Many members of the Hydrocharitaceae family (oxygen weeds, water weeds) are similar looking submersed plants with long leafy stems. Several also have the distinction of having gone from popular aquarium and pond plants to noxious weeds over the last 100 years. We describe the most common invasive species below.
Egeria densa (Brazilian elodea) is a robust invasive plant from South America with dense whorls of bright green leaves (except when growing with insufficient light, in which case the leaves are dark green with widely spaced whorls). Several characteristics distinguish it from similar plants: it is larger, it (usually) has four leaves per whorl, and each leaf is usually at least two cm long. At the plant's growing end the leaf whorls may be quite compact and bushy. Egeria densa is the large bushy plant pictured to the right. Note: it is much larger than the two Elodea canadensis plants at the top left of the photograph.
Elodea canadensis (American water weed) is a native North American plant which has also been popular with aquarists. It has become a noxious weed when introduced outside of its native range (UK for example). It is very common in Washington lakes and lake residents often find it weedy even though it is native. American waterweed is smaller than Brazilian elodea with typically three leaves per whorl. Each leaf is usually less than 1 cm long (up to 1.5 cm). When grown with sufficient light the leaves are bright green and form attractive compact whorls. There are two sprigs of Elodea canadensis in the photograph to the right. The sprig at the left side of the photograph has been growing under more shaded conditions than the much more bushy plant in the middle.
Hydrilla verticillata (hydrilla) pictured to the left is probably the most dreaded aquatic invader of the United States. It is originally from Australia, Africa, and Asia, but has been widely introduced outside of its native range. Hydrilla has two varieties, monoecious (both male and female flowers on the same plant) and dioecious (male and female flowers on different plants). Both varieties typically have five leaves per whorl and serrations along the leaf margins that cause the leaf edges to look slightly wavy. The midrib of each leaf is often reddish when fresh. Both varieties also produce tubers in the sediment and compact leafy buds called turions. The tubers and turions, unique to hydrilla, will produce new plants. The monoecious variety is smaller than the dioecious, with leaves 1 to 1.5 cm long and as little as 1 mm wide. The monoecious variety is typically found in the northern part of the US (including two lakes in Washington), while dioecious hydrilla dominates southern lakes and reservoirs.
Lagarosiphon major (African elodea) has the potential of becoming a serious aquatic pest if introduced to North America. It is native to southern Africa and is a noxious weed in New Zealand. It is on the federal noxious weed list. It can be distinguished from similar plants by its spiral leaf arrangement and leaves that curl under slightly.
Identification Characteristics of the Hydrocharataceae
|Character||Egeria densa||Elodea canadensis||Hydrilla verticillata
|Leaves per whorl||4 (3-5)||3 (2)||5 (2-8)||4-5 (2-8)||Spiral arrangement|
|Serrated leaf edges visible||With magnification||With magnification||Distinct on older plants||Distinct||With magnification|
|Leaf size||Up to 4 cm||Up to 1.5 cm||1-2 cm||1-2 cm||1.5-2 cm|
|Flowers||Male only, up to 2 cm||Tiny, male & female on separate plants||Male and female on same plants to 1 cm||Only female plants in the US, to 1 cm||Tiny, male and female on separate plants|
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