Introduced Flora

Weeds or pest plants are plants that invade and degrade natural bushland, agricultural land, waterways and roadsides and are unwanted in a given situation. When a plant is introduced outside its natural environment, its predators, competitors and disease are often missing and therefore it has the potential to become aggressive and invade sites.

The introduction of pest plants has either occurred deliberately or by invasion. Wind, water and animals may carry seeds of introduced plants. Many of the introduced plants have found there way into the natural environment through Human activities such as agricultural weeds and crops, contaminated seed, the aquarium trade and garden ornamentals.

Introduced species not only threaten biological diversity, but also the health of the ecosystems. The most serious threat is by introduced plants that grow in sufficient numbers and abundance to alter ecological processes at least at a local level and can also cause an economic loss.

The following are examples of major pest plants in the Murray-Darling Basin.

Willows
Alligator Weed
Arrowhead
Water Hyacinth
Yellow WaterLily
Parrot’s Feather
Egeria
Elodea
Noogoora Burr

Willows (Salix spp )

Willows are an introduced species that were planted to assist with navigation along the River Murray during the riverboat trade. Other reasons for the introduction have been associated with their aesthetic appearance to Europeans.
More than 100 species or varieties of willows have been introduced to Australia. They are now a major pest of waterways as they have adversely affected the ecology of the streams and rivers.

Willows spread easily and suppress native trees and aquatic animals, block access and views and can create snags. This invasion has led to a reduction in the numbers of small aquatic animals, which in turn has reduced the amount of food available for waterbirds and fish.

Willows are deciduous and drop their leaves at one time – autumn, which reach the River and release large amounts of nutrients in a short time. Their branches fall into the water and rot quickly not allowing for the establishment of a habitat for aquatic animals like native tree species. Their deep roots slow water movement, which encourages sedimentation. This allows the willow to produce suckers that may spread into clones –allowing one tree to become many and as a result the waterway is lined with willows.

The removal of willows is a contentious issue. The appropriate government authority should be consulted to ensure removal does not contradict river protection laws. Native vegetation should be planted where the willow was removed to reduce the effects of bank erosion. Care must be taken when removing willows with chemicals to prevent them from being washed into the river or onto non-target vegetation.

<Top of Page>

Alligator Weed (Alternanthera philoxeroides )

Alligator weed is a summer growing perennial plant that grows in water or in wet areas. Generally found on stationary and slow moving water bodies, creeks, channels, riverbanks and associated areas that are occasionally flooded. It has speared shaped glossy green leaves and a ball shaped white flower. This weed normally grows from a small piece of stem and can grow as a free floating raft or with its roots attached to the soil.

<Top of Page>

Arrowhead (Sagittaria graminea )

Arrowhead is an aquatic plant that is becoming an increasingly serious problem in irrigation areas. It spreads rapidly, blocks channels and greatly reduces the effectiveness of the water distribution system. It is an emergent plant that can grow up to one metre high when mature. It form varies with the most prominent form having lance-shaped or arrow-shaped leaves which are green in colour.

Arrowhead has serval methods of reproduction (eg germination of seeds, vegetative reproduction) and this allows it to spread rapidly. Control of arrowhead can be achieved with some approved herbicides. However, research is continuing into the plant’s biology to assess the best ways of control.

<Top of Page>

Water Hyacinth (Eichhornia crassipes )

Water hyacinth is a perennial aquatic plant, which is declared a noxious weed in all states in Australia. It has two leaf types some long and narrow, standing erect while the others are almost round. The plants may take root in mud in shallow water bodies, while also having the ability to be free floating in deeper water bodies.

Water hyacinths spread rapidly choking waterways. Its dense growth reduces fish populations, limits food sources and provides shelter for mosquitos and other disease spreading organisms. Control of water hyacinth can be achieved through a combination of habitat manipulation (ie denying it water), biological control agents, or the application of approved herbicides.

<Top of page>

Parrot’s Feather (Myriophyllum aquaticum)

Parrot’s feather is a perennial aquatic or semi- aquatic plant that originated from the Amazon River in South America. It arrived in Australia as an aquarium and pond plant and has since spread.

The plant has become a major weed in irrigation supply and drainage channels, lakes and water storages in NSW and Queensland. The plant is found in both static and flowing water. Growing in mud or gravel, as well as around submerged stumps and other aquatic plants.   Parrot’s Feather is dispersed vegetatively through broken stem fragments. There are no male plants in Australia so seeding does not occur.    As this plant is dispersed by stem fragments mechanical removal is not recommended. Herbicides have been used with varying levels of success.

<Top of Page>

Yellow Waterlily (Nymphaea mexicana)

Yellow waterlily is a perennial plant that causes problems by blocking waterways for irrigation and recreational uses. It is native to Europe and was introduced into Australia for its attractiveness and adaptability.

It reproduces vegetatively, allowing it to spread rapidly throughout waterways. The lily flowers from late spring to early autumn and flowers usually survive for one week. Control at present is to use glyphosate with an aquatic label this may take several years of repeated treatment to achieve adequate control. Read all directions of the product label carefully before treatment.

<Top of Page>

Egeria (Egeria densa)

Egeria or Dense Waterweed was introduced as an ornamental in the aquarium trade. The plant can form dense clumps underwater effecting recreational activities such as swimming, boating and fishing.

It is a noxious weed in WA, SA and Tasmania and has become a problem in Lake Mulwala at Yarrawonga.  Egeria is a submerged attached plant with floating flowers. It is cold tolerant and survives in some areas where winter surface water temperatures are close to freezing. Control with chemicals is limited and harvesting of plant can increase the spread.

<Top of Page>

Elodea (Elodea canadensis )
Is a perennial aquatic plant native to North America and is declared a noxious weed in some states. The plant grows completely submerged in water. It has soft green leaves, which grow in groups of three. Pieces of the plant may continue to grow while floating unattached in the water. It is generally found in stationary and slow-moving water bodies , coastal rivers and creeks , especially in colder area, irrigation channels and drains: N.S.W, Vic., Tas.

It has caused major problems in Victoria and N.S.W constructing waterways. Elodea is hard to eradicate and some of the control measures include approved herbicides and mechanical cutting.

<Top of Page>

Noogoora Burr (Xanthium occidentale)

This plant is native to North America, Mexico and Caribbean. It was reported in the Murray Valley in the 1920s and still infests the flood plain along the Murray River and its tributaries.

Noogoora Burr is a summer growing annual plant and spreads easily by wind, water and/or animals, clothing etc.

The plant is toxic to stock in its seedling stage and the burr can cause discomfit to sheep and reduce the value of wool. It also, at times, can be dense enough to restrict cattle and sheep access to watering places and effect recreational activities.

There are numerous ways of control such as chemical especially when plants are young, mechanical and even biological.

<Top of Page>


Photos and content sourced from the Murray-Darling Basin Commission and Goulburn-Murray Water