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WEED WATCH: Don’t Plant a Pest! |
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Written by Susan Donaldson
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Tuesday, 01 July 2008 |
Have you ever planted something in your garden, only to find that it does a little bit too well, and spreads to take over the entire bed? A friend once gave me some lamb’s ear, Stachys byzantina, which I planted in a bed that borders my lawn. The plants grew well, expanded to fill the area, and produced pretty pink blooms. Once the bed was full, however, the lamb’s ear kept expanding. Now I pull pesky sprouts out of my lawn each year. However, it does very well with a minimum amount of effort and water, and the sprouts are fairly easy to pull, so I’m not ready to get rid of it yet.
There’s another subset of plants with which we have not been so lucky. Like lamb’s ear, they’re not native to North America. Brought to the United States for a variety of purposes, these plants have escaped cultivation and become major pests of agricultural areas, landscapes, and wildlands. Most often, these plants are imported as seed or plant stock, and have been separated from the natural predators and diseases that helped keep them in check in their native lands. When grown in areas with suitable climate and soils, these plants can therefore run rampant. This group of weeds is often referred to as invasive ornamental plants.
If a plant is imported for use as an ornamental, why is it called a weed? The traditional definition of a weed is a plant growing where it is not wanted. Anything can be a weed if it interferes with our goals for our landscapes. For example, I do not want lamb’s ear growing in my lawn, so in that location, it is a weed. In my border planting, it is an ornamental. There are a number of attractive ornamentals that have become major invasive weed pests. These plants are considered pests no matter where they grow. Of the about four thousand nonnative plant species in the United States that have escaped cultivation, nearly seven hundred create significant problems for farmers and cost us billions of dollars annually. More than one thousand invaders threaten our native plants and animals due to their aggressive, invasive characteristics.
One of the best-known examples today is the tamarisk tree, Tamarix spp., also called saltcedar. Originally imported from the Mediterranean during the 1850s, several species of tamarisk were planted in landscapes, as windbreaks and for erosion control. Because it is tolerant of salty, alkaline soils, saltcedar thrives in arid sites in the West where other trees could not survive. In fact, at one point, many resource professionals felt it was the solution to difficult growing conditions in the West. Unfortunately, tamarisk has been too successful. It has escaped into virtually every riparian area and wetland in the West, including the Colorado River, Virgin River, lower Truckee River, and Humboldt River.
How has this tree become so invasive? Every year, each mature tree can produce as many as five hundred thousand seeds. The seeds fall into the water and are carried downstream. The plants grow rapidly, forming large shrubs in just one year. Lacking natural predators in the West, there is nothing to stop the spread of this weed. And, it also has a couple special tricks that help ensure its success.
Tamarisk is a water hog, taking in huge amounts of water through its roots, and releasing the water through transpiration from its leaves and stems. Along the Colorado River alone, tamarisk is estimated to suck up sixty-one billion gallons of water each year! As if that wasn’t enough, tamarisk will transport salts contained in the water up through its limbs and out to its leaves. When the leaves drop to the ground in the fall, the high salt content in the soil around the trees makes it very difficult for other plants to survive. You will generally find few or no other plant species coexisting in areas that are infested with tamarisk.
It is no longer legal to sell or plant tamarisk trees in Nevada. Tamarisk was listed as a noxious weed during the late 1990s by the Nevada Department of Agriculture. There are several other noxious weeds that were imported as ornamentals, only to create problems when they escaped from cultivation.
Purple loosestrife, Lythrum salicaria, is a showy, perennial noxious weed with lovely purple spikes that flower in August. Imported for ornamental use, it quickly escaped to invade wetlands, roadside swales, and riparian areas throughout the United States. Spreading by underground roots, it chokes out other vegetation, destroying habitat and food for wildlife. From 1950 to 2000, about sixteen million acres across the country became infested. Locally, you’ll find it growing in scattered gardens and landscapes, as well as in the North Truckee Drain along Sparks Boulevard and in the lower Truckee River. Gardeners have been known to dig up plants and cart them home for use in landscapes. Save yourself from years of headaches trying to control this pest. Don’t plant it!
Dalmatian toadflax, Linaria dalmatica, is another perennial introduced ornamental. It resembles snapdragon, but the flowers have a long spur extending from the lower edge, making it easy to differentiate from snapdragons. The leaves are heart-shaped and clasp the stem. This noxious weed has been found in a few scattered locations locally, as well as at Lake Tahoe. It contains a compound that can be toxic to livestock, reducing the forage value of the rangelands into which it escapes. It is one of the only plants that I’ve found thriving under sagebrush bushes.
Green fountaingrass, Pennisetum setaceum, is a showy, perennial ornamental noxious grass. It produces huge quantities of seed, allowing it to invade desert and rangeland ecosystems. Infestations will burn intensely, damaging native plants and increasing wildfire damage. To make matters worse, green fountaingrass is adapted to fire, and will often increase in density after a fire, while other native plants struggle to reestablish themselves. Green fountaingrass can become the dominant species in grasslands in as little as two to three years if not controlled. The red variety is still permitted to be used ornamentally in Nevada.
Oxeye daisy, Leucanthemum vulgare, is a cute, perennial daisy that looks a lot like Shasta daisy. It grows in places with just a bit of extra moisture. I’ve seen it invade and take over lawns. Once established, it’s very difficult to control. It’s often introduced when wildflower seed mixes are planted, so be sure to check the contents of these products before you buy them.
Gardeners nationwide are urged not to contribute to the invasive weed problem by planting invasive ornamentals. You have many plant choices. Be sure to select ones that are not invasive and detrimental to our native plants, wildlife, and environment.
For more information on invasive weeds and water quality issues, contact Susan Donaldson, University of Nevada Cooperative Extension, 784-4848 or
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Susan Donaldson is a water quality and weed specialist for the western area of University of Nevada Cooperative Extension.
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