Policy on Invasive Exotic Plants
Adopted September 1996 (PDF Version)
Native plant: any plant which is a member of a species
which was present at a given site prior to European contact.
Exotic plant: a plant which does not meet this definition
Invasive exotic plant: a plant which is able to
proliferate and aggressively alter or displace indigenous
The California Native Plant Society:
- Urges all government agencies, non-governmental
organizations, and individuals charged with land management
- adopt and implement invasive exotic plant management
- coordinate with each other at all levels regarding
non-native plant policy formulation and implementation.
- publicize the need to prevent the spread of invasive
- stop all introductions of invasive non-native plant
species into natural ecosystems which are designed to
achieve some other management objective.
- implement exotic plant control measures in such a
manner that native species and natural systems are not
- adequately fund the control of invasive exotic
- insists that all landscaping, mitigation, restoration,
revegetation, and habitat/species recovery monitoring plans
include provision for identifying and managing non-native
plants and identifying potential for damaging the genetic
structure of local native plant communities.
- advocates cooperative efforts to restrict introductions of
invasive exotic species from commercial sources, including
the agricultural, landscaping, and revegetation industries.
- supports inclusion of information regarding the effects of
invasion by exotic plants in environmental or outdoor
education programs in schools.
- encourages CNPS chapters to work for adoption and
implementation of invasive exotic policies and programs at
municipal and county levels and to raise local awareness
regarding the problem.
- encourages recognition, continuation, and expansion of the
many citizen volunteer restoration efforts around the state
as a means of habitat preservation, public education,
community building, and constituency creation.
The homogenization--blurring of distinctions--of the earth's
flora and fauna and subsequent loss of biological diversity, is
a problem of global significance which threatens livelihoods and
engenders catastrophic ecological change. The threat posed to
natural ecosystems by biological pollution--the introduction of
non-native plants, animals and other organisms--is rivalled only
by that of development. The most aggressive exotic plants are
unacceptable in natural areas because they can exclude native
plants, degrade, alter or displace natural plant communities,
promote faunal change, reduce biological diversity, disrupt
ecosystem processes, alter fire frequencies, restrict economic
return, reduce recreational values, threaten endangered species
and fundamentally alter the unique character and physiognomy of
- With the possible exception of alpine and subalpine
habitats, most areas of California contain significant
expanses of exotic weeds. To cite but a few of the most
- Vast areas of coastal dunes are occupied by iceplant (Carpobrotus
edulis ) and European beach grass (Ammophila
arenaria ), usually to the exclusion of any other
kind of plant. They deprive other plants of moisture and
nutrients scarce in this environment. Their value to
wildlife is low. They alter wind patterns that sculpt
the dunes and they bind the dunes, preventing the
natural disturbance required by some of the native
species. Because coastal dunes support biological
communities whose plant and animal inhabitants may exist
nowhere else, their degradation represents a loss of
- Many-acre stands of pampas grass (Cortaderia jubata and C. selloana ) and masses of German ivy (Delairia
odorata , syn. Senecio mikanioides ) appear
discontinuously along the coast from the Oregon border
into Baja California. German ivy forms thick blankets
which cut off light and air to plants which it covers.
Pampas grass is a robust six-foot tall grass with
sharp-edged leaves growing from a stout clump. Its
aggressive root system outcompetes plants even much
larger than itself. Tall plumes, bearing many light
seeds rise several feet beyond the leaves, dispersing
seed great distances in the wind.
- About one tenth of California (including ten million
acres of rangelands) has been invaded by yellow star
thistle (Centaurea solstitialis ) (Maddox, 1985).
The plant is toxic to horses and stout spines render it
inedible to sheep and cattle. Aside from its economic
impacts, yellow star thistle increases roadside fire
hazards and affects recreational values by invading
campgrounds, lining trails, and reducing biodiversity.
- Brooms and gorse (Cytisus, Genista, Ulex spp.)
have usurped many biological communities in the coast
ranges and Sierra foothills: grasslands (including
pasture), scrub, coastal prairie, chaparral, and mixed
evergreen forest. Brooms are highly flammable and
especially common in wildland/urban interfaces. Seeds of
all of them may be viable for decades, making
reclamation of territory they occupy exceedingly
difficult. Their ranges continue to expand.
- Giant reed grass (Arundo donax ) and salt cedar
(Tamarix spp.) have replaced riparian
communities, especially in southern California, the
Mojave Desert and the San Joaquin Valley. Salt cedar can
cause dramatic hydrologic changes, lowering water tables
and drying up streams and seeps.
- Annual grasses (e.g., Aira, Avena, Bromus, Hordeum,
Lolium, Vulpia ) and forbs (e.g., Vicia spp., Cirsium spp.) have greatly altered the character
of the remaining grasslands of California, replacing
native bunchgrasses and lessening spring and summer
- On federal lands alone, it is estimated that weeds are
claiming 4600 acres every day and dominate over 17 million
acres in the western United States, (Bureau of Land
Management, 1996) with similar expansions occurring in
Canada and Mexico.
- Control of exotic plants is expensive and control expenses
continue to escalate as the problem grows. The federal and
state departments of agriculture, national and state park
systems, and The Nature Conservancy devote large and
increasing resources to efforts to control exotic plant
species. Hundreds of grassroot groups selectively address
the problem in specific areas, but their work is dwarfed by
the magnitude of the overall problem.
- Taxpayers have spent billions of dollars purchasing and
protecting wildlands which are now being lost due to
invasion by weeds. In many cases these invasions--which will
result in permanent, effectively irreversible damage--are
allowed to proceed unopposed due to short-term budget
- Economic return is reduced in areas dominated by weeds.
For example, in addition to the above-cited California
grazing lands degraded by the spread of yellow star thistle,
a public agency was successfully sued by an adjacent
landowner because yellow star thistle invasion rendered the
- Logged-over lands are frequently invaded by non-native
plants such as pampas grass and brooms, which prevent
establishment of seedling trees.
- Biological control is expensive and time-consuming but is
the most cost-effective remedy for controlling some of the
most widespread invaders.
- One thousand and twenty-five species (17 1/2%) of the
California flora are exotic. (Rejmanek, 1994). Undesirable
plants continue to be introduced to California. Moroccan
mustard (Brassica tournefortii ), introduced into
California in the mid-1960s, has spread to cover large areas
of the Sonoran and Mojave deserts. Biologists consider it a
threat to the desert tortoise. With increased international
travel and trade, new accidental and intentional
introductions will likely accelerate.
- Ecosystem function is altered, often irreversibly, by
exotic plant invasions. The introduction of Moroccan
mustard, red brome (Bromus madritensis ssp. rubens ), cheat grass (Bromus tectorum ) and other
exotic grasses to the Mojave Desert has promoted unnatural
fuel conditions and fire cycles which have become
self-sustaining. Cheat grass causes similar impacts in
rangelands throughout the intermountain west (D'Antonio,
1992). Smooth cordgrass (Spartina alterniflora ) from
eastern North America is changing sedimentation rates in
open mud intertidal habitats (Josselyn, 1993). Other exotic
plants have dramatic effects on hydrological regimes and
- Non-native plants modify wildlife habitat, altering the
species composition, sometimes drastically. Riparian areas,
which are crucial breeding and foraging areas for both
common and endangered birds, have become dominated by giant
reed grass and salt cedar. Many species of birds don't use
stands of these species in part because they support few
insects, so food supply for insectivorous birds is poor.
Studies have shown that native birds prefer native woodlands
dominated by oak rather than groves of introduced trees such
as Tasmanian blue gum (Eucalyptus globulus )
- Fire frequencies may be altered by exotic plants, reducing
the ability of native plants to prosper and effecting
conversion of vegetation type (e.g., from native chaparral
to non-native grassland). Post-fire seeding with exotic
grasses has been shown to increase the likelihood of
premature reburn, thus promoting type conversion. This
conversion often can lead to increased erosion. (California
Native Plant Society, 1995.)
- Exotic plants further threaten many already rare or
endangered native plants by displacement of habitat:
examples include Howell's spineflower (Chorizanthe
howellii) and other dune endemics, diamond-petalled
California poppy (Eschscholzia rhombipetala),
large-flowered fiddleneck (Amsinckia grandiflora),
Morro manzanita (Arctostaphylos morroensis), San Luis
Obispo monardella (Monardella frutescens), Nipomo
Mesa lupine (Lupinus nipomoensis). At least 91 of the
plants in CNPS' Inventory of Rare and Endangered Vascular
Plants of California are threatened by invading exotics
(California Native Plant Society, 1994). Growing
infestations of non-native species are likely to drive
populations of native species so low as to require listing
by state or federal agencies. An informal analysis by the
California Department of Fish and Game found that 23% of
California's 280 plant communities are heavily impacted by
non-native plants, and another 28% are moderately threatened
by them (Keeler-Wolf, 1993).
- Exotic plants may trap nearly all the energy flowing
through the natural systems of the many areas where they
have completely displaced indigenous plants, resulting in
conversion from one vegetation type to another. This energy,
instead of entering the food chain, is channeled into
further proliferation by the invading plant, thus energizing
the cycle. Land dominated by weeds has low biological value
and is of little or no use to human societies. The land's
ability to function in a biologically stable way is
Bureau of Land Management, Partners Against Weeds, An action
plan for Bureau of Land Management, January 1996.
California Native Plant Society's Inventory of Rare and
Endangered Vascular Plants of California, 1994.
California Native Plant Society. Statement of Policy -
Seeding After Wildfire, December 1995.
D'Antonio, C. and P.M. Vitousek, 1992, Biological invasions
by exotic grasses, the grass/fire cycle, and global change.
Annual Review of Ecology and Systematics, Vol. 3, pps. 63-87.
Josselyn, M., B. Larrson, A. Fiorillo, Environmental impacts
of the invasion of Spartina alterniflora in San Francisco Bay.
Final report prepared for San Francisco Estuary Project,
Richmond, California, 1993.
Keeler-Wolf, Todd, Memo to M. Hoshovsky, 5 October 1993.
Maddox, D.M. and A. Mayfield, Yellow starthistle infestations on
the increase, California Agriculture Vol. 39 nos. 11-12, pps.
Morrison, Mike, and John Keane, Focused Environmental Study;
Restoration of Angel Island natural areas affected by
eucalyptus, July 1988.
Rejmanek, M. and J.R. Randall, Invasive alien plants in
California: 1993 summary and comparison with other areas in
North America, Madrono 41 #3, pp. 161-177, 1994.