Native to Brazil, dense waterweed (Egeria densa) has caused many problems overseas, particularly in the US. In Australia, it is a problem in a number of waterways where it is easily dispersed from broken stem pieces. This plant is commonly used in home aquariums and ponds and its appearance in some waterways has been blamed on the illegal dumping of aquarium contents.
Impacts:
Forms extremely dense, submerged masses that can seriously retard water flow, interferes with irrigation equipment, hydro-electric activity and potable water supplies, impedes fishing, boating and swimming activities
Description:
Grows in an underwater mass, with stems up to about 1.5 m long, firmly attaches to mud on the creek bed, occasionally appears as dense mats floating just below the surface, leaves are up to 4 cm long with tiny serrations on the margins, leaves grow in whorls of 4 or 5 (sometimes as many as 8) and are mostly densely clustered at branch ends near growing tips, flowers are 2 cm in diameter and have 3 white petals, roots are very fine and grow from the lower nodes
Control:
No herbicides registered, hand pulling, cutting and digging with machines effective, all methods need to be thorough to prevent re growth from pieces, infestations should be controlled while small
Source http://www.dpi.qld.gov.au