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Osmunda Ferns
by Liesbeth Leatherbarrow
by Liesbeth Leatherbarrow

Liesbeth has written for several western gardening publications, including Gardens West and The Gardener for the Prairies. She has also co-authored three gardening books: The Calgary Gardener (with the Calgary Horticultural Society), The Calgary Gardener, Volume Two: Beyond the Basics (with Lesley Reynolds), and 101 Best Plants for the Prairies (with Lesley Reynolds).

May 6, 2001

A few years ago, while in the midst of a major garden renovation project, I realized that one thing I longed for was the cool, quiet serenity and lush green foliage of a fern garden. I also knew that this desire was somewhat unreasonable given that Calgary finds itself in the middle of a semi-arid designation on most climatological maps. At the same time, I was convinced that if I chose my spot carefully for semi-shaded light conditions, amended the soil with ample leaf mould, paid strict attention to meeting moisture requirements, and was willing to engage in a certain amount of trial and error in fern selection, I would, in the end, have my fern garden. And I do. The increasing availability of ferns in local greenhouses and my philosophy of "everything is worth a try" has made it easy to experiment. I now have over a dozen species of ferns that are coming along beautifully, albeit a bit slowly, with two or three new ones taking the winter hardiness test for the first time this year. Just because the label says that a fern is meant for zone 4 or 5 doesn't mean it can't or won't survive in our zone 3 climate!
Some of my favourite ferns, and also some of the easiest to grow here in Calgary and on the rest of the prairies, are the Osmund or Osmunda (os-mun-da) ferns. With only ten species found worldwide, they represent a small genus, and, incidentally, the only cultivated genus of the family Osmundaceae. They take their name from Osmunder, Saxon name for Thor, the Norse god of thunder, weather, and crops. Botanically speaking, osmundas form a link between modern and ancient ferns that thrived during prehistoric times.
As a group, osmunda ferns make a very bold presentation in the garden, growing on average from 2 to 4 feet tall during a season, when planted in a semi-shaded, humus-rich, damp location. Their divided, feathery-looking leaves (fronds) grow in an erect manner from a basal cluster, forming large vase-shaped crowns of erect stems. In general, the sterile or foliage fronds are different from the fertile or spore-bearing fronds, which gives added visual interest to an already pleasing plant. Osmundas are some of the earliest ferns to make their appearance in the spring, and their hairy, matted, woolly-looking fiddleheads (croziers) make them easy to identify. So does their rapid rate of growth. Each spring I marvel at how quickly the fiddleheads appear (breathtakingly beautiful when backlit by the sun) and then unfurl into stately fronds. During their growth, osmundas develop massive root systems which are often shredded and sold as "osmunda fibre," a very popular orchid-growing medium.
Their love for a cool, damp location make osmunda ferns ideal for planting in a woodland garden or at the edge of a pond or stream. Use them in clumps or drifts, or interplant with other ferns and shade-loving wildflowers such as woodland phlox and primulas. Because osmundas can tolerate a certain amount of direct sunlight (as long as their roots don't dry out!) they can also readily be adapted to most perennial beds where they become dramatic accents and an excellent foil for the myriad colours of flowering plants.
The cinnamon fern, Osmunda cinnamomea (O. kin-a-mo-mee-a), forms clusters of dark green fertile (spore-bearing) fronds that emerge early in the spring. The fertile fronds gradually turn from green to a bright cinnamon brown (hence the name) and eventually wither and fall after they have shed their spores. The sterile (non-spore bearing) fronds emerge a little later in the spring and maintain their green colour throughout the season. They are generally taller than the fertile fronds, have rusty woolly stalks, and blades that are deeply cut into lance-shaped segments. In autumn, the fronds turn from russet to a golden colour. 
The interrupted fern, Osmunda claytoniana (O. klay-ton-ee-ah-na) was named after John Clayton (1686-1773), a Virginian botanist. The fronds of the interrupted fern are also divided into many deeply cut segments. Most of the outer leaves are almost exclusively foliage (non-fertile) fronds while on some of the inner leaves can be found, towards the centre of the frond a few pairs of leaflets, greatly reduced in size, brownish and covered with spores. Once the spores have been dispersed, these spore leaflets wither and drop off the plant, leaving an empty space or interruption in the middle of the frond. The rest of the fern's leaflets remain green and don't die back until the first frost.
The royal fern, Osmunda regalis (O. ray-gah-lis) is one of the largest ferns in North America, often growing from 6 to 8 feet under ideal conditions. Its leaflets are elongated and blunt-tipped, without teeth or divisions, and turn from red to green as they mature. Narrow fertile (spore-bearing) leaflets can be found clustered at the tips of inner fronds. Spore capsules covering the veins of these leaflets turn brown as they ripen and resemble the dead flowers of an astilbe plant. O. regalis 'Cristata' has "crested" fronds on which the leaflets at the tips fork into finger-like divisions. O. regalis 'Purpurascens' has young fronds that are deep copper-pink and that mature to a copper-green with purple midribs. 
Plant osmunda ferns in early spring in a humus-rich soil with their crowns at soil level. Early spring is also a good time to divide widely-spaced crowns for replanting; if closely set crowns are cut, they are not likely to survive. Ferns such as these that grow from a central crown are slow to establish but very long-lived. Getting the spacing right at the beginning, namely 2 - 3 ft apart, will help avoid the need for transplanting at a later date when the crowns may be too closely spaced and therefore not likely to survive a division. One can also propagate ferns from spores but it is important to remember that the spores of these particular ferns only remain viable for three days after reaching maturity. 
Constant replenishment of the soil with a good supply of humus avoids the need for fertilizer and helps maintain, in part, the slightly acid soil conditions these ferns love. A good layer of mulch, preferably leafy material, will help nourish the soil and retain moisture. Some protection from the wind will also help prevent dehydration. While it is important to keep your ferns well watered, it is possible to overwater them which will cause plant stress and provide a favoured environment for slugs and snails, the only possible pest to affect these plants.


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