Photo credit: Vern Wilkins, Indiana University, Bugwood.org
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Species at a Glance
Creeping water-primrose, also called floating primrose-willow, is a herbaceous, perennial, wetland plant whose sprawling stems usually grow flat along mud or a water surface. Although native to parts of North America, this species has become invasive outside of its native range, including Washington State where it is listed as a Class A noxious weed because of its ability to form dense floating mats that can displace native aquatic plants and wetland grasses.
Leaves: Alternately arranged leaves are clustered together and vary in size and shape from long and slender to round or egg shape. They are up to 9 cm (3.5 in) long. Leaf bases taper to a stalk that ranges from 2.5-4 cm (1-1.5 in) long. The leaves have smooth margins and are either hairless, or have long, soft hairs. The leaves are dark green with light green pinnate veins.
Flowers: Showy flowers have five bright yellow petals that are 1-1.5 cm (0.4-0.6 in) long and bloom late July to August. The flowers occur on long stalks arising from the leaf axils.
Fruits/Seeds: Capsules contain many small (1 mm) seeds.
Stems/Roots: Flowering stems are either floating or lying on the ground. Sprawling stems, which can reach a length of 2.7 m (9 ft), are fleshy, reddish in color, and either hairless or slightly hairy.
Similar species include the marsh seedbox (Ludwigia palustris), which has flowers without petals; true forget-me-not (Myosotis scorpiodes), which has blue flowers and a distinct midrib with less apparent branching veins; and water smartweed (Polygonum amphibium), which has similar floating leaves to water-primrose, but the flowers are thick spikes of bright pink flowers.
This species grows along freshwater shorelines and sprawls across the water’s surface. It typically inhabits still or slow-flowing freshwater habitats including the margins of wetlands, lakes, ponds, ditches, and streams in depths of up to 3 m (10 ft). It is very adaptable and can grow under a variety of nutrient, water quality, and substrate conditions. Its roots are able to absorb atmospheric oxygen, allowing it to survive in low oxygen waters, grow up on land, and tolerate dry periods.
While it produces viable seeds, creeping water-primrose spreads mostly through fragmentation. Small pieces of stem can be spread to new areas by wind, water flow, or animals where it grows roots and develops into new plants.
Creeping water-primrose is native to South America, Central America, parts of the southern United States, and possibly Australia. In the United States, it is native from South Carolina to Kansas and south to Texas and Louisiana, but it is considered invasive in all MidAtlantic states.
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.
Dense floating mats of creeping water-primrose can outcompete native plants and clog waterways, increasing the risk of flooding and affecting recreation, fishing, and navigation. This species also alters the chemistry of the aquatic environment, reducing dissolved oxygen levels by shading out submerged plants, impacting pH, phosphate, and nitrate levels, and making the habitat unsuitable for some native species.
Information for this species profile comes from the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016).