What's a weed?
You could call him a hortiphobe. When I told my Dad about this article, he said, “All I know is: a flower is something that dies right away; a weed is a plant that won’t die no matter what you do.”
Well, technically, nearly all weeds have some kind of flower—that’s how most plants reproduce—but otherwise, my dad had it pretty much right.
In your garden, you may call anything you didn't plant and don't want there, a weed. Seedlings from last year’s heirloom tomatoes may be unwelcome guests when they pop up right where you want your new lilies to flourish unharassed. Botanically speaking, however, a weed is a plant that has become widely disseminated, particularly due to human activities, and out-competes other plants. While a few native plants are weedy, non-native weeds are most problematic. Many weeds on our continent were brought here by European settlers who grew the plants in their gardens for ornamental, culinary, medicinal, or other purposes. Other weeds arrived as stowaways, in feed and in bedding for livestock.
Occasionally, a plant that is native to one region in North America may be considered a weed when it invades ecosystems elsewhere. For instance, tree lupine (Lupinus arboreus) is native to coastal southern and central California, but is considered an invasive non-native weed farther north on the west coast. Also, a non-native that’s considered relatively benign in one region where climate, soil, or plant competitors keep it in check, may prove so aggressive and successful in another region that it’s classified there as a noxious weed.
A troublesome weed may spread widely because it has many wind-blown or bird-carried seeds (like thistles), can grow in poor soils daunting to other species, grows rapidly and proliferates before other species get established, or grows easily on disturbed ground such as roadsides and agricultural areas. Some weeds (like kudzu and English ivy) also have ground-smothering foliage that keeps seeds of other plants from sprouting. When a diversity of native plants is replaced by many acres of a single non-native species, the associated diversity of native animal life is also reduced or lost.
Weeds are a big issue in agriculture, as well as plaguing home gardeners. They cost farmers money for herbicides and non-chemical controls. Several weed species have been responsible for livestock sickness, mortality, miscarriages or birth defects (e.g. tansy ragwort, Scotch broom, poison hemlock, larkspur, star thistle).
When a non-native invasive plant becomes a threat to native vegetation, agriculture, livestock or the economy, your state’s noxious weed agency may list it as a noxious weed. The federal government maintains a list of widespread noxious weeds, but state and regional lists have priority, and control is determined by state laws. Control may mean prohibiting the sale, transport, or growing of certain plants. Counties and municipalities may also maintain noxious weed lists and enact ordinances regarding their control.
Rogues’ Gallery: a few Horticultural Weeds
Two of the most serious noxious weeds in the West are Scotch broom (Cytisus scoparius) and English ivy (Hedera helix). Scotch broom is listed as a noxious weed in California, Oregon, Washington, Idaho and Hawaii. It is visible in fields, at woodland edges, on roadsides and near beaches throughout the coastal Northwest. It’s a tall shrub, woody at the base, with mostly green stems bearing very small leaves and many yellow blossoms that open in April and May.
Broom was valued as an ornamental and landscape plant by gardeners settling in California in the 19th century. It now infests an estimated 17 million acres in western Oregon alone, and is a widespread weed through much of California and western Washington. Mature plants may be as tall as nine feet and are very difficult to kill. An individual plant may live up to fifteen years, and during that time may produce thousands of tough, long-lived seeds that are spread by the tires of vehicles, by animals, by water and by the wind. It’s resistant to grazing because the leaves are toxic to mammals.
English ivy is listed as a noxious weed in Washington and Oregon, but it is not yet on the quarantine list. It has invaded thousands of acres, particularly in the Portland Columbia Gorge area, and can be seen invading forests on the Olympic Peninsula. It has also invaded parks in Seattle and other cities, seriously impacting park trees. Like broom, ivy has aggressive growing habits and is toxic, causing dermatological reactions when handled. The berries are toxic to many native songbirds, but are popular with some species, which spread the seed to new locations. Ivy smothers forest floors, suppressing all understory growth, including plants necessary to native wildlife.
Along with understory plants, new tree seedlings are unable to compete with ivy for space and sunlight. It weighs down even large trees and weakens them, sometimes leading to premature toppling. It can also damage fences and buildings. The estimated weight of English ivy removed from a single tree in Olympic National Park was 2100 pounds.
Another important weed in our region include English Holly, which has been grown agriculturally in the region for over a century, and is spread by birds that eat the berries and then deposit them with a bit of "compost" throughout our wooded areas. Tansy Ragwort (Senecio jacobea) is a yellow flower that blooms at around the same time as true Tansy. Geranium robertianum ("Stinky Bob") is a harmless-looking little thing with small bright pink flowers-- that can out-compete an acre of small native plants in a couple of seasons. Oxeye Daisy (Leucanthemum vulgare or Chrysanthemum leucanthemum) is the ubiquitous yellow-centered, white daisy, a relative of Shasta Daisy, seen filling large fields in summer, fields that formerly held an array of beautiful native wildflowers. There is no native daisy at all resembling it. Finally, Hypericum perforatum (the medicinal herb Saint John's Wort) is a robust invader that is toxic to livestock.
In the Southeast, kudzu (Pueraria montana var. lobata) has been such a pervasive noxious weed problem that the name of the plant has come to be a synonym for botanical imperialism. Originally introduced in the late 1800s as an ornamental and then actively promoted for erosion control and cattle feed in the 1930s, the plant has conquered an estimated 2-7 million acres throughout the Southeast, and has been found in small stands as far away as Washington state. Like ivy, it’s a climber that can bring down trees, fences and telephone poles.
In other areas of the country, noxious weeds include cheat grass (Bromus tectorum), star thistle (Centaurea solstitialis), salt tamarisk (Tamarix spp.), at least two species of knapweed (Centaurea spp.), donkey tail spurge (Euphorbia myrsinites), leafy spurge (Euphorbia esula), Japanese honeysuckle (Lonicera japonica), and tree-of-heaven (Ailanthus altissima). Purple loosestrife (Lythrum salicaria) is a beautiful perennial with pink-purple flower spikes. It invades moist areas, taking over large areas rapidly and crowding out native flora. It is considered a noxious weed of major concern throughout most of the US.
Getting away from invasives
Despite all this gloom and doom, it must be added that there are many non-native plants that are not weedy or invasive, are beautiful, and are perfectly fine for any garden.
Perhaps the most environmentally important thing that gardeners can do is NOT to plant invasive species, and to remove harmful species already on the site. Some invasives, those that spread primarily via rhizomes rather than seeds, can be safely grown with containment—in pots, or surrounded by concrete or metal barriers. Less-invasive or sterile cultivars (specially-bred strains) of plants like broom and ivy can be found in some nurseries. A sterile plant is one that does not produce viable seed, though it may still spread via rhizomes. Unfortunately, several “sterile” cultivars of purple loosestrife have proven to be plenty fertile.
What can you plant instead? Knowledgeable nursery and landscape professionals can advise gardeners on a broad range of alternatives to invasive plants. USDA Extension offices and Native Plant Society web sites also provide information about alternatives for environmentally benign gardens and landscaping.
GARDENING WITH NATIVES
What is a native plant?
We hear the term “native plant” a lot lately, but what does this mean and why would anyone care? Many folks assume that any plant they see everywhere must be native; so why be concerned about preserving something so common? Alas, a plant’s ubiquity these days is often due to its success as an aggressive non-native weed. In the coastal West, both Scotch broom and oxeye daisies (Leucanthemum vulgare) are widespread in woods and fields; these are noxious non-native weeds so prolific that they’re a threat to native vegetation. Purple foxglove (Digitalis purpurea) is so widespread that many people think it must be native; though not widely considered ecologically threatening, it’s a European import.
A native plant is one growing in the region where its species evolved. Some horticulturists will call a plant “native” if it is native anywhere on the North American continent, but most botanists and environmentalists use the term to refer to a region (Pacific Northwest, Great Plains, etc.) A few purists insist on genotypes specific to a very small piece of a region. To learn if a species is native to your region, your best bet is to consult a flora (a manual that lists all plant species native to a particular region) or a book on gardening with natives. Not all plants sold as "wildflowers" are natives; on the contrary, many (such as bachelor's buttons, Centaurea cyanus) are actually weedy non-natives.
Why garden with natives?
Native plants can be good choices for our gardens and landscapes:
1. Plants that evolved with the soil type and climate that obtain for your garden will have an ecological advantage and will require less care. They may also have more resistance to local disease organisms like fungi and to local plant predators like bugs. This makes the life of an organic gardener much easier. There are some great garden and landscape plants that can be called near-natives--- these may be close relatives of regional native species, with similar ecological requirements, thus conferring some of the native benefits of compatibility with climate and soil type.
2. Natives are good for wildlife: local butterflies, birds, and other animals have evolved along with the plants, and rely upon them.
3. By living with natives day to day in your garden, you get to know them better. Gardening is a creative activity, but it can also be a way of learning more about nature.
4. It is possible that in some cases the propagation of natives in gardens will help to preserve species as they face loss of wild habitat.
Many nurseries now carry at least a few native plants. Listings of these nurseries, as well as a wealth of general information on native species, can be found on the internet. Knowledgeable nursery personnel should be able to provide information on gardening with natives. The Native Plant Society, The Master Gardeners and USDA Extension sponsor some educational events focusing on natives. Conservation groups and concerned individuals in many communities have developed native plant demonstration gardens and botanically rehabilitated nature preserves.
A few Northwest noxious weeds
Cytisus scoparius- Scotch broom
Hedera helix- English ivy
Iris pseudacorus- yellow flag iris
Senecio jacobea- tansy ragwort
Impatiens glandulifera- policeman’s helmet
Leucanthemum vulgare (or Chrysanthemum leucanthemum)- oxeye daisy
Selected Resources
Weeds
Federal list: http://plants.usda.gov/java/noxious
Search by state: http://plants.usda.gov/dl_state.html
Information on English ivy and efforts to combat it: http://www.ivyout.org/
Native plant societies also often provide information on regional noxious weeds.
Native plants
Native Plant Society- at least 36 states and two interstate regions have a Native Plant Society with a web site, in addition to a few states with multiple regional and county chapters. Search the internet for your region. The Washington site http://www.wnps.org contains a wealth of information on native plants in our region.
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