AgPest » Alligator Weed

Alligator weed

Alligator weed flower

Alligator weed

Low-growing, non-woody perennial
Long horizontal stems (stolons), up to 10 metres long, can float on the surface of water, forming rafts, or grow onto banks, forming closely matted clumps
Leaves are shiny, spear-shaped, in opposite pairs or whorls, 2-7 cm long and about 1-2 cm wide
Small white papery flower heads, similar to clover heads, 1-2 cm in diameter, on 2-7 cm stalks generally appear from November to March
Stems grow to 60 cm high and have large, hollow internodes
On land, adventitious roots and thickened taproots form, stems are shorter, and internodes smaller and less hollow.

Origin

Originally from Brazil, and now a major weed problem in parts of the United States, Australia, China, India, Thailand and Indonesia, this plant has been described as one of the world’s worst aquatic and terrestrial weeds
First discovered in New Zealand in the northern Wairoa River, near Dargaville, in 1906. It is believed to have arrived with ship’s ballast water
Now widespread in Northland as a serious weed of waterways and cropping land. Common in Auckland waterways and there are several infestations in a range of habitats in the Waikato.

Ecology

Low-growing herb that grows very quickly in summer
Mats may extend for 15 m over the water surface and become so robust they can support the weight of a person
Tends to be more prostrate on land, developing a very deep root system
Does not produce viable seed, so reproduction is only vegetative
Frost and ice kill exposed stems and leaves, but protected stems can survive to support the next season’s growth
Responds to high levels of nutrients and withstands some salinity in flowing brackish water.

As a terrestrial weed it grows rapidly, particularly under warm, wet conditions, and can out-compete crops and pastures
Alligator weed will take up heavy metals from the soil and is known to be toxic to some livestock
Aquatic alligator weed can rapidly spread in streams and drainage canals, clogging them with vegetation and trapping sediments, thus increasing risk of flooding
In natural wetland areas alligator weed is a threat to native flora and fauna and is extremely difficult to eradicate once established in such a habitat
Alligator weed is listed in the National Pest Plant Accord and it is therefore illegal to propagate and knowingly spread this plant.

Ensure that alligator weed fragments are not carried on vehicles or equipment
Farmers should protect their properties by:
- insisting all contractors clean their equipment before entering the farm
- ensuring supplementary feed brought onto the farm is weed free
- ensuring aggregates, soil and sand brought onto the farm are weed free
To prevent the spread of alligator weed and other aquatic weeds such as didymo, boat operators should clean their boats and trailers when leaving waterways.