Adiantum raddianum
(delta maidenhair fern)

A. raddianum is a delicate fern native to tropical and subtropical South America. The fern grows terrestrially or on rocks and erects arching fronds, up to 50 cm high, growing out of a short rhizome. The plant has become naturalized in various tropical and subtropical islands and is considered to be invasive in Hawaii and French Polynesia. The fern readily spreads and becomes locally abundant. In Hawaii it was first observed around 1910 and is now the most common Adiantum species. It grows best in moist and shady places and appears to replace the closely related native Adiantum capillus-veneris. It also threatens an endemic species of silversword, Dubautia plantaginea subsp. humilis, and another native fern, Pteris lidgatei.

In its native range, in tropical and sub-tropical South America, from Mexico down to Argentina, the fern is found from 0 to 4000 m (Luteyn, 1999; Zuloaga et al., 2008). It has been quite widely introduced elsewhere as an ornamental and now occurs in Asia, Africa and the Pacific. In Hawaii, the fern occurs on all major islands and ascends from sea level to 4,400 m. On the Azores it is found on all islands except Terceira and Corvo and ascends to 400 m (Schäfer, 2002; Palmer, 2003).

The fern is available through the horticultural trade and sold in many nurseries. New specimens, possibly including different cultivars, are likely to be imported into many regions. Since the spores are easily dispersed and germinate readily, any specimen with fertile fronds will act as a potential source for new ferns. Dispersal of spores is an important vector in all regions with a subtropical to tropical climate where the fern can be grown outdoors.

The fern grows best in damp and shady places but does not tolerate very heavy shade. It is found in a wide range of habitats including forest floors, rock crevices, walls, roadside banks, river banks, coastal cliffs and basalt banks along trails and streams (Palmer, 2003).

Genetics

Reported chromosome numbers are 2n=60, 2n=114, 2n=58 and 2n=228 (Missouri Botanical Garden, 2012).

Reproductive Biology

As a fern, A. raddianum possesses the typical life cycle with a sporophyte generation and a gametophyte generation. The sporophyte (spore-bearing plant) is the conspicuous fern plant with fronds, roots and stems. It produces spores and its tissues are diploid. A spore germinates and forms a tiny green prothallus, which is the gametophyte (gamete-bearing plant). The prothallus is haploid and has structures on its surface producing the gametes or sex cells, e.g. sperm and egg cells. After fertilization a new sporophyte grows out of the prothallus. Fertilization is in most cases cross-fertilization. This is ensured by the large number of prothalli lying next to each other at a site where spores germinated.

The fern spreads mainly by its spores, which are produced abundantly and dispersed by wind and water. The rhizome is short but fragments may resprout if large enough. Rhizome fragments may be carried to new places by heavy rains or soil disturbances. 

Physiology and Phenology

The broad elevational range of this fern species suggests a broad environmental tolerance, including high and low temperatures, shade and bright light.

Environmental Requirements

In Hawaii, the fern has a remarkable altitudinal distribution, from 0 to 4400 m (Palmer, 2003). It grows on young volcanic soils such as moist cinder and basalt banks. In French Polynesia, it occupies a similar range between 0 and 4000 m.

Healthy plants do not suffer greatly from pests. Some generalist herbivores such as slugs, snails, earwigs and caterpillars can damage rhizomes and young fronds. Aphids are common pests on uncurling fronds in potted plants. Grey mould caused by a fungus of the genus Botrytis may occur on plants in stagnant and wet conditions (Jones, 1987).

General

• Botanical garden/zoo

Ornamental

• Potted plant

Common maidenhair fern (Adiantum capillus-veneris) is of similar size, but the frond stalk is usually shorter than the lamina. The ultimate segments are up to 3 cm wide, deeply lobed and irregularly shaped. Spore cases are not roundish and not 'U'-shaped, but longer than they are wide and arranged at the edge of ultimate segments. Brittle maidenhair fern (A. tenerum) has larger fronds (30-100 cm long) and the ultimate segments of blades are variable in shape, mostly triangular to diamond shaped. Stalks of ultimate segments are enlarged and form saucer-like discs at attachments.

Studies on the ecological impacts of A. raddianum are badly needed, as well as studies on the establishment of this fern in natural habitats. Threats to endangered plant species and its overall invasive character are largely based on anecdotal and correlative observations. Particularly noteworthy is the broad elevational range of A. raddianum on the Hawaiian Islands, i.e. 0 to 4,400 m. Here, studies on physiological tolerances or adaptations would be fruitful, especially on the effects of light and low temperatures on growth performance. Another question relates to longevity of spores, as it is not known for how long spores remain viable in the soil.

20/09/12 Original text by:

E Weber, Consultant, Switzerland