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Submodule 1: What are invasive weeds?

Where Invasive Plants Come From and How They Reach Western Rangelands

The most common pathway for the introduction of invasive plants into a new environment is human intervention. Plants have been traveling around the globe with humans as we moved from place to place and as transport due to trade increased over the course of history. As a result, invasive rangeland weeds have arrived in this country from all over the world. In general, greater numbers of invasive plants have originated in Europe and the Mideast, relating to the amount of trade between these areas and the U.S. However, examples of invasive plants that originated on each continent can be found throughout the U.S.

 

Invasive plants often enter the U.S. along with traded commodities and their packing materials– they are hitchhikers, so to speak. In some cases, seeds of invasive plants are inadvertently mixed with seeds of desired plants. When people in the U.S. purchase and plant these seeds, they end up with more than just the desired plant.

 

Let’s look at some other examples of how invasive plants arrived in this country:

• Escaped ornamental/horticultural plants – for example, it is estimated that over 70% of the invasive plants in Hawaii arrived via this route.
• Biological control introductions – several plants were introduced for wind and erosion control only to subsequently spread, leading to destruction of rangelands.
• Natural pathways – invasive species (especially their seeds) are able to travel on wind and water currents, or perhaps through the digestive tract of animals.

Several plants that turned out to be invasive actually have desirable characteristics for some applications. For example, some hilly regions that are very susceptible to erosion problems can benefit from grasses with vast fibrous root systems. These plants often reproduce by runners and stolons (vegetative reproduction) producing many established plants quickly.

 

For many invasive western rangeland weeds, researchers have a good idea of how their original introduction occurred. Here are a few examples:

• Spotted knapweed (Centaurea biebersteinii) was introduced from Eurasia as a contaminant of alfalfa and clover seed and is now widespread in the intermountain west.
• Salt Cedar (Tamarisk spp.) was introduced to the U.S. as an ornamental and for wind and erosion control in the mid 1800s, and has devastated countless waterways in the west. It is native to southern Europe and northern Africa through the Middle East and south Asia to China and Japan.
• Dyer’s woad (Isatis tinctoria) was imported into the U.S. from Europe by colonists because it contains a chemical used as textile dye. It arrived in the West as a contaminant in alfalfa seed and now threatens the production of large areas of western grazing land.
• Cheatgrass (Bromus tectroum) apparently had many pathways into the U.S. Native to Eurasia, its seed was transported in soil used as ballast in ships traveling to St. Louis, Missouri, in 1850. After this time, the grass was found along railways in western states.

 

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