A. densiflorus is a spiny perennial plant, commonly found in savanna thickets in its native environment in eastern Africa and South Africa. It has been widely introduced globally as an ornamental and has subsequently naturalised and becom...
A. densiflorus is a spiny perennial plant, commonly found in savanna thickets in its native environment in eastern Africa and South Africa. It has been widely introduced globally as an ornamental and has subsequently naturalised and become a problem in a number of countries, including the USA and Australia. The plant forms dense spiny mats, up to 2 m high in light and sandy soils, suppressing other ground flora and depleting the soil of nutrients and moisture. It may quickly invade disturbed sites in open sun or partial shade and can become a threat in coastal habitats, along river banks and in low fertility soils. It is among the most abundant invasive ornamental weeds of sandy beachfronts in Queensland, Australia and threatens natural vegetation on Lord Howe Island, Norfolk Island and a number of other islands in the Pacific Ocean. The risk assessment score for this plant in Australia is 3 (‘requiring evaluation’) and for the Pacific Islands it has a high score of 15. In Florida, USA it has been reported as displacing native ground cover, understory shrubs and the native wild coffee species Psychotria nervosa. It is also of sufficient concern for it to have been recommended for voluntary withdrawal from sale within the state (Wirth et al., 2004). In Hawaii, USA it is spreading along roadsides and invading secondary forest (PIER, 2008).
The genus Asparagus has sometimes been attributed to its own family Asparagaceae, but is now generally included in Liliaceae. Missouri Botanical Garden (2008) suggests that the correct name for this species is now Protasparagus densiflorus (Kunth) Oberm., but as this name is not yet widely accepted and many databases do not recognise it, the more commonly recognised name Asparagus densiflorus will be used for this datasheet. Also included will be information based on a number of synonyms, especially A. aethiopicus and A. sprengeri, although some sources insist that A. aethiopicus is not strictly synonymous with A. densiflorus, and the latter continues to be used especially in the horticultural trade, corresponding to the cultivar cv. ‘Sprengeri’ and others. Flora of North America (eFloras, 2008) and USDA-ARS (2008) list A. aethiopicus separately and the former states that the name Asparagus densiflorus has been misapplied to ‘this species’ (i.e. the cultivated material also known as A. sprengeri) (Green 1986; Judd, 2001). Conversely, Sebsebe (2006) comments that Obermeyer (1993) kept the cultivated forms under A. densiflorus (Kunth) Jessop. In view of the taxonomical uncertainty over which it is being reported on in the literature, they will be considered together here under the name A. densiflorus unless it is necessary to specify.
A. densiflorus is a spiny perennial plant, persisting and spreading by fleshy rhizomes and roots bearing white tubers 2-3 cm long. Stems up to 2 m long are glabrous, green to brown, much-branched and ‘leafy’ but the clusters of flattened ‘leaves’ are in fact cladophylls about 2 cm long, 2-3 m wide. True leaves are represented by small scales at the base of the cladophylls. The stems also bear scattered straight spines, about 5 mm long, just below each branch. Flowers are in groups at the stem apices, white or pale pink, bell-shaped, with a corolla of 6 tepals and orange anthers. Fruit is a red berry 5-8 mm in diameter, containing one or a few seeds 3-4 mm in diameter (Parsons and Cuthbertson, 1992).
In its native area, A. densiflora is a plant of savanna thickets, widely distributed in eastern Africa from Ethiopia to South Africa (Burkhill, 1985), however, USDA-ARS (2008) gives the native range of both A. densifolius and A. aethiopicus as South Africa only.
It has been very widely introduced throughout the world as an ornamental, in private gardens and commercially for sale as a component of cut-flower arrangements, the foliage being used as a delicate ‘fern’. The extent to which it has become naturalised is not altogether clear, but it has certainly done so and become a problem in Australia and south-eastern USA. While it is uncertain whether or not it has become naturalised in many of the other countries to which it has been introduced, it is conversely quite certain that it is already present in cultivation in many more countries than those listed - not yet any problem, but posing a potential risk.
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
This plant has been known as an ornamental and widely introduced deliberately to many countries over the past two centuries. Dates of introduction are not generally known but Parsons and Cuthbertson (1992) indicate that it was first introduced to Australia at the end of the 1800s.
Thanks to the very wide popularity of this plant as an ornamental, the risks of introduction and subsequent escape are very high indeed. Spread occurs partly from careless disposal of rhizomes in garden waste but also from dispersal of the seeds by birds eating the attractive red berries (Parsons and Cuthbertson, 1992).
The natural habitat of A. densiflorus in eastern Africa from Ethiopia to South Africa is dry to moist forest, woodland, forest edges, savanna, shady roadsides, and along riversides, also infertile, shallow, sandy soils of coastal sand dunes, beaches, cliffs and scrub. It is tolerant of salinity and may grow close to mangrove swamp (Skimina, 1980). In Australia it is a plant of warm temperate regions with temperatures of 10-20°C (DEWHA, 2008).
Chromosome number is variously recorded as 2n=40 or 2n=60. Dasgupta et al. (2007) refer to 2n=60 as ‘diploid’ but Marcellán and Camadro (1999) found this number in var. sprengeri and refer to it as hexaploid, 2n = 20 being the diploid number for A. officinalis. Sheidai and Inamdar (1992) found var. myers and ‘A. sprengeri’ to be tetraploid (2n = 40), while Kar and Sumitra Sen (1986) recorded ‘A. sprengeri’ as hexaploid, but A. densiflorus var. pyrimidalis as diploid. Missouri Botanical Garden (2008) states that A. densiflorus is only tetraploid, but tetraploid or hexaploid for ‘A. sprengeri’.
Unlike those of the crop asparagus, which is dioecious, the flowers of A. densiflorus are hermaphrodite. They are fragrant but can apparently be self-pollinated as well as cross-pollinated (Camadro, 1994; PIER, 2008). One to three seeds are formed in a red berry which is dispersed by birds.
Physiology and Phenology
Seeds may germinate at any time of the year where introduced in Australia but there is a major flush in the spring and a smaller one in the autumn. Germination can occur from below 10°C upwards, in light or darkness (Massante, 1963). Establishment is relatively slow until the roots are well developed, and tubers form on the rhizomes throughout the early summer. Flowering occurs in late winter and spring through to early summer and foliage may die back during mid-summer, with a fresh flush of rhizome and tuber development each season and fruits may be present throughout the year (Parsons and Cuthbertson, 1992).
Preliminary surveys of natural enemies of Asparagus species in South Africa have suggested a chalcid wasp, a moth larva and a rust to be of potential interest. There is also galling by the chalcids Asparagobius braunsii and a Eurytoma species (Parsons and Cuthbertson, 1992).
Greenhouse-grown A. densiflorus in Poland is attacked by the spiral nematode Scutellonema brachyurum (Wojtowicz and Szczygiel, 1990). In France it is a host to Corynebacterium fascians (Faivre-Amiot, 1967), and in Germany, to Pseudomonas tumefaciens (Stapp, 1940) and Phytomonas fascians[Rhodococcus fascians] (Pape, 1938). In USA it is believed to have been infected by Agrobacterium tumefaciens (Brown and Weiss, 1937).
The weed forms dense blankets of growth above ground and a mass of roots and rhizome below, suppressing other ground flora and depleting soil moisture and nutrients. It is among the most abundant invasive ornamental weeds of sandy beachfronts in Queensland, Australia (Batianoff and Franks, 1997). It has also become established and threatens natural vegetation on Lord Howe Island, Norfolk Island and a number of other islands in the Pacific Ocean (Parsons and Cuthbertson, 1992; PIER, 2008).
In Florida, USA it has escaped in a number of localities into e.g. scrub vegetation displacing native ground cover and understory shrubs, also overtopping native wild coffee (Psychotria nervosa) (IFAS, 2008), and it is of sufficient concern for it to be recommended for voluntary withdrawal from sale in the state (Wirth et al., 2004). In Hawaii, USA it is spreading along roadsides and invading secondary forest (PIER, 2008).
Risk assessment score for Australia is 3 (‘requiring evaluation’) and for Pacific Islands is 15 (‘high risk’) (PIER, 2008).
The main use for A. densiflorus is as an ornamental plant, whether grown as a whole plant in the garden, or for the cut foliage incorporated into floral decorations. The latter is a major commercial commodity in both local and international trade in Europe, North America and Asia. A number of different varieties are involved.
Among medicinal uses, mashed leaves are applied to cuts and tubers are given to children for stomach ache.
In Canada, the cladophylls of A. densiflorus have been the source of mesophyll cells for many detailed plant physiological studies (Guinel and Bown, 1994).
It has been regarded as a potentially useful source of resistance to pathogens such as Phoma asparagi (Tu and Cheng, 1985) and to F. oxysporum f. sp. asparagi and F. moniliforme [Gibberella fujikuroi] (Stephens et al., 1989).
A. densiflorus is superficially similar to many other Asparagus spp. including the cultivated vegetable A. officinalis. The latter has narrower, needle-like cladodes and white to pale yellow flowers on long pedicels. A. plumosus is also a cultivated ornamental but is a climbing plant with very long branches up to 5 m long and with few, down-curved spines, or none, and black berries. A. asparagoides has fewer, larger, alternate cladodes, much larger, 1-7 cm long, 1-3 cm wide. A. scandens has narrower leaves up to 1.5 mm wide, solitary flowers and a more climbing habit. A. racemosus has many longer spines up to 2 cm long.
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.
Where A. densiflorus has become established outside the intended area, digging out the rhizomes and tubers may be feasible but is unlikely to be successful without some repetition.