The Milwaukee County Parks Natural Areas staff recently discovered small populations of two uncommon invasive plant species: lesser celandine (Ranunculus ficaria) and white-top (Cardia draba). Both are relatively new to Milwaukee County, which makes their correct identification and treatment important.
Lesser celandine is part of the buttercup family with a distinct yellow flower and shiny dark green leaves. It is a perennial, spring ephemeral that grows to approximately 4 to 12 inches in height. Growth characteristics are lateral through a cluster of tuberous roots, which form large mats in low open woods and floodplains. This particular patch was found in Lake Park which is located in the City of Milwaukee. There is a very narrow window when lesser celandine treatment is effective (early April to early May). A treatment of 2% Garlon 4 in bark oil is recommended to penetrate the waxy leaves. Manual control requires the removal of all bulblets and tubers. Be careful; the disturbance of the soil could invite other invasive species to colonize so manual removal is only appropriate on smaller infestations.
White-top, also known as hoary cress, belongs to the mustard family. It grows as a perennial forb with numerous white flowers giving the plant a flat-topped appearance. The species blooms from late April through June. If conditions are right, this aggressive plant will flower a second time and produce an additional crop of seeds later in the summer.
At maturity white-top can reach a height of two feet. This particular patch contained more than 300 stalks and was found in a disturbed area near State Highway 45, adjacent to the park-and-ride on the north side of Watertown Plank Road. Control of white-top is most successful when using a 2% treatment of Roundup. Hand pulling or digging is only useful on small infestations and requires complete removal of the plant every 2-4 years.
If you discover populations of lesser celandine or white-top in southeastern Wisconsin this season, please report the locations to the Wisconsin Department of Natural Resources at: http://dnr.wi.gov/topic/Invasives/