Invasive Plants & Weeds

As the natural distribution of species shifts historically, opportunistic invasives hitching rides on the undersides of boats, insects invading shipping palettes, and dramatic climactic changes all contribute to an incredible disruption of native species.

An “invasive” species refers to a living organism not native to an ecosystem that causes extensive harm to that area. These species often reproduce quickly and spread aggressively, outcompeting other organisms and decimating native landscapes. Invasive species alter the diversity of species important for native wildlife and can quickly replace a diverse ecosystem with a strict monoculture. In some instances, invasive species can change ecosystem conditions, like soil chemistry, contributing to wildfire intensity and other conditions. 

Invasive species are responsible for the decline of approximately 42 percent of endangered or threatened species and are considered a leading threat to native wildlife. Human health and economies are directly affected by these widening shifts, with the impacts on our natural ecosystems costing billions of dollars each year. Invasive forest pests have caused the unprecedented decline of numerous tree species worldwide. Within the United States, this includes the near extinction of American chestnut, mortality of ash species, and five-needle pines die off. 

The Coconino National Forest’s stewardship team, comprised of botanists, biologists, silviculturists, and hydrologists, works vigorously to mitigate invasive species within the forest’s three ranger districts: the Flagstaff, Red Rock, and Mogollon Rim. Though there are multitudes of invasive species within the forest’s nearly two million acres, there are several distinct invasive species to be aware of.  

Treating Invasive Weeds

Coconino National Forest Herbicide Weed Treatments

The 3-forest Weed Treatment EIS identified wilderness areas and wild and scenic riparian corridors as one of the top priority areas for invasive plant survey and removal due to the rarity of these habitats in this landscape and their sensitivity to change. This EIS was finalized in June 2005, and the first treatments began in 2006. Once tamarisk is more or less a year old, manual control is no longer effective and herbicide control is necessary.

It has been shown by researchers (and our test treatments) that a systemic herbicide is most effectively applied to perennial and woody invasives during fall to mid-winter when the plants are no longer actively budding/growing but are transporting materials into the root system. We prefer to use a selective cutstump treatment technique with a low toxicity aquatically labeled herbicide. This minimizes exposure and risk for our applicators and for non-target plants.

For information about herbicide treatments please contact the forest district of which the treatment would be taking place. ADOT also has a Herbicide/Invasive Species Contact web page if you are seeking information outside of Coconino National Forest boundaries. See also, SouthWest Vegetation Management Association.

Invasive Exotic Weeds

Invasive Plants in the Verde Valley

Man uses large tool to clear blackberry bush on the side of a trail

The Coconino Botany team clearing invasive Himalayan blackberry near Fossil Springs. 

Invasive Plants & Trees

Yellow bluestem (Bothriochloa ischaemum)

dalmation toadflax

A new invader occurring primarily along roads (Beaverhead Flat, Verde Valley School, Cornville, Tissaw) and Jim Thompson trailhead.  This grass is hard to identify from other species; surveys need to occur from late August to early October when diagnostic seedheads are present. 

Dalmation Toadflax (Linaria dalmatica)

If we have large enough populations of this plant, we can use beetles to kill them, rather than dealing with hand removal or herbicides.  In the Verde Valley, we mostly see this plant in Oak Creek Canyon and along waterways.  

Scotch and Bull Thistle

close up of scotch thistle

Scotch Thistle is known from Soldier’s Wash south of 89A and from Dad Jones Tank. Bull Thistle is known to occur in West fork and Oak Creek Canyon.  The best way to tell the non-native thistles from the native thistles is that non-native thistles have a thick, spiny flange of tissue up and down the stem that prevents you from grabbing the stem without getting stung.  Stems of native thistles don’t have this so you can hold the stem without getting stung.  

Yellow Star Thistle (Centaurea solstitialis)

yellow star thistle

Yellow Star Thistle is primarily located within the old ranch sites along the Verde River.  It also occurs along road ways in Camp Verde.  Dominant spines located at the base of the flower are diagnostic. 

Giant reed (Arundo donax)

giant reed plant towering over two men

Giant reed occurs in clumps along all the main waterways in the Verde Valley.  

Russian olive (Elaeagnus angustifolia)

russian olive along a creek

Russian olive is located throughout the Verde River and lower Oak Creek corridors. The tree had a characteristic gray appearance.  Occassionally we see it growing along roadsides.

Tree of Heaven (Ailanthus altissima)

Tree of heaven plant with red flowers

Tree of Heaven invaded the Verde River corridor from seed sources of ornamental plants primarily planted in Jerome and Cottonwood. This species is seen around old homesteads and also occurs along the ditches in the Verde Valley.

Saltcedar (Tamarix spp.)

saltcedar plant

Saltcedar is present throughout the riparian corridor within Verde River Watershed. Infestations are dispersed or sparse in the upper Verde River and become more dense monocultures on the lower Verde River.


Diffuse knapweed (Centaurea diffusa)

Introduced a century ago, diffuse knapweed are incredibly hardy, persistent invasive noxious weeds that easily dominate rangeland and disturbed areas. By way of the Pacific Northwest, these weeds entered the country as an alfalfa seed contaminant. The first collection of diffuse knapweed in Arizona was on August 5, 1979 along Highway 89, seven miles north of Flagstaff. The plant now dominates landscape throughout open, disturbed fields with considerable acreage both north and east of the Flagstaff Mall.

The aptly named knapweed is also called “white,” “spreading,” and “tumble” knapweed due to its uncanny ability to cover vast areas. An acre with one hundred plants can produce more than a million seeds on dry rangeland, making this invasive weed incredibly difficult to control.

Diffuse knapweed’s white flowers are sometimes a rose-purple or lavender hue, broadly urn-shaped, and found in clusters or solitary at the end of branches. Dormant seeds may remain viable in the soil for over eight years, with studies indicating seeds may be viable for over fifteen years. These short-lived, perennial plants grow most actively from April to October, producing flowers from June to September. Seeds set from July to October.

After seeds germinate during the first year of growth, the plant takes on the form of a small cluster of gray-green, lobed leaves laying close to the ground (the basal rosette). During the second year, this basal rosette produces a densely branched, leafy stem with several small flowers blooming at the end of each stem. Flowers are wrapped in a spiny cluster of small bracts deemed the “flowering head.” Each of these flowers produced a single, small seed at the top of which are attached numerous course hairs.

Diffuse Knapweed, a significant threat to native ecosystems

In addition to diffuse knapweeds, Arizona is also home to the marauding “squarrose,” “Russian,” and “spotted” knapweeds with similar rosettes and environmental impacts.

An integrated combination of control methods will be needed to successfully sway such aggressive, adaptable, seemingly irrepressible weeds. Prevention and early detection of new population is key. Eradication and containment of existing populations all need to be addressed to achieve lasting control over invasive knapweed species. No single control method of one-year treatment program will ever effectively control an area contaminated with knapweed. The plant’s rapid growth, high seed viability, fast rate of spread, and long seed dormancy requires long-term cooperative integrated management programs and planning to prevent, contain, and reduce knapweed infestations.

The foremost method of control is prevention. Do not drive through areas infested by knapweed, as the persistent plant is a constant hitchhiker. Double check vehicles for attached knapweed plants in both tires and grills when leaving an infested area. Purchase hay free of knapweed seeds and practice sound pasture management to prevent overgrazing and area disturbance, as knapweeds thrive in such environments. When possible, reseed disturbed sites with vigorous and hardy native plants to deter knapweed establishments.

Along riparian areas, following an herbicide treatment, and on small infestations, hand pulling knapweeds is most effective. Be sure to hand pull small patches before the plant goes to seed.

Chemical controls can also be utilized. Always check with weed specialists or chemical suppliers before treatment to ensure correct dosage and application.

The infestations of leafy spurge on the Coconino National Forest are the top priority for control. Since its detection, Forest personnel have taken many actions to control the populations including mowing, sheep grazing, herbicide treatment, and biological insects. There is an area closure on some of the area containing leafy spurge, prohibiting vehicle travel in these area. The Coconino National Forest developed a plan for management of leafy spurge. For more information see the Signed Plan - October 2, 2009