I’m embarrassed to admit that when I first moved to the Pacific Northwest in 1990, before I knew much about regional native plants, I thought that foxgloves were native plants. Why? Because I encountered them in natural areas. Luckily, I know much better now and—with the exception of some infrequently traveled trails in remote corners of the world—I cannot remember a hike where I haven’t encountered invasive plants (and sometimes a terribly large number of them). Areas close to urban areas are hardest hit, but even ecosystems far from the madding crowd can suffer from their effects.
Invasive plants are nonnatives that were—and continue to be—brought here either intentionally by the nursery trade (or agriculture), or accidentally (as packing material and such). Thousands of species have been brought to North America, and many of ours have been sent abroad. All this rearranging of the earth’s flora started innocently enough centuries ago, but experts fear that it’s reached a point where biological diversity is severely threatened and essential interactions, like pollination, are damaged. Foxglove (Digitalis purpurea), as lovely as a biennial can be, may not be one of the worst offenders, but it doesn’t stay put with its countless tiny seeds, and shows up in places it doesn’t belong, basically making life miserable for the native plants that do. More problematic species often reproduce in several ways: For example, Himalayan blackberry and English ivy (shown in top photo) and its cultivars spread via rooting stems and by fruits eaten and dispersed by wildlife. Both suppress and exclude native vegetation and form dense monocultures that are unsuitable as wildlife habitat. English ivy is capable of one other feat, if left alone long enough: Killing entire trees.
Of course, not all nonnative plants pose horrendous problems, but those that do run amok are able to because whatever keeps them in balance in their native land—soils, predators, pathogens or weather conditions—are lacking here. Consequently, they do so well that they’re able to spread fairly easily from yards or agricultural areas into natural areas that support native species that can’t compete; the natives have no defense, become overwhelmed by the newcomers, and die out. This is particularly devastating for uncommon or endangered plants close to extinction. In addition, the spread of invasives (plant, animals, and pathogens) has economic ramifications.
Deadly for wildlife
While habitat loss due to deforestation, urban sprawl, livestock grazing, and agriculture is the greatest threat to the variety of life on Earth, invasive plants contribute greatly to the tragic loss of biodiversity. Since native plants are essential for native fauna (especially insect herbivores, most of which are specialists that can only use a certain plant or plants due to their chemical makeup), when natives are gone, so too are the herbivores and the higher life forms that feed on them. And, needless to say, fauna use native plants for other essentials, like nesting habitat and shelter.
Some nonnatives are also poisonous. It’s not unusual for cedar waxwings to be poisoned by the fruit of heavenly bamboo (Nandina domestica). And during a recent winter, many wild elk and pronghorn died horrible deaths in Idaho after foraging on Japanese yew (Taxus japonica), which is considered invasive in some states. Hungry bears also have been poisoned in Pennsylvania by English yew (Taxus baccata), and other animals—including livestock and people—can also be poisoned. Instead of nonnative yews, we can plant regional/local yews that wildlife coevolved with. The Pacific Northwest’s yew, Taxus brevifolia, which provides food and cover for many wild species, is the best choice from British Columbia to northern California and east to Montana, at mid to high elevations. Sadly, this attractive understory shrub that grows beneath conifers is in trouble due to over-harvesting for medicine, as well as the logging industry.
Hard work pays off
Research from the Seychelles, an archipelago in the Indian Ocean, shows that sweat and funds invested in eradication can pay off for all sorts of pollinators (bees, butterflies, beetles, birds, reptiles), for the native plants themselves, and for an entire ecosystem. Following the removal of nearly 40,000 invasive shrubs on four mountaintops on one island, researchers monitored the remaining native plants for visits from pollinators. Eight months of observation later, “Ecosystem restoration resulted in a marked increase in pollinator species, visits to flowers and interaction diversity.” Essentially, even during the rather short test period, it was found that both the number of pollinators and their interactions with plants and each other were over 20% higher in the test areas than in control plots (where the invasive shrubs had been left alone). And, the test area native plants also produced more flowers and fruit than those in control areas. Restoration works!
WHAT YOU CAN DO
♦ Eradicate them. Early detection and removal is crucial to stopping an invasive plant in its tracks, especially if you live near a natural area. To make it feasible, and if you have a variety of invasives, pace yourself—perhaps get rid of one species a week (or one a month or season, depending on the infestation). I strongly recommend forgoing pesticides (even so-called natural ones) and manually digging them out whenever possible. Digging when the soil isn’t saturated is best, to prevent destroying the soil structure that results when working wet soil. And if your arch-enemies grow on a steep slope, be sure to replace them with native erosion controllers (Oregon white oak, madrone, red alder, oceanspray, red-twig dogwood, Nootka rose, kinnikinick, salal, sword fern, etc.—whatever species are local and will do well in the light and soil conditions) as soon as you can; a biodegradable jute netting can be laid down to prevent erosion while new plants fill in.
At the very least, cut stems off at the soil level well before plants go to seed (it can happen quickly!). This method doesn’t disturb the soil (which can invite the germination of more weed seeds and steal moisture) but it can be tedious. Some species can be cut to the soil level and then be covered with a dark cloth like a dark thrift-store bedsheet to block out light (not plastic, which will prevent moisture from reaching the soil and kill soil life). Left for a year or so, it will prevent photosynthesis; afterwards, check to see if you need to dig out any live roots. Persistence usually pays off. In hard to reach places, such as beneath tree or shrub roots, repeatedly cut down or yank out leafy stems—eventually the plant will die from the lack of energy that sunlight provides. The morning glory vines that come from under a dense shrub in my yard get weaker every year because we continually pull out what we see. I seriously think this year may be their last.
One exception to the get-it-out-as-fast-as-you-can rule: If the invasive plants are providing some habitat for wildlife (nesting sites or food or cover), do a soft eviction and take them out gradually or incrementally, after nesting season, rather than all at once. This will avoid completely eliminating the habitat and causing undue stress to wildlife.
Please note: If you need to eradicate English ivy that’s climbing on a tree, cut the vines at the base of the tree but don’t pull it off the bark because bark can be damaged and possibly contribute to a tree’s death.
♦ Remember that some seeds can survive for many years. When I first started gardening in my yard, there were a lot of Robert’s geranium (Geranium robertianum) a.k.a. “Stinky Bob”. I made sure I pulled all the plants before seeds had set, but the next year they were back due to previous years’ seeds. I pulled them again and again, always before they flowered. Fifteen years later, I’m still pulling, but this year there were only two plants! Moral of the story: some seeds can stay viable a very long time, so don’t you dare let up on your weeding. But of course neglected neighboring yards can supply seeds as well, so it’s a continual process. Before planting natives, wait at least a year after the initial removal. Weed again, and then plant. It may not eliminate the seeds, but it should cut down on future seedlings and give the natives the best chance at taking control again. Growing assertive natives, those so-called “pioneer species” or “early seral” plants generally will be better at competing with weedy non-natives.
♦ Know what you’re planting. Don’t buy newly introduced plants that lack a track record, or seed mixes that may contain invasive seeds, especially ones labeled just “wildflowers.” If you want a wildflower meadow or prairie-style garden, buy only seeds that you know are native to your location and you won’t have to worry. Even though many native “pioneer species” (especially annuals) can be quite assertive, if they spread enthusiastically they won’t wreak havoc on the environment. Species from different regions of the country can be problematic, not just those from Europe or Asia, so go with only your local native plants whenever possible.
♦ Speak up if you notice plants for sale that are problematic. I’ve seen Arum italicum and Vinca minor and many others for sale at local retail nurseries, even though they’re on my city’s “Nuisance List” (and I’ve seen Stinky Bob, too!). The thing is, just because plants are deemed invasive or a “nuisance” species, doesn’t mean they can’t be sold—the only plants that are illegal to sell in a particular state are those that have been officially listed as a state noxious weed. But if enough of us educate retailers, hopefully they will pull the plants from their catalog/store.
♦ Besides eliminating invasives in our yards, we need to be very careful about what we’re dragging into natural areas on our hiking boots or sneakers. Plant material like seeds can get stuck in the tread of shoes, and some stick like velcro to laces, like the seeds of the aptly named forget-me-not. And backpacks and pant cuffs can harbor and release seeds, as well as dogs’ paws and fur. When I encountered Stinky Bob in a beautiful natural area last year in the Columbia Gorge; it had already spread over a slope as big as my back yard. No doubt someone unknowingly carried the seed there and the plant that resulted liked it there—a lot.
♦Tell others about the harm that invasives pose.
♦Join a local invasive plant eradication effort.
♦ If you see infestations in natural areas report them to the local soil and water conservation district or to an invasives hotline like Oregon’s www.oregoninvasiveshotline.org.
Depending on your location and conditions, what are some possible native substitutes for the overzealous travelers, once they’re removed? In the Pacific Northwest, to replace English ivy (and cultivars), consider salal (Gaultheria shallon), kinnikinnick (Arctostaphylos uva-ursi), sword fern (Polystichum munitum), star-flowered false solomon’s seal (Maianthemum stellatum), inside-out flower (Vancouveria hexandra), or Cascade Oregon grape (Mahonia nervosa). Himalayan blackberry might be replaced with thimbleberry (Rubus parviflorus), salmonberry (Rubus spectabilis), or black-cap raspberry (Rubus leucodermis var. leucodermis). Arum could be succeeded by false solomon’s seal (Maiantheum racemosum) or vanilla leaf (Achyls triphylla). Vinca could be ousted by piggyback plant (Tolmiea menziesii), broadpetal strawberry (Frageria virginiana), or oxalis (Oxalis oregana or O. suksdorfii). And Stinky Bob might sublet his space to Western bleeding heart (Dicentra formosa), Oregon geranium (Geranium oreganum), or licorice fern (Polypodium glycyrrhiza). Always research plants’ needs and mature sizes before planting and choose those that would occur naturally in your area.