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Simply put, if you care about our landscapes—and wildlife and wildlife habitat—you should care about invasive species. Invasive species cost us about $100 billion a year in the U.S. between the damage they cause and control efforts (US Fish & Wildlife Service).

We frequently hear about pythons in Florida, kudzu in the southern U.S. and quagga mussels in Lake Powell. But a vast number of invasive species in the U.S. are plants—1,230 as reported by the Invasive Plant Atlas ( In Arizona, more than 400,000 acres of national forest land are infested—and the rate of infestation is increasing by about 20% annually (USDA). At the Arboretum at Flagstaff, we are frequently involved in invasive weed surveying, treatment, and education through grants from the Forest Service and Arizona State Forestry.

What is the difference between native and non-native species? Species native to North America are generally recognized as those occurring prior to European settlement. Many of our familiar ornamentals and food crops are non-native or “exotic,” which means that humans brought them here during settlement. Since colonization, an estimated 3,500 species of plants have escaped cultivation in the U.S. and are now part of our natural landscapes (National Park Service). Daffodils, lilacs and wheat (the cereal grain that we consume) are but a few of those non-native species; however, they are not invasive.

An invasive is “a plant that is both non-native and able to establish on many sites, grow quickly and spread to the point of disrupting plant communities or ecosystems” (USDA). Invasive species can be characterized by robust vegetative growth, high reproductive rates, high germination rates and longevity. According to the Plant Conservation Alliance, they impact natural ecosystems by:

• Displacing or destroying native species

• Competing with rare or endangered plant species

• Reducing native biological diversity

• Altering soil characteristics

• Altering fire intensity and frequency

• Interfering with natural succession

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• Competing for native pollinators

• Replacing complex communities with monocultures

Some invasive species cause substantial changes to the invaded habitat, to the point where they have been termed “landscape transformers” (UC Agriculture and Natural Resources). These include plants such as saltcedar or tamarisk and yellow starthistle, two invasives common in Arizona. Other local invasives include Russian olive, Siberian elm, knapweeds, Scotch and Canada thistle, Dalmation toadflax, field bindweed and cheat grass.

What can a property owner do? First and foremost, learn to identify local invasive species—and control them on your property. Most invasive plant species are opportunistic and will be the first to grow in disturbed soil or overgrazed areas. Pull the invaders (much easier to do during monsoon), including as much of the root system as you can. Bag and dispose of any with seed heads. And then repeat!

Many invasives have extensive root systems, and may have spread seed as well. If they keep returning, you may have to resort to herbicides. If you do, use a professional for application. Remember, herbicides are poisons.

Use native plants for landscaping. Talk to your local nurseries and ask them to not sell invasive species. Note major weed infestations and report them to the Southwest Vegetation Management Association ( Finally, talk to your neighbors or home owners association about controlling invasive species in your neighborhood.

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Lynne Nemeth, Executive Director of the Arboretum at Flagstaff, is the editor of Gardening Etcetera. To reach her with articles, ideas or comments, please email


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