Photo credit: © Tony Rodd, https://flic.kr/p/8rVsGC
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Species at a Glance
European water-starwort, also known as common or pond water-starwort, is a small, delicate, aquatic perennial herb that roots to the substrate in shallow water. The appearance of this species can vary slightly depending on its habitat and growing conditions.
Leaves: Opposite leaves are arranged in pairs along the stem and are both floating and submerged. Floating leaves are oval-shaped, have 5-7 veins, and are up to 2 cm (0.8 in) long. Submerged leaves are typically narrow and linear, have a single vein, and grow 4-10 mm (0.2-0.4 in) in length; however, they can also be broader and more closely resemble the floating leaves.
Flowers: Tiny, simple flowers don’t have sepals or petals and are typically located right next to one another in the leaf axils. The flowers can have 0-4 (but usually two) small, white bracts at their bases, which may help the flowers float on the water and aid in pollination.
Fruits/Seeds: Rounded fruits are 1.5-2 mm (0.06-0.08 in) thick with a thin wing extending from the base to the head of the fruit. Four compartments, each containing one seed, do not split when ripe.
Stems/Roots: Elongate and branched stems can grow from 10-30 cm (4-12 in) in length, rising or sprawling to the surface.
May be confused with other water-starworts (Callitriche spp.); however, the mature fruits of other species are not round in shape and lack the distinctive wing on the fruits. Due to the variability in leaf shape and size, mature fruit must be examined for positive identification of all water-starworts. European water-starwort may also be confused with other opposite-leaved delicate plants when not in fruit, such as mudwort (Elatine sp.) or horned pondweed (Zannichellia palustris).
This species is typically found in shallow waters of lakes, ponds, rivers and streams. It can tolerate some salinity such as brackish waters or salt marshes and flats.
European water-starwort was first recorded in New York in 1861, most likely brought over in the ballast water of ships. Once introduced, it became a popular aquarium plant and subsequent escapes and releases have contributed to its spread throughout North America. It spreads through both seeds and plant fragments. Seeds can pass through the digestive tracts of birds and be released in new locations, and plant fragments and stems can easily attach to boats, trailers, and other aquatic recreational equipment.
Introduced from Europe and North Africa, this species is found in the Pacific Northwest, in the Mid-Atlantic region, and has scattered populations in Tennessee, Alabama, Montana, and Wisconsin.
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.
European water-starwort is capable of forming dense mats of vegetation that may crowd out and displace native aquatic vegetation.
Information for this species profile comes from the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016).