Adiantum hispidulum
(rosy maidenhair fern)

A. hispidulum, the rosy maidenhair fern, is known to be invasive and weedy in the main Hawaiian Islands where it has escaped from cultivation. In Hawaii it is a serious weed generally of mesic slopes and gulch bottoms and is often abundant along intermittent and perennial streams. It is capable of invading intact plant communities and pristine areas. A. hispidulum usually begins to colonize in areas where there is some type of natural disturbance such as landslides, tree falls, disturbance by feral ungulates, or even a single dislodged rock. The dense clumps and rhizome mats of this fern prevent establishment of many native taxa including rare species. A. hispidulum can also overrun other ferns and herbs (Wilson, 1996; Palmer, 2003; H. Oppenheimer, Hawaii Plant Extinction Prevention Program (PEP), USA, personal communication, 2013). It is naturalized locally in a few other parts of the world including the southeastern USA and parts of its native range including eastern and southern Africa, Malaya, and Singapore.

A. hispidulum is widespread and presumed to be native from Australia and Africa to Asia, Malesia and various Pacific Islands (Palau, Fiji, New Caledonia, Vanuatu), and eastward to Polynesia (Austral Islands (Rapa), Society Islands (Tahiti), Cook Islands (Raratonga), and New Zealand. However, it is sporadic throughout its native range, and in addition to native plant communities it often occurs in disturbed habitats. For example around villages, along trails, on roadside banks, rock walls, or in the understory of forestry plantings, suggesting a potential for invasiveness. Australia is the type locality and presumably the centre of diversity for A. hispidulum, as it occurs in all States and Territories. Bostock et al. (1998) recognize four taxonomic varieties in Australia. The widespread var. hispidulum is terrestrial or lithophytic in a variety of habitats, including open forest and as relictual populations in gorges in arid areas. The other three varieties are said to be more localized and endemic to Australia. In this treatment, however, a single variable species is recognized without infraspecific taxa (subspecies, varieties, or forms), following International Plant Names Index (IPNI) (2013) and Tropicos (2013).

A. hispidulum is sporadic throughout its native range, but is known to escape from cultivation via spores. A. hispidulum may colonize native plant communities as well as disturbed habitats such as around villages, along trails, on roadside banks, rock walls, or in the understory of forestry plantings. This species is potentially invasive in tropical and subtropical moist to wet environments and most likely spreads from cultivated plants by spore dispersal.

A. hispidulum occurs in dry to mesic, sunny to shady, usually moist rocky slopes, cliffs, woods, gulch bottoms, trails, roadside banks, rock walls, and understory of forestry plantings and native forest.

Genetics

Based on chromosome counts of material from Australia and Ceylon, A. hispidulum is an apogamous hexaploid (has six sets of chromosomes), with a chromosome number of n = ca. 170-180 (Manton and Sledge, 1954). The base chromosome number in A. hispidulum appears to be 29 rather than 30, the usual base number for the genus (Brownlie, 1977).

Reproductive Biology

The apogamous (asexual) life cycle of certain ferns means that they produce 32, larger than normal diploid spores per sporangium, instead of the normal 64 haploid spores (Moran, 2004). Other apogamous taxa with higher polyploid chromosome numbers (triploid, tetraploid, hexaploid, etc.) cannot reproduce sexually either. When their spores germinate the resulting prothallus (gametophyte or gamete producing phase in a fern’s life cycle) does not produce sex organs but proliferates vegetatively without fertilization. The resulting sporophyte (spore producing) plantlet grows to maturity and in turn produces fronds with apogamous spores. Apogamous reproduction is advantageous in ferns that grow in dry habitats for two reasons: firstly apogamous ferns lack swimming sperm and there is no need for water in reproduction; and secondly their prothallus matures faster than those of sexually reproducing ferns (Moran, 2004). Apogamous ferns generally have wider geographic distributions than their sexually reproducing relatives. This appears to be the case for A. hispidulum, thus explaining its wide distribution, success in colonizing dry to mesic, disturbed habitats, and invasive tendencies.

Physiology and Phenology

Apogamy may explain the wide distribution, success in colonizing dry to mesic, disturbed habitats, and invasive tendencies of A. hispidulum. In addition to this the growth rate of all life stages of A. hispidulum is much greater than those of native fern species on Hawaiian islands.

General

• Botanical garden/zoo

Ornamental

• Potted plant

This species can be detected by visual identification, facilitated by using a reference book (Heath and Chinnock, 1974) or key (Palmer, 2003).

A. hispidulum may be distinguished from related maidenhair species (e.g. A. capillus-veneris, A. aethiopicum, A. raddianum) by it fronds that repeatedly fork at 45º angles, stipes clothed with short, dark fibrils and hairs, and pinnules with short, light brown hairs.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

21/12/2013 Original text by:

David H. Lorence, National Tropical Botanical Garden, Hawaii, USA