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Weed Watch: Invasive Trees |
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Written by Susan Donaldson
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Monday, 24 August 2009 |
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If you agree with the common definition that a weed is a plant that is growing where it is not wanted, then sprouts of many types of plants could be considered weeds when they are growing in garden beds or lawns. We tend to think of weeds most often as grasses or pesky herbaceous plants, but in fact, some trees can also be invasive in many environments.
If you agree with the common definition that a weed is a plant that is growing where it is not wanted, then sprouts of many types of plants could be considered weeds when they are growing in garden beds or lawns. We tend to think of weeds most often as grasses or pesky herbaceous plants, but in fact, some trees can also be invasive in many environments.
Nevada has only one tree species on its noxious weed list, which is the list of plants that landowners are required to control. That tree, saltcedar (Tamarix spp.), has infested riparian areas throughout the western United States, where it outcompetes other types of vegetation and has the ability to use large quantities of water—as much as 200-300 gallons of water per mature tree per day. It is illegal to plant saltcedar in Nevada.
As the name suggests, saltcedar is tolerant of salty soils and will move salts through plant tissues and out onto the leaf surfaces. When the leaves drop in the fall, a salty layer is created around the tree, making it difficult for other plants to grow.
Saltcedar is difficult to control mechanically as it easily sprouts back after it has been cut or burned. Most often, trees are cut and an herbicide is immediately painted on the cut stump. An insect has been introduced to help control existing populations of saltcedar. This beetle devours the foliage, and after several years of repeated defoliation, the trees begin to die.
While saltcedar is the only tree on Nevada’s noxious weed list, another troublesome species is the Russian olive (Elaeagnus angustifolia L.), which has been planted in many parts of Nevada. A native of western Asia, Russian olive was introduced to the United States in the late 1800s for use in windbreaks and as an ornamental tree. It is well suited to Nevada’s dry, poor ground, as it tolerates salty soils and extreme temperatures.
Russian olive has striking, slivery foliage that provides a visual accent in the landscape. Its cherry-like fruits are eaten by birds, which help to spread the plant. At maturity, the small trees are characterized by long, sharp thorns that help protect the plants from damage by predators, but also can result in cuts and punctured bicycle tires.
Russian olive tends to spread quickly and is currently invading Nevada’s streamside areas, choking irrigation canals and forcing out native vegetation. Despite the fact that Russian olive provides food for birds, ecologists have found that bird species’ richness is greater in areas with a higher concentration of native vegetation. Both New Mexico and Utah have named Russian olive noxious, and California has labeled it invasive. In 2006, then-President George W. Bush signed the Saltcedar and Russian Olive Control Demonstration Act in support of long-term management of both invasive tree species.
If you’re looking for a native tree with an appearance similar to Russian olive, consider the native silver buffaloberry (Shepherdia argentea). This tree is also deciduous, produces an edible fruit and is tolerant of poor soil, drought, wind, cold, and salty soils. It is used in conservation plantings and windbreaks, and provides good cover and nesting sites for many birds. Its berries were used by Native Americans and can be used to make jellies. In most cases, it’s preferable to plant native species, as they rarely create as many problems as introduced species.
For more information on weeds and water quality issues, contact Susan Donaldson, University of Nevada Cooperative Extension, (775) 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. |
