Not to be semantically confused with :
Pondweeds (Potomogeton species) or Duckweeds Lemna species [aquatic plants with similar names]
Easily (?) distinguished from : all other Elodea waterweed species by its large, curly leaves, such as
Canadian Waterweed (Elodea canadensis), Nuttall's Waterweed (Elodea nuttallii),
South American Waterweed (Elodea callitrichoides) the very rare [RRR]
Esthwaite Waterweed (Hydrilla verticillata) and Large-Flowered Waterweed (Egeria densa).
Curly Waterweed is a perennial sold in garden centres as a plant for oxygenating ponds, often mis-labelled as the non-existent plant 'Elodea crispa'. It is a non-native and comes from South Africa, and was first discovered naturalised in the UK in 1944. When it gets out into the wild, it is a problem plant of freshwater, growing in shallow ponds (up to 3m deep), canals, open watercourses, and slow-moving streams and rivers.
It likes to grow in chalky or alkaline water. Because of the way it uptakes carbon, it will increase the pH of the water it is growing in until the soluble bicarbonate starts to precipitate on the leaves as calcium carbonate, forming a white marl of lime-rich mud. In small ponds photosynthesis by this plant can increase the pH of the water up until it reaches 10.4 (which is the limit of bicarbonate uptake). Since many other plants cannot tolerate such high pH values eventually less competitive plant species will be eliminated. By this means it displaces both
Canadian Waterweed Elodea canadensis and Nuttall's Waterweed (Elodea nuttallii).
It is a long plant, and can grow in water up to 3m deep, but no longer. The leaves float on the surface when they have grown long enough from the bottom to reach the surface. They branch too.
It is a dioecious plant, with male and female flowers on separate plants. In the UK it is thought that all occurrences are female. It thus cannot reproduce sexually in the UK, but reproduces vegetatively instead. It is thought that since birds cannot carry the heavy plant about, it propagates in a stream when bits of the brittle stem break off and floating downstream to take root elsewhere. Carelessly discarded excess from fish-ponds may also contribute to its spread.
The plants are all female in the UK, and have flowers with 3 pinkish sepals and petals (as do many other aquatic plants). They are borne on a single but long stem which grows from the leaf axils which holds the flower several inches aloft above the water. The petals can act like sails in a slight wind. See Nuttall's Waterweed for the major similarities of the flowers between the two. The only apparent differences between the two (Curly Waterweed and Nuttall's Waterweed) is in the number of stamens on the male flowers - but no plants in the UK are male!).
It can be controlled by the application of Dichlobenyl (2,6-DichloroBenzonitrile) or of Terbutryn (a sulfur containing substance with a Triazin ring) in March or April. The first is only slightly toxic to humans but kills young seedlings of both dicots and monocot species. The second is likely poisonous to humans. Both will kill most submerged vegetation, so need to be used with care.