A member of the primrose family (Onagraceae), Floating primrose willow is a floating aquatic perennial. Early growth is a rosette of leaves. These plants are most conspicuous in bloom, when bright, five petaled flowers emerge on stems held above the water.
Leaves are alternately arranged, appearing whorled from above, oval to lance-shaped, shiny, glabrous and up to four inches in length. (3)
Flowers are five petaled, bright yellow primrose-type flowers an inch in diameter and are often held on long stalks above the water. (3)
Floating primrose willow is an aggressive invader of the freshwater shorelines of lakes, ponds and streams, although it will colonize any slow-moving body water. Capable of traveling great distances via waterfowl, this species can establish new colonies from both stem fragments and seeds. Dense aggregations can shade out plants growing below it, thereby altering the diversity of the communities it invades. Furthermore, this species is allelopathic, inhibiting the germination of vulnerable plants. The thick vegetative mats formed by large infestations can cause anoxic conditions, particularly in the summer, reduce water flow and stabilize sediments, and increase sulphide and phosphate levels causing a cascade of ecological changes. (1)
The flea beetle, Lysathia flavipes, reportedly causes heavy damage to Floating willow primrose, although it is not target specific. (8)
Manual or Mechanical Control
Pulling / Digging Up: Not advisable. Although mechanized mass pulling on very large populations in conjunction with herbicide usage can help reduce seed rain. (7)
Mowing: Not advisable. Mowing or cutting can leave easily dispersible fragments than can establish new populations elsewhere. (7)
Girdling: Not applicable
Prescribed Fire: No information available
Prescribed Grazing: Not applicable
Soil Tilling: Not applicable
Mulching: Not applicable
Solarization: Not applicable
Hot Foam Spray: Not applicable
The pesticide application rates and usage herein are recommendations based on research and interviews with land managers. When considering the use of pesticides, it is your responsibility to fully understand the laws, regulations and best practices required to apply pesticides in a responsible manner. At times, the pest you seek to treat may not be on a pesticide label, requiring a 2ee exemption from NYSDEC. Always thoroughly read the label of any pesticide and consult the NYSDEC or a licensed pesticide applicator with questions.
Foliar Spray: There are a number of herbicides available for use on Floating primrose willow A wetland safe 1-2% solution of Triclopyr or glyphosate is effective at managing populations of Marsh dewflower. Treatment should be performed prior to fruiting/seed set. 2 4-D is also suitable for use, as is Imazapyr and Imazamox. Follow up treatment may be necessary in subsequent years. (7)
Cut Stump: Not applicable
Basal Bark: Not applicable
Stem Injection: Not applicable
Pre-Emergent Spray: Not applicable
General management overview and recommendation
As with any other invasive infestation, large occurrences of Floating primrose willow are best managed via a combination of mechanical and chemical means. Small populations can be hand pulled but care must be taken to remove all plant material. Dense patches must be sprayed to attain good control. All managed infestations should be monitored to prevent reinvasion from nearby populations, or re-sprouting of vestige fragments. Any new occurrences can be sprayed.
Post treatment monitoring
Any infestations managed by chemical means must be revisited in 2-3 weeks to check for treatment efficacy. Follow up spot treatments may be necessary for at least one year after initial treatment.
Any pulled for fragmented plant material must be bagged and disposed of in a landfill or thoroughly dried and composted.