Plant Invasions II


1. How do they get here?

a.  Unintentional introductions: many invasive plants have been introduced to new territories as hitchhikers within vehicles or parcels of trade. For example, filaree (Erodium) seeds were brought to the coast of California as ballast on Spanish ships, in packing material, and fur of animals (livestock). Plants established near the shore of California were spread by birds on great migratory flyways. The plants spread inland before people did. Tumbleweed (Salsola) came in the bales of flax that Mennonites brought with them while fleeing religious persecution in Russia. They were dispersed along the spread of railroad lines out west, actually carried by rail and establishing in the disturbed ground adjacent to the tracks and around depots.

Railroad depots and sea ports were likely places for initial plant introductions. With the advent of global travel there were no longer any barriers to plant dispersal. Cars, trains, ships, airplanes have overcome the traditional barriers of transport, such as mountain ranges and oceans.

Once dispersed to new lands, invasive plants may establish and maintain populations in the presence of perennially disturbed lands. In the west and southwest U.S. cattle grazing has had disturbing influences on the generation and persistence of weeds such as cheatgrass and red brome (Bromus tectorum and B. madritensis, respectively). Once established, increasing populations of these annual grasses have maintained their populations through an accelerated wildfire regime to which native plants are unaccustomed.


b. Intentional introductions: agriculture, nurseries, horticulture, (escaped plants, naturalized plants) etc.

Many plants that were intentionally introduced to new locations have, in subsequent years, escaped from their original planting sites and have become established in the wild.

i.                    Russian olive (Elaeagnus angustifolia) was introduced from Russia to  South Dakota as an ornamental and hedge plant. In the 20th century it was used as a landscape plant in Arizona and Utah. It is now a dominant plant in riparian canyons in the Sonoran and Great Basin deserts.

ii.                  Saltcedar (Tamarix ramosissima), a native of Eurasia, was being advertised by U.S. nurseries by the 1820s. It was initially sold as an ornamental plant that provided a great deal of shade. By the 1880s, Tamarix had escaped cultivation, and by 1900 was common in river bottoms of Arizona. Today, it is a major pest in riparian areas throughout the southwestern U.S.

iii.                Landscape architects may need to specify to their clients and nurseries which plants may be invasive, since their purchases may be large enough to affect what nurseries choose to supply. As with others, landscape architects are often unaware that plants they encourage clients to grow are invasive and potentially  harmful. Fountain grass (Pennistum setaceum) is an attractive bunchgrass from Africa and the Middle East, currently sold in local nurseries, has been establishing itself in the wild within the Lake Mead National Recreation Area, crowding out native plants.



2.     Effects of plant invasion

a. Ecosystem effects

          1. Altering soil chemistry: some invaders alter soil chemistry, making it difficult for native species to survive and reproduce. Ice-plant (Mesembryanthemum crystallinum), planted on hillsides along the California coast for erosion control, accumulates large quantities of salt, which is released after the plant dies. The increased salinity prevents native vegetation from reestablishing.

          2. Altering hydrology (Tamarix) - Wetland and riparian invaders can alter hydrology and sedimentation rates. Tamarisks invade wetland and riparian areas throughout the southwestern U.S., and are believed to be responsible for lowering water tables at some sites. They might reduce or eliminate surface water habitats that native plants and animals need to survive.

          3. Altering the rate and intensity of wildfires (Bromus) – Cheatgrass (Bromus spp.) is a European annual that has invaded millions of acres of rangeland in the Great Basin and Mojave Deserts, leading to widespread increases in fire frequency from once every 60-110 years to once every 3-5 years. Native shrubs do not recover well from more frequent fires and have been eliminated or reduced to minor components in many of these areas.

          These deserts ordinarily have infrequent fires because there is little fuel to feed fires. Consequently, most native Mojave desert plants have not evolved in association with fires, and have  no adaptive resources to survive fires. Cheatgrass, on the other hand, is a spring annual, and avoids the hot, dry season by setting seed and dying. Heavy infestations of cheatgrass, caused mostly by grazing and other disturbances, fill the inter-shrub spaces with dry, unpalatable tinder, and carry fires from shrub to shrub. Cheatgrass is able to withstand fires because they are buried in the soil, which dampens the effects of the heat.

          Cheatgrass was first observed in the region in 1900 in western Nevada. The seed evidently was brought in the coats of sheep driven from California, where cheatgrass had arrived a few years earlier.


4. Sedimentation and Erosion (Centaurea) – Invasive plants can alter an ecosystem’s rate of sedimentation and soil erosion. The spotted knapweed (Centaurea maculosa) moves into grasslands and disturbed areas where it displaces native bunchgrasses and herbs. The root system of knapweed (taproot) is far simpler than native bunchgrasses (densely fibrous root system), and do not slow runoff from rainstorms in the same way. The runoff and consequent erosion from invaded hillsides are far greater, and sediment loads in streams far higher, which negatively impacts fish in streams: degrades spawning habitats). Additionally, it has been found in certain areas (Glacier Nat’l Park) that spotted  knapweed has been associated with 96% less cryptogamic crust than uninfested grasslands.


b. Displacement of native species Tamarix and Salix. Where saltcedar has taken hold in many riparian areas of southwestern U.S. (especially the lower Colorado), it has diminished populations of native willows (Salix spp.) and cottonwoods (Populus spp.). the replacement of native plants with exotic plants will also affect populations of wildlife, insects, and microbes.


c. Threatens biodiversity – Over the past several decades there has been a heightened concern at the national and international level about impacts of invasive species on biodiversity. Human induced biological invasions are occurring on a global scale and are beginning to blur the Earth’s biota. The distinctiveness, which has developed over the past 180 million years as a result of the isolation of the continents, maintains biodiversity. Unlike chemical pollutants, which degrade over time, invasive organisms can become established and reproduce. Once established, they can spread from site to site, and region to region, often without further human assistance.


d. Grazing land degradation – invasions of tumbleweed (Salsola iberica) and Halogeton glomeratus fanned out into the rangelands, traveling to the western states as a railroad annual. They are both from Russia, and both benefit from soil disturbance and removal of competition by grazing. Halogeton , from the Caspian Sea area, showed up in Wells, NV in 1934. It had spread widely along roads and in rangelands before it was discovered to be toxic to sheep.. It has killed thousands of sheep in the Great Basin. Halogeton leaf litter enriches the soil surface with salt, which favors its own seedlings over competitors.

Relate story of MX missile slated for the Great Basin.



Weeds and Invasives


1. Erodium cicutarium (filaree, heron’s bill) – the introduction of these plants actually preceded the Spanish. Seeds were brought in via ballast on ships, in packing material, or fur of animals. Plants established near the shore of California were spread by birds on great migratory flyways. The plants spread inland before people did.

Found in California when the first missions were built, and was plentiful enough to be incorporated into the adobe bricks (along with Rumex crispus and Sonchus asper).

Filaree was later used as fodder on ranches, and became a fairly popular range plant, promoted by the Agricultural Extension bulletin in the late 19th century.


2. Cynodon dactylon (Bermuda grass) – considered a scared plant in the Veda, the sacred writings of ancient India (known as “preserver of nations” or “shield of India” because of its forage value). It has a long history in African medicinal lore and was probably introduced to Africa on Arab merchant ships before 500 AD. It is now worldwide, found on every continent except Antarctica.

Introduced in the U.S. in 1751 (Georgia), it spread rapidly and covered the roadsides of the east and southeast within 50 years. Sold by the flat ($5) in San Francisco in 1850; by 1860 it had become a troublesome weed in southern California. Rivers and canals were ideal dispersers, and by 1911 it had become a serious problem as it spread to fields and canals.


3. Salsola iberica or S. kali (Tumbleweed) – introduced to South Dakota probably by Mennonite farmers escaping persecution in Russia. One of the crops the Mennonites planted was flax, but flax seed was contaminated with tumbleweed seed, also known as Russian thistle or “Wind Witch” or “Leap the Field”.

From South Dakota, tumbleweed spread rapidly to Nebraska, where in no time at all it became a noxious weed. The Nebraska Extension Service published a bulletin in 1892 with a ten-point plan for eradicating tumbleweed, including a directive to familiarize “…every child in the public schools, with the appearance of this pest in order that he may destroy it wherever he finds it.”
The plant was spread west via the railroad, and its initial distribution pattern shows it moving along railways and roadways. Wind was also a good dispersal method, especially on the
Great Plains with its high winds traveling for miles.

By 1894 it was estimated to have caused over $2 million in damages to wheat fields in the Great Plains states.


4. Tamarix ramosissima (Saltcedar) – This species appears to have originated in Asia/Africa, where it spread out to Asia and the Middle East. It is a species that appears in the Bible under the name Eshel, and in ancient Arabic literature as Asul. It was valued for its manna, a sweet exudates produced by a scale insect.

The species was advertised in nurseries as early as 1820s as an attractive, shade-bearing tree. By the 1880s, in the west, it had already escaped cultivation, and by 199 was found to be common in river bottoms in Utah and Arizona (Salt River). Tamarix naturalized rapidly from the 1930s to the 1960s, most often in areas disturbed by human activity, such as upstream and downstream of dams.


[One thing to consider is the great enthusiasm for plant introduction, without any consideration for the possible negative consequences that seem obvious to us today. It can rightly be said that plant introductions has made major changes in lifestyles of virtually every person on earth – from introduction of the potato from the Americas to Europe to the introduction of wheat to the Americas. It has been said to be one of the important civilizing influences at work throughout our history. In the case of the few invasive pest species, however, plant introductions has also wreaked havoc with native species throughout the world.]


What are salt cedars? (Tamarix ramosissima from the Tamaricaceae)

Salt cedars are many-branched shrubs or trees, usu. Less than 25’ tall, with small scale-like lvs. Leaves have salt glands, and salt crystals can often be found on the leaves. Widely distributed throughout the southwestern riparian areas, especially along the Lower Colorado. Saltcedar is abundant where surface or subsurface water is available for most of the year, including streambanks, pond margins, springs, canals, ditches, washes, rivers. Disturbed sites, including dammed areas and burned areas, are particularly favorable for salt cedar establishment. It survives where many native species – woody riparian species, cannot.

Where does it come from?

From much of Central Asia, from the Near East around the Caspian Sea, through western China to North Korea. It may have been introduced into North America by the Spaniards, it didn’t gain recognition in the western U.S. until 1800s, when it was mainly planted for erosion control, as windbreaks, for shade, and as an ornamental.

How does it propagate?

It spreads by seed and vegetative growth. Individual plants can produce 500,000 tiny seeds per year (100 seeds per square inch), which are easily dispersed long distances by wind and water. Seeds are about .17 mm in diameter, and small hairs at the apex of the seed coat facilitate wind dispersal. Germination can occur within 24 hours in warm, moist soil. Seed production occurs over a 5.5 month period, with a major and minor peak. They flower after the first year of growth. Roots also sprout adventitiously. Plants can regenerate from cuttings that fall on moist soil.

Plants can grow 3-4 meters in one growing season.

What problems does it cause?

There is still a debate whether saltcedar is to be seen as a consequence or a cause of environmental changes associated with its presence and proliferation. The presence of saltcedar is associated with dramatic changes in:


          geomorphology – trapping and stabilizing alluvial sediments, which results in narrowing of stream channels and more frequent flooding.

groundwater availability – blamed for lowering water tables because of its high transpiration rate, and on a regional scale, dense salt cedar groves use far more water than native riparian plant associations. Actually, at the leaf level, the WUE of Tamarix is very similar to the willows, but the much greater canopy of Tamarix transpires far more water than willows do. At the canopy and regional level, then, Tamarix uses far more water.

soil chemistry – soil salinities increae as a result of inputs of salt from glands on slatcedar leaves. The dome-shaped glands consists of at least two cells embedded in the epidermal pits. Increased salinity inhibits growth and germination of native riparian species.

fire frequency – Leaf litter from drought-deciduous saltcedar increases the frequency of fire.

plant community composition - Saltcedar is capable of resprouting vigorously following fire and, coupled with changes in soil salinity, ultimately dominates riparian plant communities

native wildlife diversity – although saltcedar provides habitat and nest sites for some wildlife (white-winged dove: Zenaida asiatica), it has very little value to most native amphibians, reptiles, birds, and mammals. The endangered southwestern willow flycatcher, a native songbird, uses the tamarisk bosque, along rivers and washes, because of its proximity to water.


How to get rid of it? It is easily spread but difficult to eliminate.

          Prevent site disturbances that contribute to its success (fire, increased soil salinity, ground disturbance dams, grazing). Seedlings will continue to establish as long as saltcedar infestation persist upwind or upstream of the target area.

          Physical control – manual/mechanical methods. Difficult to kill with mechanical methods as saltcedar is able to resprout vigorously following cutting (coppicing) or burning (doesn’t kill roots). Seedlings and small plants can be uprooted by hand (labor intensive).

Flooding – flooding thickets for 1-2 years can kill most saltcedar plants in a thicket. Some forest and river managers propose the periodic release of water impounded by dams, or those with senior water rights may be able to mimic floods of a natural river. If the timing of the flooding is accurate it can help to establish native riparian species. E.g. cottonwood trees let their seeds fly a week before tamarisk. By wetting the mudflat and slowly drying it out after cottonwood germination, the tamarisk may not have a chance to get established. Cottonwoods evolved to release their seeds right at the peak of spring runoff from the mountains.

Biological control – insects and fungi. Natural enemies of saltcedar have been recommended for release in the U.S.. A mealybug (Trabutina mannipara) from Israel, and a leaf beetle (Diorhabda elongate) from China.

Chemical control – Herbicides have been used in an aerial spray onto the canopy, but the most frequently used method is to cut the shrub off near the ground and apply Garlan (triclopyr) within a minute of the cut, before the cells (vessels) close. (Garlan must be translocated throughout the shrub, esp. to the roots).


5. Pennistum setaceum (Fountain grass)

A coarse perennial grass with a densely clumped growth form and erect stems usu. to 1-1/2’ to 5’.  Inflorescences are prominent, nodding , and feathery, resembling bottlebrushes, 6-15” long, with many light pink to purple flowers.

Where found – in riparian areas and now making its way around the lake in the Lake Mead National Recreation Area.

Where does it come from? – Native to Africa and Middle East, it has been introduced to our area, and has spread, largely because of its popularity as an ornamental plant. It is easily dispersed by vehicles, humans, livestock, and, over short distances, by wind, water, and birds.

Its remarkable spread over broad ranges is attributed to its ability to adapt, physiologically and morphologically, to different environments.

What problems does it cause?

Fountain grass is well adapted to fire, and plants can recover to pre-burn density, even increase in density, following a burn. Fire may also contribute to the spread of fountain grass. Fountain grass raises fuel loads, which increases the intensity and spread of fire and results in severe damage to native species less adapted to and less tolerant of fire regimes.

Thick infestations of fountain grass interfere with the regeneration of native plant species. It often grows faster (more biomass) and out-competes (higher net photosynthesis) native spp.

How does it grow and reproduce?

Reproduces by either fertilized or unfertilized seed (apomictic).It can reproduce asexually by producing seeds from the cells of female plants other than egg cells.

Seeds may remain viable in the soil for several years (7 yrs.).

How to get rid of it?

Difficult to eliminate. Long-lived seeds make control difficult, and continued monitoring is essential.


6. Bromus madritensis ssp. rubens (foxtail chess or red brome) and B. tectorum (cheatgrass) – annual grasses found in cultivated lands and waste areas in its native range. Common weedy species of grasslands and scrub habitats. In arid and semi-arid regions of the southwestern U.S., especially those areas that have been disturbed by wildfire, livestock grazing, off-road vehicles, or agriculture. It is especially dense where moist, nutrient-rich microsites occur, including areas beneath perennial shrubs, crevices on rocky outcrops, and margins of roads and washes.

Where did it come from and how does it spread?

Native to southern Europe, northern Africa, and southwestern Asia, where it occurs from sea-level to 4260’. Established in California by 1848, naturalized by 1890s. Long distance dispersal of foxtail chess is accomplished by seeds that lodge in animal fur and in loosely woven clothing. Short-distance dispersal is aided by wind, which blows seeds along the ground until they settle in eddies behind shrubs or rocks or in depressions in the ground.

What problems does it cause?

Altered pattern of wildfire - Chess is highly inflammable and promotes wildfires in desert plant communities where fires historically have been infrequent.

microhabitat characteristics – desert plant communities are being converted into annual grassland dominated by foxtail chess and other exotic annuals. Nutrient cycling caused by foxtail chess and competition for soil nutrients and light negatively affect annual plant populations and revegetated plants.

Altho foxtail chess is sometimes grazed by livestock (when a seedling), it is not considered a good forage plant and is generally regarded as having no economic value. Dried florets become entangled in wool, reducing its value, and lodge in the digestive tracts of some livestock, sometimes causing death.

How does it grow and reproduce?

Reproduces by seed only. A winter annual, it emerges in early winter following rainfall and remaining largely quiescent until early spring, when rainfall and higher temperatures stimulate growth and flowering. Plants continue to flower until water stress kills them, typically by the middle or end of May. Populations increase during years of average to high rainfall. During years of low rainfall a high percentage of seedlings die prior to reproducing, thereby depleting the seedbank. In desert regions, stems can remain rooted and upright for 1-3 years following the death of the plant.