Invasive Species Factsheet: Water Primrose

Water primrose forms creeping aquatic mats on water surfaces and has upright stems when found on shorelines. It is characterized by its willow-like foliage spaced alternately along the plant stem.

Young water primrose plants have leaves that are more ovular in shape. The stems are reddish in color and tend to root freely at the nodes.

This plant has noticeably bright yellow flowers, each with five petals about 1-2" wide. Flowers of water primrose begin blooming in late July and finish by early September.

In North America, water primrose is found mostly in the southern United States. However, populations have also been observed in a few western states as well as portions of the mid-Atlantic region, as far north as New York.

In Pennsylvania, water primrose has only been documented in the southeast portion of the state and is therefore considered a high priority species in the Commonwealth. Any suspected findings should be reported immediately.

Water primrose prefers to reside in  riparian  and  lacustrine  habitats with slow moving or still water and is tolerant of water level fluctuations. Look for this plant in places such as ponds, lake margins, wetlands, and ditches.

Water primrose spreads vegetatively via stolons (i.e., runners) that stretch out on land and water surfaces. Their initial growth always begins along the shoreline of a waterbody, and over time, spreads out from the rooted and/or floating parts of the "parent" vegetation.

Water primrose can also reproduce via small plant fragments. If given the opportunity to travel on wildlife or recreational equipment, these plant fragments can establish new colonies.

To help prevent the spread of this and other aquatic invasive species, clean your boats, shoes, and other recreational gear and properly dispose of or leave on site any plant fragments you find.

Depending on its life stage, water primrose may be confused with other native and exotic floating and emergent  Ludwigia  species known from Pennsylvania including:

•  Creeping water primrose  (Ludwigia peploides)
•  Seedbox  (Ludwigia alternifolia)
•  Wingleaf primrose-willow  (Ludwigia decurrens)

Water primrose can be distinguished from these species by referencing the information provided below:

Creeping water primrose: Also an exotic invasive species in Pennsylvania, the flowering stems of this plant are creeping or floating, whereas water primrose has erect stems. Its flowers are smaller, with petals usually less than 1.5 cm long (greater than 1.5 cm for water primrose). Leaves tend to be wider and more rounded at the tips as compared to water primrose.

Seedbox: This native species has four petals and four sepals, whereas water primrose has five petals and sepals.

Wingleaf primrose-willow: Flowers have four petals and four sepals whereas water primrose has five petals and sepals. This plant is native to Pennsylvania, though rarely found; known only from Lancaster County.

Water primrose is capable of invading natural areas where it rapidly forms a floating mass that chokes out native flora and clogs waterways. It shades nearby aquatic vegetation and uses  allelopathy  to outcompete other plants. Water primrose further degrades aquatic habitats by reducing dissolved oxygen and growing over open water habitat.

In areas where water primrose is well established, biodiversity of native plant and animal species is severely threatened, often forcing their removal or decline.

Water primrose causes recreational activities to be impaired in affected waterbodies. In addition, it can negatively impact irrigation, fish passage, and flood control. Attempts should be made to recognize this species in the wild and remove it where possible.

Background photos: Both of these images are of the same waterbody showing a population of invasive water primrose. The image on the left was taken in October 2012; the image on the right was taken in July 2015. As you can see, the extent of the infestation is exponentially larger just three years later. Drag the left and right arrows (swipe feature) to view each image in its entirety.