A long-lived plant forming massive clusters of leaves 1-2 m high and 2-4 m across. Its very large leaves (1-2 m long and 15-25 cm wide) are somewhat fleshy and are rarely bent backwards these leaves are bluish-grey to greyish-green in colour. The leaf margins have numerous prickly teeth and the leaves end in a dark-brown spine (2-3 cm long) when fully mature this plant develops a massive much-branched flower cluster on a robust flowering stem 5-12 m tall. Its upright flowers (7-10.5 cm long) are yellow or greenish-yellow in colour and have six very prominent stamens. Its large capsules (4.5-6 cm long) eventually split open to release their seeds.
Spreading century plant (Agave americana var. expansa) is naturalised in south-eastern Queensland.
A weed of roadsides, railways, cliffsides, road cuttings, disturbed sites, waste areas, riparian vegetation, open woodlands and disturbed urban bushland.
A very large and long-lived (i.e. perennial) rosette-forming plant, growing 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 produce numerous suckers which form a large clump or colony. When fully mature, plants produce a massive flower cluster on a robust flowering stem 5-12 m tall.
Century plant (Agave americana) is regarded as an environmental weed in Victoria, New South Wales, South Australia, Western Australia and Queensland, and as a potential environmental weed or "sleeper weed" in the Northern Territory. It was recently listed as a priority environmental weed in at least one Natural Resource Management region and is thought to pose a significant threat to rangeland biodiversity in Australia. Though this species grows and spreads slowly, and is largely seen as an invader of roadsides and disturbed sites, it is also found growing in natural vegetation and eventually forms dense almost impenetrable thickets. Century plant (Agave americana) is certainly taken seriously in New South Wales, where it appears on numerous local and regional environmental wed lists (e.g. in Warringah Shire, Randwick City, Blue Mountains City, the Sydney North region, the wider Sydney and Blue Mountains region and the NSW North Coast region). It is regarded as a threat to coastal environs in New South Wales and is present in national parks and conservation areas in the Far South Coast Region of this state (e.g. in several areas within the Southern Foreshore Reserve at Narrawallee Beach and subject to a control program in Ben Boyd National Park and Bellbird Creek Nature Reserve). In Queensland, century plant (Agave americana) appears on the list of the top 200 most invasive plants in south-eastern Queensland and is listed as a high priority environmental weed in the Noosa Shire. This species is also listed as an invasive garden plant in the Greater Adelaide region and is known to be a serious coastal weed in South Australia. It has also been recorded in conservation areas in this state (e.g. Anstey Hill Recreation Park, Agery Reserve and Clinton Conservation Park) and the infestation in Clinton Conservation Park, on the Yorke Peninsula, is noted to be spreading through coastal shrubland where it is displacing native saltbushes, grasses and samphires. Century plant (Agave americana) has also been recorded in conservation areas in Victoria and appears on several environmental weed lists in this state (e.g. in Banyule City, Moyne Shire, Nillumbik Shire and the Goulburn Broken Catchment). It is also seen as a potential weed threat in the Katherine region in the Northern Territory. While century plant (Agave americana) is not seen as a significant threat in Western Australia, it occupies valleys between sand dunes and limestone ridges and grows in disturbed natural vegetation in this state. It has also been actively controlled in Francois Peron National Park and other conservation areas in the Shark Bay region, where it is not yet widely established.
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 (i.e. lanceolate) and arranged in a rosette. They may be upright (i.e. erect or ascending) or spreading in nature, and are occasionally bent backwards near their tips (i.e. reflexed towards the apex). These leaves (1-2 m long and 15-25 cm wide) are usually rigid and somewhat fleshy (i.e. succulent). They are greyish-green or bluish-grey (i.e. glaucous) in colour. The leaf margins coarsely toothed (i.e. serrate), with prickly teeth (5-10 mm long) borne at intervals of about 2 cm. The leaves have a pointed tip (i.e. acute apex) topped with a relatively large dark-brown coloured spine (2-3 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 (i.e. they are terminal panicles). Individual flowers are borne in an upright (i.e. erect) position on stalks (i.e. pedicels) 2-4 cm long. These flowers (7-10.5 cm long) are yellow or greenish-yellow in colour with their six 'petals' (i.e. 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 (i.e. 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. Flowering occurs from summer through to autumn. The fruit is a large oblong capsule (3.5-8 cm long) with a pointed tip (i.e. 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.
This species produces seed, but it mainly reproduces itself vegetatively via suckers.This plant spreads laterally via suckers 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.
Three forms of this species are known to be naturalised in Australia. They can be distinguished by the following differences: Agave americana var. americana has bluish-grey or greyish green leaves that are often bent backwards at their tips (i.e. they are reflexed). The spines at the tips of its leaves are relatively large (3-5 cm long).Agave americana var. americana 'Marginata' has variegated leaves (i.e. they are green with yellowish margins) that are often bent backwards at their tips (i.e. they are reflexed). The spines at the tips of its leaves are relatively large (3-5 cm long).Agave americana var. expansa has bluish-grey or greyish green leaves that are mostly borne upright (i.e. they are rarely reflexed). The spines at the tips of its leaves are relatively small (2-3 cm long). Century plant (Agave americana) may be easily confused with the sisals (i.e. Agave sisalana and Agave angustifolia) and the false agaves (i.e. Furcraea foetida and Furcraea selloa). These species can be distinguished by the following differences: century plant (Agave 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 (i.e. bulbils) on the branches of its flower clusters.