are quickly becoming the premier threat to California's unique
native plants and plant communities. They can invade and quickly alter areas
otherwise protected from impacts. CNPS urges agencies and others involved in land
management to develop and implement invasive exotic control
programs. CNPS chapter members participate in a wide variety
of local programs to help with this problem.
Although the term 'exotic' somehow brings up the image of a
plant holding a drink with a little umbrella, the reality is
that exotic means 'out-of-place' in the California ecosystem.
Although there has been debate about how long a plant has to
have been resident in California to be considered native, a
practical working definition employs pre-European-contact as the
cut-off point, while those introduced since that time are
considered exotic or non-native.
The problem develops with those exotic plants that spread
into the surrounding ecosystems and displace the native plants.
They do this either because they are free of their home-range
diseases, more aggressive in their growth habits, or because
they put out more seed that lasts longer in the soil, or because
there is nothing to eat them or compete with them in the
California ecosystem that is being invaded. These exotic plant
properties may cause a crisis in the web of life of the invaded
ecosystem, for the newcomer is not a food source and may support
no life, while the displaced native plants take with them the
pyramid of life that used the plant as the prime recycler of
To the casual viewer there is seldom anything to see that
would indicate an invasion of exotic plants is taking place. The
green hills of spring are usually the green grasses of Europe
that were introduced by the Spanish with their cattle. The
feathery plumes of pampas grass on the hills of Big Sur might
look like they have always been there. However, the sad fact is
that the invasion is sometimes fast; so fast that within a
decade the ecosystem of old has completely vanished.
been working with the California Invasive Plant Council (Cal-IPC)
and the California Department of Food and Agriculture to
identify those plants which are putting California's flora at
greatest risk, and to find methods to eradicate the menace.
Agencies such as the Bureau of Land Management, the National
Park Service, US Forest Service, and California state parks are
also active participants in the fight against pest plants. Weed
Management Areas are springing up all over the state, usually
covering one or two counties. They are coalitions of private
landowners, public agencies, non-profit organizations, and
grassroots activists formed to combat a common enemy.
The need to protect natural ecosystems from invasion has run
on a parallel and much less well-funded path than programs that
are designed to control agricultural pest plants, although the
two paths are beginning to converge. Agriculturists have
actually opposed including non-crop-damaging exotic plants in
state and federal eradication programs, as those programs may
ban the shipment or interstate travel of pest-contaminated
products, and obviously cause them some problems. However the
swift occupation of wildlands and ranching lands by pests such
as yellow starthistle, artichoke thistle, and whitetop have
caused agriculturists and habitat conservationists to cooperate.
What are the solutions to some of these plant invasions?
Plants can sometimes be removed by hand, especially where
invasions are freshly started. The policy is to always work from
the outside of the population, forcing the invader into a
smaller and smaller perimeter. Small outlying populations should
be removed early in the process. Piecemeal removal simply does
not work. Sometimes the plants resist hand removal, due to a
pervasive root structure or to their ability to reroot from
small fragments. Two troublesome plants of the latter type are
Cape (German) ivy and giant reed (arundo). Cape ivy, which was
brought here from South Africa, is choking the summer-cool areas
of the state, particularly the riparian corridors of coastal
valleys; it may be capable of also invading inland riparian
habitats. Giant reed, a bamboo-like grass that grows to the
height of a house, invades riparian areas. Both the ivy and the
reed can root from small segments, and therefore the worst thing
you can do is hit them with a weed-whacker. If you pull the
plant, bits and pieces break off, and the root remains to
resprout. It can be removed by very intense manual labor, but
this is not possible for multi-acre invasions. The only answer
seems to be chemical, and there are some relatively benign
herbicides (Roundup, for example) which seem to do the trick.
Herbicide should be used with caution, but in many cases there
are seldom viable alternatives, given the limited labor pools
and finances of conservation organizations and public agencies.
CNPS Policy on Invasive
California Invasive Plant Council (Cal-IPC)
Take a look at CalFlora's expanded weed photos
and mapping www.calflora.org
Notice of Intent to adopt a Negative Declaration for
a proposal to introduce a non-native bumblebee, Bombus impatiens, into
the agro-ecological system of the state of California for crop pollination (PDF 302K)
Internet Links on
Citizens in Action: The Broom Education and Eradication Program in Forest Ranch