Agave americana (Century Plant)

Scientific name

Agave americanaL.

Three separate taxa are sometimes recognised in Australia: Agave americana L. var. americana; Agave americana L. var. expansa (Jacobi) Gentry; Agave americana L. 'Marginata'.

Synonyms

For Agave americana var. expansa: Agave expansaJacobi

For Agave americana 'Marginata': Agave americana L. var. marginata; Agave americana L. var. picta (Salm-Dyck) A. Terrac.; Agave picta Salm-Dyck

Common names

Agave, agave cactus, aloe, American agave, American aloe, century plant, century-plant, maguey

Family

Agavaceae

Origin

Native to northern and central Mexico and some parts of southern USA.

Naturalised distribution (global)

Locations in which Agave americana is naturalised include mainland Australia, southern Europe, the Canary Islands, southern China, southern Africa and New Zealand.

Introduced, naturalised or invasive in East Africa

Agave americana is invasive in parts of Tanzania (Henderson 2002). It is also cultivated and naturalised in parts of Kenya, including Nairobi and Central Provinces.

Habitat

This species is often naturalised around old habitations and along roadsides in temperate, subtropical and semi-arid regions. However it also grows in pastures, grasslands, open woodlands, coastal environments and in riparian zones (banks of watercourses).

Description

Agave americana is known as century plant but it typically only lives between 10 and 30 years. It can grow 1-2 m high and 2-4 m across. Older individuals may sometimes develop a short woody stem at the base of the plant and commonly produces numerous suckers (adventitious roots) which form a large clump or colony. When fully mature, A. americana develops a massive flower cluster on a robust flowering stem 6-12 m tall.

The stems of this plant are very short and woody. However, massive flowering stems are eventually produced. These are green or greyish green with alternately arranged bract-like leaves.

The very large leaves at the base of the plant are long and narrow (lanceolate) and arranged in a rosette. They may be upright (erect or ascending) or spreading in nature, and are sometimes bent backwards near their tips (reflexed towards the apex). These leaves (1-2 m long and 15-25 cm wide) are usually rigid and somewhat fleshy (succulent). They are normally bluish-grey (glaucous) to greyish-green in colour, but forms with variegated leaves are relatively common. The leaf margins coarsely toothed (serrate), with prickly teeth (up to 1 cm long) borne at intervals of 2-6 cm. The leaves have a pointed tip (acute apex) topped with a large dark-brown coloured spine (1.5-6 cm long).

The massive flower clusters (1-8 m long) are borne at the top of a very robust flowering stem. These flower clusters are much-branched, with the branches being further divided towards their tips (they are terminal panicles). Individual flowers are borne in an upright (erect) position on stalks (pedicels) 2-4 cm long. These flowers (7-10.5 cm long) are yellow or greenish-yellow in colour with their six 'petals' (perianth segments or tepals) being fused together at the base into a short tube (8-20 mm long). The flowers also have six very prominent stamens, consisting of stalks (filaments) 6-10 cm long and yellow anthers (2.5-3.5 cm long). They also have a large ovary (3-4.5 cm long) topped with a style and three stigmas.

The fruit is a large oblong capsule (3.5-8 cm long) with a pointed tip (beaked apex) and consists of three compartments. These capsules turn from green to brown or blackish in colour as they mature and eventually split open to release their seeds. The seeds (6-8 mm long) are black in colour and shiny in appearance.

Reproduction and dispersal

This species produces seed, but it mainly reproduces itself vegetatively via suckers which allow it to spread laterally and can form very large and dense colonies over time. Young plants produced in this manner can be dispersed downstream during floods. The seeds are also dispersed by both wind and water. Plants are most commonly spread into bushland areas in dumped garden waste.

Similar species

Agave americana may be easily confused with the sisals (Agave sisalana and Agave angustifolia) and the false agaves (Furcraea foetida and Furcraea selloa). These species can be distinguished by the following differences:

  • A. americana has very large greyish or variegated leaves that are usually 1-2 m long on adult plants. These leaves have numerous, relatively large prickles (5-10 mm long) along their margins. Its flowers are borne in an upright position and are yellow or yellowish-green in colour. This species produces large capsules and usually doesn't develop plantlets (bulbils) on the branches of its flower clusters.
  • A. sisalana (sisal) has relatively large dark green or greyish-green leaves that are usually 0.5-1.3 m long on adult plants. These leaves do not have any prickles on their margins. Its flowers are borne in an upright position and are yellow or yellowish-green in colour. This species usually doesn't produce capsules, instead developing numerous plantlets (bulbils) on the branches of its flower clusters.
  • A. angustifolia (Caribbean agave) has relatively large green, greyish-green or variegated leaves that are usually 0.5-1 m long on adult plants. These leaves have many, relatively small prickles (2-5 mm long) along their margins. Its flowers are borne in an upright position and are yellow or yellowish-green in colour. This species produces large capsules and sometimes also develops plantlets (bulbils) on the branches of its flower clusters.
  • F. foetida (Cuban hemp) has very large pale green leaves that are usually 1-2 m long on adult plants. These leaves usually have some prickles along their margins, but mainly near the bases of the leaves. Its flowers are borne in a drooping position and are whitish or greenish-white in colour. This species doesn't produce capsules, instead developing numerous plantlets (bulbils) on the branches of its flower clusters.
  • F. selloa (variegated false agave) has very large variegated leaves that are usually 1-2 m long on adult plants. These leaves have numerous relatively large prickles (7-8 mm long) along their margins. Its flowers are borne in a drooping position and are whitish or greenish-white in colour. This species doesn't produce capsules, instead developing numerous plantlets (bulbils) on the branches of its flower clusters.

Economic and other uses

Agave americana is sometimes grown as a hedge. Its leaves are sometimes used medicinally or grown for their fibres. It is also used as an ornamental plant and its flowering stems can be used for fencing.

Environmental and other impacts

Agave americana is regarded as an environmental weed in several parts of the world. The prickles along the leaf margins and sharp spines on the tips of the leaves can cause injury to people and animals (both domestic livestock and native animals). Large clumps can have an impact on pastures, as these dense colonies can prevent the growth of more suitable species and restrict the access of livestock.

A. americana has been included in the Global Invasive Species Database (GISD 2011).

Management

The precise management measures adopted for any plant invasion will depend upon factors such as the terrain, the cost and availability of labour, the severity of the infestation and the presence of other invasive species. Some components of an integrated management approach are introduced below.

The best form of weed control is prevention. If prevention is no longer possible, it is best to treat the weed infestations when they are small to prevent them from establishing (early detection and rapid response). Control is generally best applied to the least infested areas before dense infestations are tackled. Consistent follow-up work is required for sustainable management.

Plants should be killed before flowers appear on tall flowering stem and follow up should be done at least annually. Small plants can be dug out and left to rot. There are several ways to control larger plants chemically including the following: stems of leaves of larger plants can be injected with 4-10 jabs of a suitable herbicide (depending on the size of plant) and left to rot; the plant can be cut at ground level and the stump painted with a suitable herbicide. Leaf fragments can resprout so these fragments should be disposed of either by soaking in herbicide or deep burying. When using any herbicide always read the label first and follow all instructions and safety requirements. If in doubt consult an expert.

The editors could find no information on the prospects for biological control of this species.

Legislation

Not considered noxious by any state or government authorities in Kenya, Tanzania or Uganda.

References

GISD (2011). Global Invasive Species Database online data sheet. Agave Americana (herb, shrub). www.issg.org/database. Invasive Species Specialist Group. Accessed March 2011.

Henderson, L. (2001).  Alien weeds and invasive plants. A complete guide to declared weeds and invaders in South Africa.  Plant Protection Research Institute Handbook No. 12, 300pp. PPR, ARC South Africa.

Henderson, L. (2002). Problem plants in Ngorongoro Conservation Area. Final Report to the NCAA.

Weber E. (2003). Invasive Plant Species of the World. A Reference Guide to Environmental Weeds. Wallingford, UK: CABI Publishing.

Editors

Agnes Lusweti, National Museums of Kenya; Emily Wabuyele, National Museums of Kenya, Paul Ssegawa, Makerere University; John Mauremootoo, BioNET-INTERNATIONAL Secretariat - UK.

Acknowledgments

This fact sheet is adapted from The Environmental Weeds of Australia by Sheldon Navie and Steve Adkins, Centre for Biological Information Technology, University of Queensland. We recognise the support from the National Museums of Kenya, Tropical Pesticides Research Institute (TPRI) - Tanzania and Makerere University, Uganda. This activity was undertaken as part of the BioNET-EAFRINET UVIMA Project (Taxonomy for Development in East Africa).

Contact

BioNET-EAFRINET Regional Coordinator: eafrinet@africaonline.co.ke