Noxious weeds infest about 100 million acres of North America and conquer more than 3 million acres each year. You can help stop the invasion of the non-natives by paying attention to what’s in your own yard.
A few years ago I saw a bumper sticker that said, “All who wander are not lost.” As someone who has moved around a lot, I liked the slogan. It made me feel better about all the places I’ve lived and roamed. I certainly know what it’s like to be a non-native.
For better or worse, it seems modern humans have always been adventurers looking for a better place to call home. Settlers have forged their way across wilderness, more often than not at the peril of those who were there before them. Today we move around the globe farther, faster, and more often than at any other time in our history. Despite our restless searching, however, we also like to bring a piece of our roots with us wherever we go. The evidence lies, at least in part, in the fact that most of the cultivated flowers, vegetables, fruits, grains, grasses, and clovers—and nearly 70 percent of the weeds—have come to the United States from other parts of the world.
Many plants introduced to the United States are such a part of our cultural heritage and familial traditions that most of us would never dream that they’re not native species. Where would we be without wheat, potatoes, and many varieties of apples? How many times as a child did you twirl a bright yellow buttercup between your fingers? Who isn’t at least tempted every December to purchase a bright red poinsettia? And perhaps you enjoy kale as a dinner vegetable, or a salad of lettuce and cucumbers. None of these are native to this country.
Invasion on U.S. soil
The introduction of non-native plants has been on the rise in recent years as a result of human population growth, increased international travel, and international trade. The problem becomes especially thorny when introduced plants become invasive and displace local native plants and animals, sometimes dangerously altering entire ecosystems. When a plant from another home is brought to a new environment, often the natural controls that limit its growth don’t exist. Aggressive invaders reduce the amount of light, water, nutrients, and space available to native species and alter hydrological patterns, soil chemistry, moisture-holding capacity, and erodability.
Consider hydrilla, a native of Asia, Australia, and Africa. This aquatic plant, introduced in the 1950s, has been choking generating stations and municipal water supplies in Florida and clogging rivers and lakes in California with thick green carpets that can grow as much as ten inches a day. Or consider purple loosestrife. First introduced in the early 1800s from Europe for its striking flowers, purple loosestrife quickly traveled across the United States. Today, known as the “purple plague,” it has grown into dense stands covering thousands of acres of wetlands, crowding out native plants and animals in the Northeast and Midwest.
Or kudzu. Originally from Asia, kudzu was brought to the southeastern United States about a hundred years ago to help prevent soil erosion. In her book Despicable Species (The Lyons Press, 1999), Janet Lembke explains that by the mid-1990s, kudzu had laid claim to more than 7 million acres—almost 11,000 square miles—of the South—and it seizes another 12,000 acres every year. Understandably, it has become known as the vine that ate the South.
You mean these aren't native?
If you’re a gardener, you probably visit your nursery each spring on the lookout for new varieties to make your garden more unique and colorful. Or perhaps you need help with soil erosion. Or maybe you’re too busy to spend time working in your yard and want something tolerant that will virtually grow itself. As a result, nurseries search out plants they think their customers will like, plants that establish and grow quickly, or ornamentals with unusual flowers. Consequently, in his book Out of Bounds: Bioinvasion in a Borderless World (W.W. Norton & Company, 1998), Chris Bright explains that “horticulture, which is a minor industry in terms of economic size, has helped unleash some of the world’s worst plant invasions.” In the continental United States and Canada, garden introductions are estimated to account for about half of the 300 or so really serious pest plants, Bright attests.
Colorado State weed coordinator Eric Lane notes, however, that the horticulture industry seems genuinely interested in finding solutions to the problem of invasives. The American Nursery and Landscape Association is working with The Nature Conservancy and other organizations to develop codes of conduct to help stop the spread of invasive non-native plants. Lane warns, however, that without national bans on problem plants, individual states are left to decide which plants to ban, and that creates a lot of confusion. For example, purple loosestrife is banned as a noxious weed in many states yet is still available for sale in others.
Regardless of where we call home, there are useful strategies to help prevent the introduction of invasive non-native plants. Begin by looking over your existing yard and garden and becoming familiar with the plants there. Then educate yourself about invasives in your region. Lane recommends contacting your county extension agency or your local native plant society. Either of these organizations can give you the shortlist of which plants are a problem in your area and what to avoid at the nursery.
If it’s too late for prevention and you’ve already got invasives in your landscaping, you’ll want to decide how to manage and remove them. Lane gives the following advice: “The most important thing in weed management is persistence, persistence, persistence.” This is especially true, he explains, if you choose not to use herbicides. Manual and mechanical methods range from simple hand pulling to the use of weed-pulling tools, mowing, and girdling.
Unless you’re managing weeds in a small, contained flowerbed, it’s important to limit soil disturbance, which exposes seeds to sunlight and aids in germination. Lane explains it’s also important to know the life cycles of your unwanted plants. If it’s an annual, for example, hand pulling can work very effectively. If it’s a biannual, it’s important to get the root; if it snaps off when you pull it, you can be sure it’ll be back next year. In fact, if your weed has a rhizomatous root system—roots that spread horizontally beneath the soil surface—hand pulling can actually stimulate plant growth. If it’s a perennial, you may need a more complex strategy. And in many cases, be careful because mowing can make things worse. Laying down weed fabric barriers and mulches can work for a small area, but they’re not selective and will kill everything beneath them. Contact your county extension agent and native plant society for tricks of the trade in your region.
We can’t all return to the places we were born, nor would we want to. But we can learn to respect where we are. As Ernest Callenbach explains in Ecology: A Pocket Guide (University of California Press, 1998), “All living beings, including humans, have complex interactions with the climate, altitude, soils, geology, and landforms of the places where they live.” So take a slow walk in your yard, your neighborhood park, your community, the closest open space land. Walk with a curious eye and ear. Get to know the place you call home.
The Gardener’s Weed Book: Earth Safe Controls (Storey Books, 1996), by Barbara Pleasant, is full of practical suggestions for non-chemical weed-management strategies. Following are a few of her suggestions:
• Use cultivated plants to smother unwanted weeds by sowing a specially selected companion crop that forms a weed-suppressing ground cover. Ideally the companion crop should sprout quickly, be short in height, and have broad leaves.
• Use natural mulches such as grass clippings or fall leaves to prevent sunlight from germinating new seeds and encouraging weed growth. Commercial bark mulches work well also.
• Make sure your garden isn’t too large for you. The bigger it is, the more work it will be to stay on top of weeds.
Halting the invasion
The following strategies for preventing the spread of invasive non-native weeds are from Alaska invasive plant program assistant Marta Mueller and Colorado State weed coordinator Eric Lane.
• Avoid digging up or collecting seeds from plants growing along roadsides or while on vacation.
• Don’t give away plants escaping your garden unless you know for certain they’re native, and then only to someone who lives in the same region.
• Ask your nursery retailers about the plants they sell. If they don’t know whether they’re invasive, don’t buy until you can find out.
• Be careful of plants or seeds whose literature says they’re good in all climates, rainfalls, and soil types. These are the qualities of invasive plants.
• Make sure your seed packages are clearly labeled. Many wildflower mixes contain seeds for non-native varieties. Even if a seed packet is clearly labeled, pay close attention to what sprouts. If you don’t recognize it, pull it. Each state has different regulations about seed packet contamination.
• Tell your local nursery owners your concerns regarding invasive non-native plants. It’s important they understand that, as a consumer, you care about this issue. And be prepared to help educate them; they may not know which plants are cause for concern.