by Mark N. McCann
Creeping buttercup, Ranunculus repens may be the most insidious species of the non-native invasive ilk, that a maritime Northwest gardener will ever face. This Eurasian invader has many relatives in the Northwest, but on disturbed sites the R. repens can often be found forming unbroken mats. Aiding the buttercup is an allelopathic compound it produces called protoanemonin. This compound can also cause blisters and skin irritation in some humans and is hazardous to livestock that may browse on this common weed, though if cured in hay the poison becomes inert. In theory, this substance discourages other plant species’ competition nearby, but some native species seem to be unaffected. Skunk cabbage (Lysichiton americanum) can be seen popping up amidst large patches of R. repens, as well as the ubiquitous salmonberry (Rubus spectabilis), which may produce its own allelopathic defenses.
The R. repens can tolerate marshy, continually wet conditions to seasonally dry, heavy clays, just as salmonberry and skunk cabbage, but it is rarely found in deep shade or on soils that are well drained. Wet sites are seldom found in deep shade because few species that produce deep shade can tolerate “wet feet.” Hemlock (Tsuga heterophylla), Sitka spruce (Picea sitchensis) and the Western red cedar (Thuja plicata) are the deep shade, native conifers best suited for wet sites; however, even these may not survive in the boggy conditions where R. repens can be found thriving.
The tall buttercup (R. acris) has similar leaves and flowers to its cousin, R. repens—an upright growth habit is the greatest difference—but this species can be controlled with the simple “tug and lug” method. Additionally, R. acris survives on drier sites than the R. repens. Mowing or cutting down the R. acris before it can produce the large burr-like seeds will eventually clear an area of this pest, but this method requires vigilance.
R. repens reproduces by runners, seeds and vegetatively. Even a tiny portion of the root (1 mm) left in the ground can produce a fully formed plant within a year.
R. repens and R. acris are easily identifiable. The three-lobed leaves are unique to the genus and both are usually found in disturbed soils. Disturbed wetland or boggy areas readily support R. repens, and the sunny fringe areas often hold R. acris. The three leaflets are toothed and fringed, making these species easy to spot. R. repens often sport pale silver marks on the upper side of the leaves and can be found hugging the ground, in spots where frequent mowing occurs, or reaching upwards through competing species to heights of several feet.
All the Ranunculus species have the small, yellow flowers that endow them with the common name, buttercups.
Many sources recommend herbicides to eradicate R. repens but other methods exist. First, let’s examine the methods that are counterproductive for its removal.
The “tug and lug” method for this species is like building a castle with sand; even a small portion of the remaining rhizome can play into a full-sized plant. A rainy ride gardener would have to remove a foot or more of soil for each plant to insure success.
Turning the soil under may be the worst eradication method as this spreads the rhizomes and stems. Plus, this creates an ideal condition for seed germination.
Covering an affected area with a biodegradable light-proof material, such as cardboard or discarded carpet, may be the best method to keep R. repens at bay. Throw soil and straw on top of the covering and plant native ground cover to expedite the breakdown of the underlying material. Plastic should be avoided as a cover for two reasons—if the integrity of the plastic is compromised, the buttercups will find a way through the hole, and plastic is forever.
The R. repens will not tolerate prolonged dry periods or well-drained soils. Most rainy side gardeners find that clays dominate their soils, thus holding water longer than sand or gravel soils. Improving your garden’s drainage can be an important step to keeping R. repens in check.
The only function R. repens might serve is erosion control. Many native species can be used to this same end. Kinnikinick, Arctostaphylos uva-ursi or A. nevadensis, can hold steep banks, with the former better adapted to dry sites. Two blueberry species, Vaccinium caespitosum and V. uliginosum, grow well in boggy, sunny sites and produce palatable berries. Three additional species are bog cranberry (Vaccinium oxycoccos), crowberry (Empetrum nigrum) and bog laurel (Kalmia microphylla). For the gardener who prefers showy blooms, the monkey flower (Mimulus sp.) will become a passion. M. guttatus, the yellow species and M. lewisii, the pink flowering one, are the most commonly found of the genus.
The previously mentioned skunk cabbage and coltsfoot (Pestasites frigidus) should be reserved for the most avid native species gardener. One will smell just like its namesake and the other can take over a site.
One last native offered is the wapato (Sagittaria latifolia). This is for the gardener with a site that remains wet year round. The wapato was disappearing from the wild as were the wetland sites where it once flourished. Today, hunters will often plant wapato in ponds and wetlands to lure in geese and ducks, which are fond of the starchy tuber.
No known native fauna consume or use the R. repens or R. acris and it is poisonous to livestock.
Mark N. McCann is currently working on a book about Green Space and Stream Stewardship within the Vancouver-Seattle-Portland-Eugene (VSPE) Corridor. Based on his five years of experience removing invasive species, and replanting and propagating natives species within the Tryon Creek State Natural Area (TCSNA) as a member of the Adopt-A-Plot program. He has been a Citizen Member of the Tryon Creek Watershed Council, Friends of Tryon Creek and presently is a volunteer at Berry Botanic Garden, specifically in the Native Plant’s Trail and in the Propagation Department.
Photograph by Debbie Teashon