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European Water Clover
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Species at a Glance
European water clover, also called European water fern or water shamrock, is a perennial herbaceous fern that gets its name from its clover-shaped leaves. It poses a threat to ecosystems as dense stands form and crowd out native wetland plants.
Leaves: Leaves are divided into four triangle-shaped leaflets that are hairless and approximately 0.6-2.5 cm (0.25-1 in) long. They are either slightly submerged, floating, or emergent. Both the upper and lower surfaces of the leaflets are pale green or bluish-green. The upper leaflet surface is smooth while the lower surface may have short fine hairs. Emergent leaflets may fold together at night and spread outward during the day.
Flowers: This species is a fern and has no flower.
Stems/Roots: Thin petioles are typically 5-10 cm (2-4 in) long and form from creeping rhizomes. They are straw-colored to light green, circular in cross section, and are usually smooth. Submerged petioles often curve upward, while emergent petioles have a tendency to lean or sprawl.
Spores: Near the base of the petioles are small spore-bearing bodies called sporocarps. They are .35-.5 cm (0.1-0.2 in) long, oval, thick, reddish-brown, dark brown, or dark purple in color, and somewhat flattened in shape. They are typically arranged in groups of 2-3, although there can be anywhere from 1-5. Young sporocarps are hairy but become smooth as they age.
European water clover may resemble species of wood sorrel (Oxalis) or clover (Trifolium), but it is a sporeproducing fern rather than a flowering seed plant. Wood sorrel and clover can be distinguished by their trifoliate leaves and terrestrial habits, while European water clover has quadrifoliate leaves and is primarily aquatic.
Although this species is capable of growing on wet ground, it is typically found growing in shallow and slow-moving waters of lakes, ponds, and creeks.
European water clover is a popular water garden plant, giving it the potential to spread into natural areas through intentional release or accidental escapes. Once introduced, it can easily spread by rhizomes and sporocarps that can float downstream on water currents or be moved to new locations on boats, trailers, or on waterfowl. The sporocarps can remain dormant for decades until conditions are favorable to release the spores.
Native to parts of southeastern Europe and Asia, this species was introduced into North America during the 19th century as an ornamental plant. Currently, this fern can be found in several northeastern and Midwestern states, including New York, Pennsylvania, New Jersey, Delaware, Maryland, and North Carolina.
Note: Distribution data for this species may have changed since the publication of the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016), the source of information for this description.
While ecological threats of the European water clover are somewhat unknown, this species is capable of forming monotypic stands that can outcompete native aquatic plants for available sunlight and habitat. These stands can also persist during the winter seasons because of the underground rhizomes.
Information for this species profile comes from the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016).