Invasive Species and Responsible Gardening - August 10, 2005
Jeff Schalau, County Director, Associate Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County

One of my educational responsibilities is teaching people about threats posed by invasive/noxious weeds. In general, these are plants that pose threats to people, other organisms, and/or the environment. In past Backyard Gardener columns, I have written about several invasive/noxious species including yellow starthistle, hoary cress (whitetop), puncture vine, diffuse knapweed, giant salvinia, and sweet resinbush. Gardeners and nurseries have a role to play in invasive species prevention and management.

As a reminder, invasive and/or noxious weeds are non-native plants whose introduction causes, or is likely to cause, economic or environmental harm or harm to human health. Human actions are the primary means of invasive species introduction. The word “noxious” is a term that means the plant has been designated as particularly undesirable by law or regulation. Noxious weeds are found on lists compiled by state, federal, or land management agencies. Invasive weeds may have no legal designation, but are often in the process of being listed as noxious.

You may wonder why a plant can become invasive or noxious. Often it is because they have been removed from their native environment and introduced to areas where their natural enemies are not present. These natural enemies are often insects that feed on them, but can also be plant diseases, herbivorous animals, or other organisms that keep plant populations in check.

A classic example is prickly pear cactus when it was introduced to Australia from Brazil in 1788 for the production of red dye from cochineal scale. The introduced cactus escaped cultivation and displaced native plants because natural enemies were not present. In 1926, a prickly pear-eating moth (Cactoblastus cactorum) was introduced from Argentina to control the invasive cactus. Within six years, most of the thick stands of prickly pear were gone and invaded lands were reclaimed. However, prickly pear is still a listed noxious weed in Australia.

Ironically, the Cactoblastus moth was also introduced to the Virgin Islands to control prickly pear, but was discovered in Florida in 1989 where it was feeding on rare native cactus species. Since then, it has spread north and west and it is feared that it will reach the desert southwest where it will certainly impact native prickly pear populations. Cactoblastus is another reminder that introduced species can have unforeseen impacts on native plant communities. Note: today, intentional releases of biological control insects are always tested under controlled experimental conditions where researchers observe their behavior to prevent unforeseen ecological impacts.

Many noxious/invasive weeds have been unintentionally introduced to the United States as seed contaminants, livestock feed, and in ship ballasts. Like Cactoblastus, invasive/noxious plants are often introduced (often unknowingly) by well-intentioned people planting them because they are attractive or functional parts of cultivated landscapes or used in conservation efforts. Two examples are Russian olive (Elaeagnus angustifolia) and salt cedar (Tamarix ramosissima). These two species have severely impacted western riparian areas (lakes, rivers, and streams) and are costing taxpayers millions of dollars each year to control. I’ll also remind you that many of the recent wildfires in Arizona’s desert areas were fueled by introduced invasive weeds.

Recently, the nursery and floral industries have been participating in discussions with the Federal Animal Plant Health Inspection Service (APHIS) to find common ground in the plant screening and introduction process. While these discussions are difficult, they are an important step toward responsible land stewardship. Pesticides, biological control organisms, and genetically engineered crops go through rigorous screening processes to determine if they may have harmful effects to people, non-target organisms, and the environment. We may want to consider the potential impacts of ornamental plant introductions in the same context.

In my opinion, citizens cannot have too much information regarding threats posed by invasive/noxious weeds. You may not care about this issue today, but everyone is impacted by invasive/noxious weeds. By increasing our awareness, each of us can make a difference.

The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site:

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: August 4, 2005
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