Rorippa sylvestris aka Creeping Yellow Cress

Rorippa sylvestris, also known as creeping yellowcress, keek, or yellow fieldcress, is an invasive plant species that is believed to have entered the United States from Europe before 1818 through contaminated nursery seed stock or ballast water. Native to Europe and Asia, it has now widely established throughout North America. This perennial plant forms dense colonies and can be confused with the native annual Bog Yellow-cress. However, the flowers of Rorippa sylvestris are more than twice the size of R. palustris, and its mature fruit is longer and more slender. The herbaceous stems can grow up to 35cm tall and are ribbed, glabrous, and green or purple in strong sunlight. The leaves are deeply pinnatifid, alternate, petiolate, and glabrous or with very few short hairs. The inflorescence is a terminal and axillary raceme that can reach up to 10cm long. The siliques are cylindric but slightly compressed, glabrous, and up to 1cm long, with a beak up to 1mm long. The flowers have four distinct, spatulate, yellow petals that are up to 4mm long and six erect stamens. The plant spreads by creeping, white, fleshy roots and is very difficult to control once established. Physical control methods can exacerbate the problem, as the plant readily regrows from rhizome fragments. Pre-emergent herbicides only suppress top growth and do not kill the rhizomes, while selective post-emergent herbicides are not available. Glyphosate is marginally effective, but the best way to identify Rorippa sylvestris is by comparing its roots to those of other similar species.