Wee, modest, crimson lopped flower,
Thou’s met me in an evil hour;
For I maun crush amang the stour
Thy slender stem:
To spare thee now is past my pow’r.
Thou bonnie gem.
Robert Burns, “To a Mountain Daisy”
It is late winter as I walk along a coastal trail in northernmost California. Spanish heath (Erica lusitanica) in all its glory is growing up against a canopy of scarlet-berried cotoneaster (Cotoneaster franchetii). The Spanish heath has tall spires of pink tinged buds that open up into tiny, white, bell-shaped flowers atop stems of fuzzy green leaves—a distinct contrast to the dark green foliage of the cotoneaster. From the trail, a steep path down to the beach is flanked by the white heaths, some of the bushes six feet high.
Further along I see the waving plumes of pampas grass (Cortadera jubata and C. selloana), tall and stately—a spectacular addition to the landscape. At my feet are patches of ice plant (Mesembryanthemum carpobrotus) with its succulent leaves and, at this time of year, an occasional pink or yellow flower. Catching at my clothes are the brambles of Himalayan blackberry (Rubus discolor) with their promise of succulent fruit in the summer, and scattered among the brush are stands of Scotch broom (Cytisis scoparius).
Alas, they are miscreants all.
Spanish heath, pampas grass, ice plant, Himalayan blackberry, Scotch broom, and cotoneaster are all listed by the US Fish and Wildlife Service as among the thirteen worst invaders and most harmful weeds of Humboldt County. Since Spanish heath is a favorite of florists, and cotoneaster and pampas grass are popular landscape plants, it is not easy to persuade us that they present a threat. The fact is that, if unmolested, they will take over huge tracts of land and wipe out our less aggressive native plants as they spread. This is a scenario that will lead to vast areas of the country covered in monocultures of exotic weeds.
In early summer here, the most dramatic examples of take-over are fields and roadsides of dazzling Scotch broom. Covering the sand dunes are colonies of tall plumed pampas grass and yellow bush lupine (Lupinus arboreus), which is actually native to the central coast of California. All are handsome in themselves, but an overwhelming threat to our local natives.
Scotch broom is exceedingly aggressive and not only crowds out native plants but is difficult to remove completely, as any remaining roots will resprout. Pampas grass, native to South America, thrives on coastal dunes. It is also difficult to remove and produces millions of viable seeds annually. Fortunately, it abhors shade and, if any roots remain after the plants have been dug out, a covering of black plastic will inhibit further growth.
The yellow bush lupine is partial to the sand dunes where it crowds out the native blue lupine (Lupine littoralis). Its history is a familiar story of good intentions undermined by inadequate information. Early in the twentieth century, seeds of the yellow lupine were spread along the dunes to stabilize them and to prevent sand from covering the railroad tracks. The tracks have long gone, but the yellow lupine flourishes, as does European beach grass (Ammophila arenaria) planted for the same purpose and probably even more destructive to the native dune community.
Where do these interlopers (or exotics) come from and why can’t our native plants hold their own against them? Some of these exotics have become established from seeds accidentally dispersed through human activity since the country was first settled. Others have been brought in intentionally for landscape purposes. Most of our foreign invaders are harmless and co-exist with native species, but others that have no natural enemies here reproduce rapidly and crowd out native plants. The Himalayan blackberry creates impenetrable thickets (to which many a berry picker can attest) and smothers indigenous growth. Ice plant raises the salinity of the soil, whereas Spanish heath and yellow bush lupine can change a soil’s pH, making it less hospitable for natives. In addition, yellow bush lupine and European beach grass stabilize the sand on the dunes so that the delicate flowers of the natural dune mat (???), which depend on shifting and drifting sand, are unable to survive. Unfortunately, the mild climate of our coastal regions creates a haven for many exotics.
The flowers that comprise the dune mat, which we are rapidly losing, have a fragile, understated charm. Yellow sand verbena (Abronia latifolia) has a delicious smell. The common beach buckwheat (Eriogonum latifolium), with its rust colored pompoms, and beach strawberry (Fragaria chiloensis) are but a few of those at risk. The Humboldt Bay wallflower (Erysimum menziesii subsp. breviflora) and the rare pink sand verbena (Abronia umbellata subsp. breviflora) are both federally listed as endangered species. Since dunes are limited in California, representing only twenty-three percent of the coastline, they present a critical challenge.
Invasive plants are not just a local concern but a national problem of enormous proportions, ranging from kudzu vine (Pueraria lobata) galloping across the south to purple loosestrife (Lythrum salicaria) filling wetlands in the north. The Bureau of Land Management estimates that every day 2,300 aces of its land are lost to invasive plants. Such plants invading agricultural and natural areas cost the country over $20 billion a year.
We are in a dire situation. However, contributing their widow’s mite, local volunteers have toiled to create an oasis here and there where native plants can flourish. Our local organization, Friends of the Dunes, works with the California Native Plant Society to organize work parties for dune restoration at Manila and Lanphere-Christensen Dunes, to arrange for interpretive walks, and to otherwise educate the public. The war on weeds is also waged by the Fish and Wildlife Service, Bureau of Land Management, and within the state parks of California. Even Caltrans, the state’s transportation agency, now has a policy for planting native species along its roadsides.
Scientists, too, are contributing. At Michigan State University, experiments are underway to rear galerucella beetles (Galerucella calmariensis and G. pusilla) that feed on the leaves of loosestrife, in an attempt to halt the rapid spread of the plant in Michigan. Work continues on biological controls, but these sometimes have unforeseen consequences and, as yet, are only used in test areas.
Should we not leave nature to do its thing instead of imposing on it our perception of what is best? Disturbances of the land and the intrusion of exotic plants are not new developments, but the rate of spread of these invasives over the past two hundred years has increased dramatically and is certainly not a natural trend. Since humans have been responsible for this, it is not unreasonable to expect us to rectify it.
And what can we gardeners do? We can avoid growing these and other potentially invasive plants in our gardens and keep a close eye on the nurseries we patronize. Three of the worst culprits (Scotch broom, Spanish heath and pampas grass) are still on sale at most nurseries in the state.