West of Alppiruusulaakso (Rhododendron valley), underneath Serbian spruce (Picea omorica) on a damp ditch bed, the sensitive fern attracts us with its cute rounded leaves which is significantly different from native Finnish ferns. The sensitive fern is the sole species of the genus Onoclea. The species name sensibilis means sensitive and refers to its sensitivity to frost: the first autumn frosts turn the fronds black. Though the fronds may be frost sensitive, the rhizomes are not; the sensitive fern has thrived in Mustila for decades.
While ferns in general have their sori on the underside of the fronds, the sensitive fern has distinct fertile fronds like the sstrich fern native to Finland. The fertile fronds keep over winter and release their spores in spring before the growth of new fronds. The sensitive fern spreads via its rhizome and grows a dense foliage at a favourable location. In its original habitat it may tolerate drought in deep shade, but the sunnier the site, the more crucial becomes the dampness of the soil.