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 we’ve sent many of ours 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 billions of tiny seeds, and shows up in places it doesn’t belong, basically making life miserable for the plants that do. More problematic species often reproduce in several ways: For example, Himalayan blackberry and English ivy (shown above) and its cultivars, both spread via rooting stems and by fruits eaten and dispersed by wildlife. Both suppress and exclude native vegetation to form dense monocultures, entirely unsuitable as wildlife habitat. English ivy is capable of one other feat, if left alone: taking down entire trees.
Of course, not all nonnative plants pose horrendous problems, but those that do run amok do it because whatever checks—soils, predators, pathogens—kept them in balance in their native land 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. 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 specialist species that can only use a certain plant or plants), animals suffer from the demise of native plants that they may use for forage or other purposes. And some are actually poisonous. It’s not unusual for cedar waxwings to be poisoned by the fruit of heavenly bamboo (Nandina domestica). And this past 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
Recent 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 the whole 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 (go on, git!). 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 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 slope, be sure to replace them with native erosion controllers (Oregon white oak, madrone, oceanspray, red-twig dogwood, Nootka rose, salal, sword fern, etc.) as soon as you can.
At the very least, cut stems off at the soil level well before plants go to seed (it can happen quickly!). Some species can then be covered with a dark sheet (not plastic, which will prevent moisture from reaching the soil and kill soil life), to block out light. Left for 6 months to a year, it will prevent photosynthesis; afterwards, check to see if you need to dig out any more live roots. Persistence usually pays off. In hard to reach places, 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 incrementally, rather than all at once. This will avoid completely eliminating the habitat and causing undue stress to wildlife.
♦ 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”. Dutifully I pulled all the plants before seeds had set, but the next year they were back due to the previous year’s 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 season after the initial removal. Weed again, and then plant. It may not eliminate the seeds, but it should cut down on future sprouts and will give the natives the best chance at taking control again.
♦ 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 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!) You see, 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/stores.
♦ 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 a 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? For 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).