Noxious Weed Programs

Spotted Knapweed (Centaurea Maculosa) Eradication Project

Spotted knapweed, an A-rated pest, is a biennial or short-lived perennial weed of very limited distribution in eastern Fresno County. It has been detected in the Sierra mountain range in the eastern portion of Fresno County. Spotted knapweed grows and spreads very rapidly in disturbed areas. There are some indications that it releases chemical substances that inhibit the growth of adjacent plants. Spotted knapweed can reach 1 to 3 feet tall. It has solitary flower heads at the end of branches. The pinkish-purple flower heads have stiff bracts at the base with a dark comb-like fringe giving it a spotted appearance. Flowering occurs June to October.

Once established spotted knapweed produces a very stout, and deep taproot and becomes difficult to control.

Negative Impact and Importance of Eradication

Spotted knapweed has the potential to become the yellow starthistle of the high Sierras. If allowed to spread, native vegetation and associated wildlife would be negatively impacted. Timber harvest activities would be delayed or curtailed entirely. Infestations of meadows and grazing areas would have a negative impact on the cattle industry. Due to the limited distribution of spotted knapweed, early detection and treatment is critical.

Early Measures to Control Knapweed

Spotted knapweed was introduced from Eurasia. It was detected in Fresno County on August 1, 1986, along Highway 168, east of the Big Creek turnoff at an elevation of approximately 6,000 feet. The infestation was treated, and subsequent surveys were negative.

• On August 7, 2001, spotted knapweed was detected by U.S. Forest Service (USFS) personnel on Dinkey Creek Road east of Glen Meadow at an elevation of approximately 6,000 feet. Five adult plants in full bloom were hand pulled. Surveys since 2001 have been negative.
• The latest find was on August 22, 2005. The infestation site was approximately 1/2 acre at an elevation of 5,664 feet. All stages of growth were present. On August 23, blooming plants were hand pulled. The remaining rosettes were treated.
• Follow-up surveys in 2006 and 2007 found single flowering plants, which were hand pulled. Subsequent surveys in 2008 through 2010 were negative.

Current Measures

Each year, survey for spotted knapweed begins the last week of July in the Shaver Lake area.

Spotted knapweed flower head with dark tip bract (left) rosette stage (right)

Spotted knapweed flower head with dark tip bract (left) rosette stage (right)

Rush Skeletonweed (Chondrilla Juncea) Eradication Project

Rush Skeletonweed is an A-rated weed of moderate distribution in Fresno. Rush Skeletonweed is a non-native invasive noxious weed species from southern Europe that was introduced into San Luis Obispo County in 1965.

Negative Impact and Importance of Eradication

Rush skeletonweed can be a serious agricultural pest because it can reduce crop yield and choke harvesting machinery. Due to its limited distribution and its ability to rapidly spread, delimitation surveys and treatment to eradicate this weed is critical. Each rush skeletonweed plant can produce up to 20,000 seeds and be carried by wind, vehicles, clothing and animals. Once rush skeletonweed is established, it produces a very extensive and deep root. Control methods for rush skeletonweed include chemical spraying and repetitive plowing.

Early Measures to Control Rush Skeletonweed

The earliest known find in Fresno County was in July, 1981. Rush skeletonweed was found in an alley in downtown Fresno near the railroad tracks. Subsequent finds in the 1990s occured west of downtown Fresno in a residential area.

An eradication project began in 1997.

Currently, rush skeletonweed is being found in the southeastern portion of Fresno City and southeastward into in urban and agricultural areas of Fresno County. It continues to spread southeastwardly. Recent finds are within 15 miles of the entrance to Sequoia National Forest on Highway 180.

Current Measures

Due to the elimination of the weed control program at the state level, Fresno County Department of Agriculture staff will perform a limited survey and treatment program for rush skeletonweed in May of each year, which may continue through the month of October.

Physical Characteristics and Habitat

Rush skeletonweed grows to be 1 to 4 feet tall. It is a perennial. The lower 4 to 6 inches of the stem have downward bent red hairs. Milky juice is exuded when the leaves are torn. The flowers are yellow with flat strap-shaped petals with distinct lobes. In the Fresno area, flowering is usually June through the first frost.

Rush skeletonweed will infest well-drained light-textured disturbed soils. Infestations are found along roadsides, in rangelands, grain fields, vineyards, orchards and pastures. The small leaves and the rigid, wiry stems make herbicide control difficult. The root system is extensive and deep. Rush Skeletonweed can reproduce vegetatively from adventitious buds in the roots. Plowing just once can be problematic; as the roots are broken into small pieces they can develop into new plants.

Mature rush skeletonweed (left). Rush skeletonweed flowers and seedheads (right).

Heart-Podded Hoary Cress (Cardaria Draba) Lens-Podded Whitetop (Cardaria Chalepensis) and Hairy Whitetop (Cardaria Pubescens) are B-rated weeds of very limited distribution in Fresno County. Hoary cress is a non-native invasive noxious weed species that is native to Europe.

It will readily infest disturbed soils. Hoary cress is a perennial pest in alfalfa. Livestock that ingest large quantities can develop digestive problems. It is very common in alkaline soils and is highly competitive with other species once it becomes established.

Current Measures

Yearly, during February, the Fresno County Department of Agriculture staff will begin noxious weed surveys for hoary cress. Known hoary cress infestations are northwest of Firebaugh and in the Dinkey Creek area.

Physical Characteristics

All species are deep rooted perennial herbaceous plants that grow up to 2 feet tall. They will reproduce by seed and root fragments. In the early spring, the plants emerge from the roots as a rosette. Flowering occurs early summer to fall. Hoary cress forms a cluster of small white flowers with four petals. The C. draba flower produces a heart shaped bladder-like seed capsule containing two reddish-brown seeds. C. chalepensis produces a lens-podded seed capsule, and C. pubescens produces a hairy globe-podded seed capsule. Seeds are set by mid-summer. All hoary cress species are distinguished from perennial pepperweed by the leaf attachment to the stem. Hoary cress leaves clasp the stem while perennial pepperweed leaves do not (see image).

Perennial pepperweed (Lepidium Latifolium) is a B-rated weed of limited distribution in Fresno County. It is a perennial herbaceous plant that grows to 1 to 5 feet tall. Pepperweed forms a deep-seated rootstock making the plant difficult to control. The root system can become extensive with depths greater than 10 feet. It will reproduce by seed and root fragments. In the early spring, the plant emerges from the roots as a rosette. Flowering occurs early summer to fall. Pepperweed forms dense clusters of white flowers near the ends of branches. Each flower produces a seed capsule containing two seeds. The seeds are believed to be short-lived and do not germinate well.

Current Measures

Each year, during March, the Fresno County Department of Agriculture staff will begin noxious weed surveys for perennial pepperweed and hoary cress, a B-rated noxious weed of limited distribution in Fresno County. Known infestations are west of Firebaugh, around Mendota, north of Helm, east of Clovis and in the Shaver Lake area. Staff will survey and treat as needed.

Physical Characteristics and Habitat

Perennial pepperweed is a non-native invasive noxious weed species that is native to southeastern Europe to southwestern Asia. The first recorded detection in California was in 1936 in Stanislaus County. It is fairly well established in Northern California, and spotty throughout the rest of the state.

Negative Impact and Importance of Eradication

Pepperweed can survive at elevations over 9,000 feet. Contaminated hay or straw bales are believed to be the source of recent infestations. Plowing or discing only breaks up the root and increases the number of plants. Perennial pepperweed will infest waste lands, ditches, roadsides, pastures, meadows, roadsides, and wet areas. If perennial pepperweed has infested a stream bank or canal fragmented roots can float downstream to establish new infestations. This invasive weed can readily overtake and transform a native plant community. It can choke out native plants and trees and render crop land useless.

Perennial pepperweed flower cluster (left), and an infestation in the Sierras east of Clovis (right)

Infestations along roadsides (left) can spread into adjacent cropland (right) greatly reducing crop production.

Early Measures to Control

Purple Starthistle (Centaurea calcitrapa) was first detected in eastern Fresno County in May of 2001.

From 2001 to 2007 the site was surveyed, and all of the plants were hand pulled and disposed. No plants were detected in surveys from 2007 to 2010. In 2012 mature plants were detected at the same site. Approximately 1.5 acres of the 32-acre site were treated.

Negative Impact and Importance of Eradication

Management methods are currently ongoing. Purple starthistle is a B-rated invasive noxious weed. It is a biennial weed that will infest rangeland and open grasslands. Just like its close relative yellow starthistle, it can degrade the forage quality of rangeland and hinder access to humans and livestock. Mature plants can grow over 6 feet tall. The bracts of the flowers have toothpick like spines over one inch in length.

Mature purple starthistle along canal bank (left) and flower heads and spines (right)

Purple starthistle flower (left) and root system (right)

Early Measures to Control

Water hyacinth (Eichhornia Crassipes) is a B-rated weed of limited distribution in Fresno. Waterhyacinth is a non-native invasive noxious aquatic weed species that was introduced into the United States as an ornamental. It infests localized areas in California.

Negative Impact and Importance of Eradication

Water Hyacinth can block water flow in irrigation and drainage canals. More importantly, it can clog waterways, interfering with natural water flows and navigation. Water hyacinth will crowd out native plants with little benefit to wildlife. The dense stands it forms along shorelines, or in open water, inhibit migrating water birds from landing. The large floating mats reduce light and oxygen levels underwater, harming fish and other aquatic life.

Physical Characteristics and Habitat

Water hyacinth seeds germinate in April. Overwintering mature plants will begin regrowth as the weather warms. It can root in mud, but generally it floats on the water surface forming dense mats. The mature plant can be 1-2 feet tall with dense dark colored fibrous roots. It usually reproduces vegetatively by stolons that form daughter plants. The process repeats over and over again. During the summer water hyacinth can double its coverage every 30 days.

Previous water hyacinth infestations on the San Joaquin River north of Poso Dam (left); and east of Highway 99 (middle). The leaf surface of water hyacinth is bright green and glossy (right). The leafstalks are filled with air cells and look inflated and bladderlike.