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Monterey County Weed Threats

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Fertile Capeweed Arctotheca calendula
  • Rated as an "A" species by the State Department of Food and Agriculture.
  • Detected in May, 1999, near Gonzales and Pesante Rd., Prunedale.
  • Fertile capeweed plants initially form rosettes, and under ideal conditions, develop into low plants up to about 6 inches high and 16 inches across.
  • Annual seed-producing.
  • Aside from the production of seeds (which are enclosed in pinkish-brown balls of long hairs), it is differentiated from sterile form by its non-creeping growth habit and having flowers with dark purple to black centers. The sterile form has flowers of uniform yellow.
Cal-IPC: Arctotheca calendula

French Broom Genista monspessulana
  • An invasive woody shrub-like plant which is native to the Canary Islands.
  • Member of the pea family and grows up to 9 ft. in height.
  • During the spring it is covered with profuse yellow blooms about a half-inch long.
  • It is found primarily along the coast and northern Monterey County.
  • French broom is a prolific seeder and in some areas, seedlings will appear to carpet the ground.
  • French broom is relatively easy to control by flaming or herbicide treatments of seedlings. When the ground is moist, entire large plants can be pulled up by their roots using a weed wrench. Once a treatment program is started, it is important to maintain control efforts, as the French broom seeds will germinate for several years.

Cal-IPC: Genista Monsepssulana

Cape Ivy Delairia odorata

Also known as German Ivy Senecio milkanioides side_body_Cape_Ivy

  • Member of the sunflower family with bright green leaves shaped somewhat like ivy. The flowers are small and yellow and occur in dense clusters.
  • It is an invader from South Africa that has become or is rapidly becoming an ecological disaster in most of the riparian or stream-side areas of the County, especially along the coast.
  • This plant is capable of forming a dense vine-like growth that completely smothers all underlying vegetation.
  • It is very difficult to control without destroying any and all associated plants along with it. Any small root fragments left unkilled are capable of regrowth. Currently, USDA researchers are investigating a number of bio-control agents in Africa that may be introduced here in the future.


Cal-IPC: Delairea Odorata

Arundo Arundo donax
  • This invasive weed, native to North Africa, is becoming a dominant plant along the Salinas River, where it is crowding out native species.
  • Arundo is spread by the breaking up and movement of root fragments. Where it occurs in a river, it can restrict stream flow and enhance flooding.

Cal-IPC: Arundo donax 

Pampas Grass Cortaderia selloan
  • Native to South America
  • Considered the less invasive of the species’ of Pampas Grass in the county.
  • These grasses form large clumps of tough wiry and sharp-edged leaf blades that are difficult to remove once they reach a certain size.
  • It has the potential for prolific seed production, and does so by asexual means
  • Available in the nursery trade and plants have been widely planted. It was thought that the true pampas grass was safe, because it required male plants to pollinate, and supposedly none of the plants being sold were male. In fact, male plants are now being found in the wild.
  • In the past, this grass was recommended for use as an erosion control, especially where road cuts had been made. It has unfortunately proven to be a serious threat to native plant habitat.

 Cal-IPC: Cortaderia Selloana 

Purple Pampas Grass Cortaderia jubata
  • Considered to be more invasive and more prevalent in this county than other species of Pampas Grass
  • Most purple pampas grass infestations are seen along the coastal areas.
  • Similar in appearance to true Pampas Grass, explained above.
Cal-IPC: Cortaderia jubata 






Yellowstar-thistle Centaurea solstitialis
  • Unquestionably the most serious rangeland noxious weed in the County.
  • The flower of this plant is yellow, and is armed around the base with long sharp spines. The rest of the plant, including stems and leaves are spineless. Mature start-thistle plants can grow over 3 feet tall and 3 ft. across under ideal conditions.
  • It is able to colonize different areas by having a deep root system and an adaptable growth form, both tall and slender in grassy areas, or broad and bushy in less competitive situations.
  • Yellow star-thistle is a very adaptable plant and is quite successful at out-competing native plants. In some areas, it forms almost impenetrable stands that are so thick the even it cannot grow again for awhile.
  • Fortunately, a number of introduced bio-control agents have been released here against it which are becoming established and thriving. Some of these agents are being made available for redistribution in the County.
Cap-IPC: Centaurea solstitialis  


Veldt Grass Ehrharta calycina

  • Native to southern Africa. It was introduced to Davis, California as a drought-resistant grass for rangelands, but it was unable to withstand heavy grazing.
  • It flowers in the spring. The leafy stems reach to 3 feet in height and are adapted to light sandy soils.
  • Veldt grass occurs in several locations in Monterey County near the ocean. It can form dense stands and crowds out native vegetation.
Cal-IPC: Ehrharta spp.


Taurian Thistle Onopordum tauricum
  • Rated as an "A" species by the State Department of Food and Agriculture.
  • It is a large thistle, capable of growing up to 8 ft. tall under ideal conditions. Mature plants have been found in very dry years that were only a few inches tall, but with a fully developed small seed head.
  • The flowers are purple-red, and are surrounded by numerous pointed bracts. The leaves are spiny along their edges, and the stems have spiny wings up their entire length.
  • First found in 1973 on rangeland along Paraiso Springs Rd. near Soledad. In subsequent surveys it was found at Paraiso Hot Springs and down the drainage creeks from there all the way to Arroyo Seco Rd.
  • The Taurian thistle seeds are capable of long dormancy, usually being stimulated to germinate by scarification or scratching of the seedcoat, stimulating the development of plants in areas where they had not been seen for many years.
  • Eradication treatments are made on Taurian thistle two times a year. Treatments are made by shovel and backpack sprayer, using Roundup and Transline. Plants gone to seed have their seed heads clipped and bagged, to be disposed of later. The entire drainage creek from Paraiso Hot Springs to Arroyo Seco Road is surveyed, mostly by foot.
Calflora: Onopordum tauricum


Puna Grass Achnatherum brachychaetum
  • This grass is native to the Andes regions of South America
  • Under ideal conditions, it can grow up to 3 ft. tall, and 3 ft. across. The plants have tough fibrous roots, but no creeping rhizomes.
  • It is unusual in that it forms seeds at the base of the plant within the stems, so-called cleistogamous seeds. These seeds are capable of propagating the plants even if the tops are cut back or mown.
  • Puna grass was detected in 1995 in a pasture in Greenfield. Subsequent surveys revealed the weed at 10 locations in Greenfield and isolated locations north to Gonzales. Most of the finds were in permanent pastures, the heaviest being along Third Ave., where 5 acres of pasture are heavily infested.
  • Treatments are made 4 times a year, with shovels being used in pastures with livestock, or by backpack sprayers using Roundup where animals can be moved off for a few weeks, or in non-crop areas. Some experimental treatments were also done with propane burners, which did not give acceptable control. The burner did kill the grass, but required an unacceptable amount of time per plant.
Cal-IPC: Achnatherum brachychaetum

Skeletonweed Chondrilla juncea
  • Rated as an "A" species by the State Department of Food and Agriculture.
  • It can reach a meter in height and it produces extensive branches that often form a weedy thicket.
  • The plant reproduces by cloning itself at the root thereby avoiding clipping. Tilling and chopping even helps the plant spread, making it troublesome in many areas.
  • Successful in competition with other plants for water and nutrients.
  • Found along the railroad south of Bradley and one roadside infestation near Soledad.
  • It has been detected and spot eradicated at various location since the 1970’s.
Cal-IPC: Chondrilla juncea L.


 Scotch Thistle Onopordum acanthium
  • Rated as an "A" species by the State Department of Food and Agriculture.
  • The flowers are purple to white, one to two inches across with numerous spiny bracts. Most mature plants are found less than 6 ft. in height.
  • The leaves and stems have woolly hairs giving them a characteristic fuzzy whitish appearance. Its stems have spiny wings.
  • There’s been an active infestation since the 1960’s originally near Priest Valley, but it is spreading downstream via Lewis Creek to San Lorenzo Creek as far as King City. One plant was found in the Salinas River about a half mile downstream from the San Lorenzo Creek confluence.
  • As with Taurian thistle, seeds are capable of long dormancy, germinating when the seed coat is scratched or broken.
  • Eradication treatments are made on Scotch thistle two times a year. Treatments are made by shovel or herbicide application. Plants gone to seed have their seed heads clipped and bagged, to be disposed of later. Known infested areas are surveyed by foot and ATV.
Cal-IPC: Onopordum acanthium


Sticky Eupatorium Ageratina adenophora
  • Cal-IPC rating moderate, Federally listed as a noxious weed. Movement is regulated by CDFA in California.
  • Invades coastal shore and mountain habitats in Monterey County; disturbed areas, coastal canyons, riparian areas and scrub.
  • Escaped from cultivation, native to Mexico
  • Perennial in the sunflower family, up to seven ft. tall, with sticky hairs on leaves; produces abundant seed which is dispersed by wind, water, mud on shoes, and by clinging to animals and people.
  • Toxic to horses, causing a lung disease.
  • Other common names are: Crofton weed, Sticky snakeroot and Thoroughwort
Cal-IPC: Ageratina adenophora