Invasive plants in the Ponziane islands - Agave americana

Invasive plants in the Ponziane islands - Agave americana

25 June 2021

Agave americana is native to tropical and subtropical areas of South America but is widespread in the Mediterranean region. Spanish and Portuguese explorers probably brought Agave in Europe, but it was the growing interest for collections in the 19th century that caused the import of many species and varieties.

It is a succulent plant, up to 8 metres tall, and with a woody, enlarged rhizome (subterranean plant stem, which sends out roots and shoots). The plant has a large basal rosette of evergreen, strong leaves, which grows around the short, hardly visible stem. Leaves are blue-greenish, sometimes with a white or yellow stripe. They have thorns along the margin (1 cm) and a heavy spike (about 3 cm) at the tip. The plant develops a single inflorescence at the top of a tall scape starting from the centre of the rosette. A series of horizontal, overlapping branches carry several yellow-greenish flowers. Pollination is by insects. The fruit is an obovoid capsule with black, thin seeds.

Agave americana grows for about 15 years by adding new leaves to the basal rosette, flowers in summer only once in its lifetime, and dies. However, it is easily propagated by offsets from the rhizomes and from the scape. Asexual (or vegetative) growth enables the plant to spread efficiently and form vigorous populations rapidly, which are difficult to remove manually because of the spines on the leaves.

 Agave americana is mainly ornamental but has many traditional uses: the flower stalks are squeezed to produce a sweet juice that can be drunk or fermented to make an alcoholic beverage (pulque); the fibrose leaves can be used to make ropes or coarse fabrics, or even paper; the sharp thorns may serve as pins and needles.

In Italy, Agave americana was introduced in the 16th century and is especially present in the central and southern regions. In the Ponziane islands, it is widespread in different environments in Ventotene and Santo Stefano, including the coastal cliffs, where it may interfere with habitats of Community interest (Habitats Directive 92/43/EEC), such as the vegetated seacliffs with endemic Limonium (habitat 1240), the low coastal formations with Helichrysum (habitat 5320) and the maquis with Euphorbia dendroides (habitat 5330).