Doing their best to hide their disappointment at having pitched up in a colder climate than that in which they generally occur, several species of tropical ferns find just enough warmth in the Far North of New Zealand and (more intriguingly) in the localised heat of geothermal areas in parts of the country that would otherwise be too chilly for their needs. One of these tropical interlopers, Cyclosorus interruptus, is an attractive ground fern that forms creeping mats through wetland habitats such as the dune swamp pictured below (at Karikari Peninsula).
Although this species has been available for many years (primarily through the efforts of Oratia Native Plant Nursery), it is still very uncommon within cultivation – undoubtedly due (at least in part) to a dearth of gardeners and landscape architects furnished with a burning desire for finding just the right creeping fern to adorn their swampy hollow.
That said, with the rise in use of rain gardens and other means of addressing water flows, our built-up areas positively abound in swampy hollows when compared with bygone years – when sitting water would have generally been viewed as a deficiency in one’s drainage. There is also an increased interest in the beauty and ecological value of wetlands (and their attendant species), to the extent that species like Cyclosorus interruptus should find more scope for inclusion within a wide range of gardens and public spaces.
Far from the classic image of sand dunes, a wide array of habitats develop within these dynamic environments – including the counter-intuitive presence of vast wetlands in spaces where the back dunes effectively block the path of water (where it might otherwise drain towards the sea). In many places, this natural sequence has been obscured by historical changes wrought upon landscapes, whether for agriculture or coastal development, or where extensive stretches of coastal highways now run straight over the top of former wetland systems.
At the large swamp pictured within this profile (on Karikari Peninsula), Cyclosorus interruptus and another ‘tropical’ fern, Thelypteris confluens, occur together within shallow water at the back of the dunes, where they create a vibrant green mantle at the base of raupo and sedges during the warmer months. The manner in which these ferns add a seasonal layer of interest to natural plant communities (in terms of both texture and colour) points towards their potential role within gardens and public plantings – in which they can be deployed amidst diverse drifts of sedges and herbs in damp gardens.
Despite what one might logically assume about wetlands, parts of swamps can be dry for extended periods of time (especially towards their margins). Accordingly, Cyclosorus interruptus is capable of enduring months of dry conditions within planted areas – a fact borne out of our experience of having grown this species within seasonally dry positions.
Within New Zealand, C. interruptus occurs in geothermal areas near Rotorua and Taupo (only in close proximity to heat sources, including within surprisingly hot water1) and in coastal wetlands from Kawhia and Tauranga north (notably in Northland’s warm climate). Outside of our shores, it is found in many tropical parts of the Pacific, including Australia, the Cook Islands (and other Pacific Islands) and into south-east Asia.