Ferns for the Garden Landscape: The genus Osmunda
Ferns have become an indispensable addition to the modern garden, especially for those gardeners who have shade issues or utilize water features. Each year, more new fern cultivars seem to arise. The fronds may be quite coarse or very lacy, in a multitude of green shades. More recently reddish and silvery-grey foliaged cultivars have arisen. The genus Athyrium, commonly called the lady ferns, are among the most important fern genera, both historically as well as in recent times. However, the focus of this article, as a beginning to a series of articles devoted to ferns, are the so-called flowering ferns, the genus Osmunda.
At this stage it might be useful to describe some fern terms. The leafy portion of ferns are called the fronds and the leaf stalk are called stipes. The fronds are typically divided into smaller leaflets called pinnae. Ferns do not reproduce by seed, rather they produce dust-like spores that drift for miles on the gentlest of breezes. These spores are produced on the undersides of the fronds in structures called sporangia. In more advanced ferns, the sporangia have a umbrella-like fruit cover called the indusia which protect the often round to crescent shaped sporangia call sori (commonly these are called fruit dots). Ferns produce two types of fronds: fertile and sterile. The fertile fronds house the fruit dots, typically along the frond margins or on the underside of the frond. Sterile fronds lack fruit dots. Often both the fertile and sterile fronds look alike but in some fern the two types of fronds can look quite different and are referred to as being dimorphic fronds.
So now back to the genus Osmunda. The name ‘Osmunda' may have arisen from two sources. The Nordic God Thor was also called Osmunder, a possible source of the genus name. Alternatively, the name may have arisen from the Latin ‘os' meaning mouth and ‘mundare', to clean, referring to the use of the leaves as a mouthwash.
The genus Osmunda contains 12 species whose species range from the arctic circle to the tropics. They are considered one of the more primitive ferns. All are relatively large with bold, coarse fronds arising in a crown pattern from stout rootstocks. Their roots are characteristically black and wiry. The main characteristic which separate Osmunda from other ferns are based on differences in their reproductive structures. Among Osmunda, the sporangia lack indusia and rather than appearing as dots on the undersides of the leafy fronds, the spore cases are globular, densely-arranged and are found on modified fertile fronds. These fronds appear green initially, but once mature, they turn reddish-brown. The leafy sterile fronds last all season, but the fertile fronds are very short-lived, usually shrivelled and dropped by mid-summer.
Other features of Osmunda include their rich yellow, orange to cinnamon-brown fall colour (none of the Osmunda we grow in the north are evergreen). The newly emerging fiddleheads (croziers) are densely covered in loose wooly scales which are shed as the fronds unfurl. This lends the ferns a silvery appearance in early spring.
Early spring croziers (fiddleheads) of typical Osmunda
Cultivation is quite simple; moist, organic-rich soil is the main prerequisite. If the soil is adequately moist, they can tolerate full sun but they may also be used in shadier situations. However, as a rule, they are not as shade-tolerant as most standard ferns. Propagation is from division, which can be a chore since the root-stocks are quite large and tough, or from spores. If propagating from spores, it should be noted that unlike most ferns, Osmunda spores are short-lived and need to be sown fresh. For gardeners in temperate regions, we are concerned with three main species; the royal fern, O. regalis; the cinnamon fern, O. cinnamomea and the interrupted fern, O. claytoniana. All three species are hardy to zone 3.
Of the three, the most unique is the royal fern. This fern is quite variable in size, ranging from 60 cm to nearly 2 m! Its distributional range is cosmopolitan, among the most widespread of all ferns. In the wild, they commonly grow in rich fens and along stream banks, even growing into the water. This makes them usable as a marginal plant in water features. The fronds are more or less triangular with the pinnae arranged in a panicle pattern. This makes them among the most unique ferns and lends them a particularly attractive form. The fertile, spore-producing pinnae, which are located at the tips of the leafy fronds, shrivel shortly after releasing their spores. Besides the wild form, there are three royal fern selections. ‘Crispa' has crisped margins on the pinnae while ‘Cristata' has fine crested margins. ‘Purpurascens' has bronzy-purplish new fronds whose stipes (frond stalks) remain purple all season. The roots of royal fern are the source of osmunda fibre, a media commonly used historically when cultivating orchids.
Details of the royal fern; these pictures clearly show the fertile pinnae located at the tips of the sterile fronds
The most common of the Osmunda ferns in my local area is the cinnamon fern. This fern has dimorphic sterile and fertile fronds. The fertile fronds appear first and are located in the center of the crown-like sterile fronds. They are spike-like and erect, first green but then becoming cinnamon-coloured (hence the common name). These quickly shrivel and die by mid-summer. The common name is in reference to the fertile fronds which turn cinnamon-rust while the spores are being shed. The somewhat glossy, sterile fronds are lance-shaped and may reach 90 cm in length. This species is found in both North and South America as well as east Asia, where they inhabit sphagnum bogs, fens, swamps and swales. This species is the most sun-tolerant of the three species.
Mature cinnamon fern showing the separate fertile and sterile fronds, along with close-up details of the fertile fronds before and after the spores are shed
The third species is the interrupted fern. This species looks superficially like the cinnamon fern. However, the fronds, which may exceed 90 cm, are mat-green rather than glossy. Located centrally on each fertile frond is four or more pairs of dark green leaflets which bear the spore cases in dense clusters. When mature, these leaflets turn rusty-red to brown, then later shrivel and die. This results in an interrupted space along the stem of the fertile frond, hence the common name. These fertile fronds are taller and more erect than the gently arching sterile fronds. This species is found both in North America as well as China, commonly growing along streams but also rocky slopes. This species can tolerate drier soils than the other two.
Details of the interrupted fern; the left picture shows the fertile pinnae set along the stems between the sterile ones.
For those gardeners living in zones 7-8 there are a few other Osmunda you could try, if lucky enough to find a source. These include the Asian species O. gracilis, O. japonica, O. lancea and O. schraderi. All of these look superficially like royal ferns in regards to the sterile fronds but differ slightly in regards to their fertile portions.
For a bold look created by foliage, the Osmunda are second to none. They combine beautifully with Ligularia, Hosta, Cimicifuga, Polygonatum and other moisture-loving, shade-tolerant perennials. And unlike some ferns which can be a little invasive, these ferns remain as discrete clumps. If you can find a source, I can highly recommend them.
I would like to thank hczone6 for use of the royal fern pictures and croziers of cinnamon fern; equilibrium for the picture of the potted interrupted fern and gmarr for the garden picture of interrupted fern
This article was originally published on November 1, 2008. Please note that the author may not be available for answering questions.
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