Shape: A ancient colony forming fern with fronds that are broadly triangular, the fronds usually reclining giving an overall "formless" shape.
Sterile Fronds: The sterile fronds are pale sea-green in color and turn yellow in autumn. They may be 8 to 40 inches long. They are easily killed by frost and thus comes the common name "sensitive". The sterile fronds are pinnatifid (the frond is once-divided with the divisions not reaching to the rachis [central stem]) to 1-pinnate (see photo for illustration). There can be up to 12+ pairs of sterile pinnae, arranged opposite to nearly opposite each other.
Sterile Pinnae: The pinnae of sterile fronds are quite broad with the central vein depressed and the smaller veins form a "netted" network which is unusual in the fern world. The rachis is smooth and the lower pinnae and next to the lowest are usually the longest, giving the triangular shape to the frond. The upper pinnae are not cut to the rachis but have wings of tissue connecting them. Margins are wavy and indented but not toothed.
Fertile Fronds: Spores are produced on separate erect fertile fronds which are green in the growing season and turn brown in the autumn. The fertile fronds persist through winter and release their spores to the wind the following spring before new leaves form.
Fertility: The sori (spore producing organs) are bead like and arranged toward the top of the fertile fronds on short upward ascending pinnae. The pinnae edge rolls over to cover them, thus replacing the need for an indusium.
Fiddleheads: Spring fiddleheads are a pale red with fine whitish hair.
Habitat: The plant grows from a creeping rhizome in moist to wet soil, neutral to acidic and in sun or shade. They are fast growers and spreaders, almost invasive, (see photos below), thus needing frequent division if you wish to control them. In the garden they mix well with Siberian iris, hosta, or turtlehead. The fertile fronds work well in dried flower arrangements or provide winter interest in the landscape. Sensitive Fern grows in hardiness zones 2 to 10.
Names: The genus name Onoclea, is from the Greek onos, meaning 'vessel' and kleio, meaning 'to close' and refers to a closed vessel, referring to the sori enclosed in the rolled margins of the fertile pinnae. The species name, sensibilis, refers to the plants sensitivity to frost. The author name for the plant classification - 'L.' refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy.
Comparisons: Sensitive Fern has some resemblance to the Netted Chain Fern, Woodwardia areolata, (not found in Minnesota) which also has netted veins, but there the pinnae have fine teeth on the wavy margins, the pinnae are not opposite, but alternate, and the sori are produced on fertile fronds that have pinnae resembling those of the sterile fronds.
Above: Various shapes of the fronds. Note in 2nd photo, some of the pinnae do not divide all the way down to the central rachis - referred to as "pinnatifid".
Below: Spring fiddleheads of pale red color with unfolding pinnae
Below: 1st photo - The "netted" network of veins along the depressed central vein. 2nd photo - The base of the fronds, emerging from the rhizome. 3rd photo - The fertile frond with developing "sori" in early Fall.
Below- The fertile fronds: 1st photo - in the forming state in early August. The individual roundish structures are the "sori" which contain the spore forming organs and are enfolded by the pinnae margin. 2nd photo - The developed fertile frond turning to the brown color of autumn (this one in early September). This frond will overwinter and release spores in the spring.
Below: Three views of the over-wintering fertile frond showing the mature round sori still enclosed by the rolled over edge of the pinnae.
Below: Sensitive Fern can be invasive in certain circumstances. The photos below show an area of a large marsh in western Minnetonka MN that has extensive growth of the plant, almost to the exclusion of other plants within the growing area except for Water Smartweed. Winter view on top, spring view below.
Notes: Sensitive Fern is indigenous to the Garden area. Eloise Butler catalogued it on May 25, 1907. She planted 6 more in Oct. 1918, sourced from a bog off Superior Ave. near the Garden. Martha Crone added plants in 1946. It has been present on all Garden census' and was most recently planted in 2006. It is found in North America from the Rocky Mountains eastward. In Minnesota it is native to most counties except those in the drier, less wooded West and South particularly the SW quadrant. O. sensibilis is the only species in the genus. Fossils of the species have been found dating back 60 million years - with little change in appearance.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"