Prickly Pear

The first recorded introduction of prickly pear was attributed to Governor Phillip at Port Jackson in 1788. The reason for introducing the plant was to create a cochineal industry in the new colony. Cochineal is an insect that feeds on certain species of cactus and from which a scarlet dye is obtained. This dye was used to colour the distinctive red coats of the British soldiers at that time. (Source: www.daf.qld.gov.au)

Prickly pest pears are drought tolerant plants, which can impede movement and displace native vegetation and pasture species. (www.naturalresources.sa.gov.au)

Water Hyacinth

Water hyacinth is a floating waterweed with a fibrous root system and dark green rounded leaves up to 5 cm in diameter. Flowers are light purple with a darker blue/purple and yellow centre. They are carried in dense spikes projecting above the plant.

Rampant growth of water hyacinth can destroy native wetlands and waterways, killing native fish and other wildlife. 

Originally from Brazil, water hyacinth was introduced to the Brisbane metropolitan area as an ornamental pond specimen in the early 1900s. Valued for its floral presentation, it was released into ponds and lagoons in public parks throughout Queensland. Flooding then spread the plant into creeks, rivers and dams where, having no natural predators, it flourished and quickly became a nuisance.(Source: www.moretonbay.qld.gov.au)

Blackberry

By the early 1840s, blackberry had been deliberately introduced from Britain into NSW for its fruit and for making hedgerows. Shortly after it escaped into the wild and by the 1880s was recognised as a significant weed.

Blackberry forms dense thickets that exclude native species, leading to its complete dominance of the understorey and eventually the canopy. The thickets also impede access, alter fire regimes and dominate the landscape.

Blackberries are spread mainly by birds and foxes and in water, such as creeks. (Source: http://www.environment.nsw.gov.au/)

Bridal Creeper

Bridal Creeper is a South African plant introduced in the 19th Century as a garden plant and it's one of the worst weeds in Australia. 

It is a climber or creeper that forms a dense, almost impenetrable, mat five to ten centimetres below the soil. The cover it creates prevents native plant seedlings from establishing and birds eating fruits spread seeds of the weed.

Para Grass

Para grass is a semi-aquatic plant native to tropical Africa. It was introduced into Queensland around 1880 to reduce soil erosion along the banks of waterways. (Source: https://www.daf.qld.gov.au) 

It can grow in water up to a metre deep from which it excludes all other species, establishing itself as a dense monoculture which completely obliterates open water. Para grass spreads rapidly, even into rainforest next to flood plains, carrying fire into these areas in dry times. (http://www.savanna.org.au/)

Olive Hymenachne

Hymenachne was first imported into Australia in the 1970s, with the aim of using it in ponded pastures too deep for para grass. Ponded pastures are used to provide stock feed during the dry season when other sources of protein have been used up. 

Hymenachne is a semi-aquatic grass that was introduced as fodder in ponded pastures of central Queensland. It was planted in tropical wetlands of northern Queensland and the Northern Territory, and has since escaped from cultivation and seriously threatens northern wetlands. Hymenachne invades permanent water bodies by blocking waterways, potentially causing flooding and threatening drinking water. (Source: http://www.environment.nsw.gov.au/)

Lantana

Lantana is a Weed of National Significance. It is regarded as one of the worst weeds in Australia because of its invasiveness, potential for spread, and economic and environmental impacts. Lantana forms dense (thick), impenetrable thickets that take over native bushland and pastures on the east coast of Australia. It competes for resources with, and reduces the productivity of, pastures and forestry plantations. It adds fuel to fires, and is toxic to stock. 

It is a problem in gardens because it can cross-pollinate with weedy varieties to create new, more resilient forms.