Asplenium rhizophyllum, Walking Fern - 5 (2)

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by Nelson Maher

On a warm sunny day at the end of winter it's not hard to think about walking fern (Asplenium rhizophyllum). I know that under a great depth of snow all those walking ferns that I saw last year are in excellent shape, deep green and leathery. This species is just one of the 15 evergreen ferns that I look forward to seeing each spring as I take my first walk on the Bruce Peninsula without skis or snowshoes.

Searching for ferns is a ritual of spring for many people along the Niagara Escarpment in Grey County. The limestone rock - more specifically dolostone, which is the harder top crust of the escarpment with its higher magnesium content - is the preferred habitat of many ferns. Centuries ago, large blocks fell away from the escarpment into the shade of the talus slope. A heavy layer of moss formed on their surface creating the ideal habitat for walking ferns. Sometimes you will find the dolostone rocks quite a distance from the escarpment, pushed there by glaciation. The bigger the rock in a wooded ravine, the better the chance of finding walking ferns growing on its shady side.

Fanciful nature lovers might imagine the walking fern will follow them out of the woods, but the plant actually gets its name from its unusual reproductive strategy. The walking fern produces offspring when the slender tips of its fronds touch the plush moss and send out new roots. As many new plants grow, this creates a mat with overlapping fronds arching into the moss, covering the top (and often spreading down the sides) of limestone rocks. Although many artists envision walking ferns as growing by themselves on rocks I have never found this to be true.

Few people have seen this fern that is unique among the ferns of eastern North America. The frond is simple-cut, 10-30 centimetres (four-12 inches) long. It is cordate (heart-shaped) or auriculate (with ear-like lobes) at the base prolonged into a slender apex with a bud at the tip, most often producing a new plant in a blanket of moss. The veins are areolate (netted or chained like a fish-net). Sori or spores are elongated along the veins and appear scattered on the underside of the frond. The six to seven fronds grow from a small, erect, unbranched rhizome with dark brown scales. The stalk is reddish-brown at the base shading to green near the blade. Fertile fronds are somewhat larger than sterile ones.

Asplenium rhizophyllum is the symbol of the Bruce Trail, the Niagara Escarpment hiking trail that extends from the Niagara River to Tobermory. This lovely fern follows the escarpment in leaps and jumps, then seems to peter out north of Lion's Head Promontory only to reappear on Manitoulin Island at a few stations on the dolomite rocks.

Once a member of the genus Camptosorus, the walking fern was moved to the Asplenium genus due to the frequency of hybridization between it and other Aspleniums such as ebony spleenwort (A. platyneuron), maidenhair spleenwort (A. trichomanes), hart's tongue fern (A. scolopendrium) and others which inhabit the same ecosystem.

Many of the limestone rocks that are home to the walking fern harbour other fern species. The fragile fern (Cystopteris fragilis) is the earliest to put out new growth (and the first to die back in September), quite often on the same rock where a multitude of bulblet ferns (Cystopteris bulbifera) may appear a week later. The common polypody or rock cap fern (Polypodium virginianum) can form a dense mass on top of a boulder, easily able to endure the dry exposed conditions. If the rock is large enough it will have the marginal shield fern (Dryopteris marginalis), a large fern that is plentiful from one end of the Niagara Escarpment to the other.

Hunting out the dolomite rocks where I have found the walking ferns in the past is like visiting old friends. To find the plants in good shape and spreading further across the rock is cause for great elation. But sometimes I am sad to find a rock stripped by an ignorant person who does not understand that these ferns cannot be removed like an orange peel and put back on another limestone rock in their garden and expected to grow. A walking fern may have taken up to five centuries to adapt to that particular rock. Its roots extend into all the little pockets in the rock, modifying the soil beneath the moss. No other fern creates a symbiotic relationship like this.

I have been trying to grow walking ferns from spores, with little or no success until this last year. I now have about three dozen sprouts under 10 centimetres (four inches) in height. It should take two years from starting the spores to planting outside. I will surgically cut them into the moss on a 10-ton dolomite rock with plenty of shade from maple trees. About one-fifth of the rock is buried in the ground so as to hold moisture in the years to come. I will be ecstatic if I have a 50% success rate in three or four years.

Nelson Maher is a hobby naturalist and lifelong fern lover.

For information on other ferns or instructions for growing ferns from spores consult A Guide to the Ferns of Grey and Bruce Counties, Ontario published by the Owen Sound Field Naturalists, Box 401, Owen Sound, ON N4K 5P7.