Interrupted Fern is a vase shaped fern, to 3+ feet in height, whose shape is very similar in appearance to the Cinnamon Fern, but unlike the Cinnamon, the Interrupted Fern frond can bear both sterile and fertile pinnae (pinnae is the plural of the smaller leafy side branches that grow off the main stem - individually they are pinna, and if the pinna itself has one or more distinct secondary segments, as does this species, these are called pinnules). The fertile fronds are more erect, the sterile fronds more spreading. The frond blades are pinnate-pinnatifid.
Fronds - fertile: A fertile frond is interrupted in the middle by 2 to 5 pairs of smaller spore producing fertile pinnae that resemble brown clusters of very small grapes. These clusters are the "sporangia". These fertile pinnae, during their time on the plant, remain small and are thus interrupted in their growth. Above and below that "interruption" the pinnae are the normal yellow-green color, widest at the base, but those below the interruption are much shorter and ascending in position, more widely spaced. Fertile fronds are taller than the sterile fonds and more erect.
Fronds - Sterile: The sterile fronds are those without any fertile pinnae; they are broadest in the middle area, cut into 10 to 20 pairs of pinnae, woolly at first, then losing the hair. There is also a lack of hair at the base of the sterile pinnae underside which helps distinguish this fern from the Cinnamon Fern whose sterile fronds look similar to the sterile fronds of Interrupted Fern.
Pinnules: The pinnules become wider toward the central stalk (costa) of the pinna. The main vein of the pinna costa does not branch into the individual pinnules. The tips of pinnules are bluntly rounded and the bases are not quite cut clear to the costa of the pinna.
Fertility: The 2 to 5 pairs of smaller spore producing fertile pinnae have "sporangia" that resemble brown clusters of very small grapes. Instead of sori housing the sporangia, the fertile pinnae of this and other Osmundas have bare sporangia on their fertile pinnae. These small brown clusters of sporangia, are light green initially, then dark green at maturity at which time they open and release their spores, then they turn dark brown after they have discharged their spores and fall off in late summer.
The fiddleheads (Croziers) are stout, with their woolly white hair, typical of Osmundas, and are some of the earliest to emerge in spring. Mature fronds are smooth and free of this hair. Croziers were once considered edible but are now known to be carcinogenic. If only Eloise Butler was aware of that! (read her note at bottom of page).
Habitat: Interrupted Fern grows in both shade and open areas. The plant prefers alkaline, drier soils, well drained, not marshy. Roots are a tangled mass of wiry black growth growing from a large rhizome.
Names: The genus Osmunda has an obscure derivation. Some believe it is named after an early Scandinavian writer Osmundus and some say it is named for Osmunder, a Saxon name for a Celtic god Thor. The species name, claytoniana, was an honorary named for the English born Virginia botanist John Clayton, (1694-1773) who lived in Williamsburg. His name is on several other plants, including Spring Beauty, Claytonia virginica. The author name for the plant classification, 'L.' is for Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy.
Comparison ferns: There are three that could be confusing - Royal fern (The fertile pinnae have the sori at the tips of the fertile green fronds), Cinnamon Fern (The fertile fronds are distinctively separate with the pinnae appressed to the rachis) and Ostrich Fern (fertile frond is also distinctively separate but the fertile pinnae arch outward from the rachis, not appressed like the Cinnamon).
Above: 1st photo - Sterile and fertile fronds are hard to distinguish until the fertile pinnae appear just below mid-blade (2nd photo), causing an 'interruption' in the frond. These will fall away in late summer. Below the interruption, the sterile pinna are smaller.
Below - The fertile pinnae: 1st photo - as they mature the sporangia are a greenish brown color. 2nd photo - Reaching maturity they release their spores and they turn brown. 3rd photo - After maturity the sporangia dry up and fall away from the frond.
Below: 1st photo - Note the veins of pinnules do not join the costa of the pinna (the side branch - horizontal in the photo). 1st and 2nd photo - Each pinnule on the pinna becomes wider toward the central stalk (costa) of the pinna and the pinnules, each slightly offset from the opposite one, are not cut cleanly all the way to the costa - said to be pinnatifid.
Below: Detail of a mature sterile frond.
Below: Frond Development
Below: 1st photo - The fiddleheads with their woolly white hair, typical of Osmundas, emerge early in spring. Mature fronds are smooth and free of hair. 2nd photo - The fertile fronds developing with the light green sterile pinnae above and below the fertile pinnae - now a dark green color. 3rd photo - The fertile pinnae at maturity - a greenish-brown color.
Below: A views of the hillside that is covered by Interrupted Fern at Guidebook station #11.
Below: Photo from early October with the leaf canopy thinning out and the bed of Purple Trillium in the foreground lying dormant. By late Summer the fronds of Interrupted Fern will start to die back and the entire area will be bare. In 1938 however, Garden curator Martha Crone reported that on Sept. 30th, the fern back looked as lovely as it did in May. Each season is different.
Notes: Interrupted Fern is indigenous to the Garden area. Eloise Butler catalogued it on April 29, 1907. It was planted extensively in the Garden between 1958 and 1960 when the Fern Glen was developed. It was the most common fern put into the new area. Martha Crone planted 501 plants in 1956, 210 in 1957 and 375 in 1958. Ken Avery added another 150 in 1960 when he finished the Glen. It has been planted again as recently as 2006. While native to Minnesota throughout all wooded areas, in North America it is known in the entire eastern half of the continent except Louisiana and Florida in the south.
In North America there are only 4 main species of the Osmunda genus: O. cinnamomea, the Cinnamon Fern; O. claytoniana, the Interrupted Fern; O. ruggii, a sterile hybrid found only in Virginia; and O. regalis, the Royal Fern. The latter has two varieties but only one is found in North America - var. spectabilis. O. cinnamomea has had four varieties described in North America but current authorities such as Flora of North America do not distinguish them. All three main species are represented in the Garden. The hybrid in Virginia is described as - Osmunda ×ruggii R. Tryon [claytoniana × regalis].
Eloise Butler wrote: "The white starchy bud of the interrupted fern was a delicious morsel well worth long and hard digging to procure. It has a taste peculiar to itself and I think it would make an excellent salad." from Children's Forage Plants in the Wild Garden, Jan. 1915, unpublished [BUT see note above on toxicity] and "Indeed, the most spectacular features of the garden is a hillside densely clothed with the Interrupted Fern. . ." from Ferns in the Wild Garden, June 1915, unpublished.
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"