only parts of the plant visible through the winter.
Sensitive fern can be used as an ornamental in gardens and landscaping. If you are considering planting sensitive fern, keep in mind that it can spread easily with its creeping rhizomes and might become somewhat weedy.
This species is native to North America and eastern Asia. People have introduced it to western Europe.
Sensitive fern is unusual for its relatively long fossil record. Well-preserved specimens that can be determined as this exact species, with confidence, have been recovered from sandstone and mudstone deposits in Canada that are of Paleocene age, more than 55 million years old.
Some types of insects feed on the leaves, stems, roots, or sap, but overall, few animals rely on this plant for food. It is listed as toxic to livestock and horses, if eaten in large enough quantities. Deer apparently avoid eating it, and their browsing of other nearby plants can indirectly help the fern to spread through reduced competition.