Invasive Plant Species Identification & Control Guide
Selected Weeds of Scott Valley, California
Invasive plant species pose risks to our local biodiversity by reducing or out-competing native plant populations, changing the intensity and spread of wildfires, altering aquatic ecosystems, and reducing forage for livestock and cropland for ranchers and farmers. Once noxious or invasive weeds become established it is extremely difficult to eradicate them and bring back the native plant communities that have been displaced. The intended purpose of this guide of some of the most invasive weeds in Scott Valley, California is to promote awareness of invasive species, as well as assist land users in identifying, reporting and controlling invasive plants.
"Noxious (or invasive) weed" means any species of plant that is, or liable to be, troublesome, aggressive, intrusive, detrimental, or destructive to agriculture, silviculture, or important native species, and difficult to control or eradicate, which the Secretary of the California Department of Food and Agriculture, by regulation, designates to be a noxious weed. (Definition from the California Food and Agriculture Code.)
Isatis tinctoria L.
Family: Brassicaceae | Origin: Native to Europe. Was cultivated for centuries in Europe as a medicinal herb and source of blue dye, also cultivated by early settlers in eastern states. | Habitat: Disturbed and undisturbed sites, roadsides, fields, pastures, grain and alfalfa fields, forest and rangeland. Often grows on dry, rocky or sandy soils. | Cal-IPC Inventory: Moderate Invasiveness
Dyer's woad is an erect biennial, sometimes winter annual or short-lived perennial that grows to heights of 1'-4'. Leaves are bluish-green, with a pale mid-vein, and covered with a powdery white film. Rosette leaves are long and narrow, mostly 1.5"-7" long, 0.4"-1.5" wide, with weakly toothed to wavy margins. Stem leaves are broad to narrowly arrowhead-shaped, alternate and clasping the stem. Plants exist as basal rosettes until flower stems develop at maturity.
Dyer's woad reproduces only by seed. It flowers in spring (typically May locally), producing umbrella-shaped panicles with small, bright yellow, four-petaled flowers. Most seeds fall near the parent plants, but some disperse short distances with wind and to greater distances with water, and as a seed and hay contaminant. New seedlings emerge in fall and early spring. Anecdotal evidence suggests the seedbank may persist for several years. Therefore, several consecutive years of control are generally necessary.
Local distribution: Large infestations along roadways and within unused pastureland occur throughout Scott Valley.
Perennial pepperweed (tall whitetop)
Lepidium latifolium L.
Family: Brassicaceae | Origin: Native to Eurasia | Habitat: Many different areas and habitats, including wetlands, riparian areas, meadows, vernal pools, salt marshes, flood plains, sand dunes, roadsides, irrigation ditches, ornamental plantings, and agronomic crops including alfalfa, orchards, vineyards, and irrigated pastures. Most typically found on moist or seasonally wet sites in the west and most problematic in riparian or wetland areas. Tolerates saline and alkaline conditions. | Cal-IPC Inventory: High Invasiveness
Impacts: Perennial pepperweed can rapidly form large, dense stands that displace desirable vegetation and wildlife. Populations easily spread along waterways and can infest entire stream corridors, riparian areas and irrigation structures. Roots do not hold soil together well, allowing erosion of river, stream, or ditch banks. Flooded streams often wash away roots growing along the streambank, and new infestations develop downstream. Perennial pepperweed reduces forage quality in hay and pasture by extracting salts from deep soil and deposits them on the soil surface, inhibiting the germination and growth of other species that are sensitive to salinity.
Perennial pepperweed is an erect perennial to 6' tall. The crown and lower stems are weakly woody. The foliage lacks hairs and is green to gray-green, often dusted with powdery white caused by a rust fungus. Basal leaves are larger and wider than stem leaves, to 1' long and 4" wide, with serrate margins. Dead thatch can persists for several years. Roots are long, thick, minimally branched, and vigorously creeping. Most grow in the top 2' of soil, nut some can penetrate to a depth of 10' or more.
The inflorescences are rounded to pyramidal and consists or numerous small white flowers. Flowers have four petals, producing small pods (about 2 mm long) with tiny reddish brown seeds (about 1 mm long). Perennial pepperweed is a prolific seed producer; however, seeds do not appear to remain viable in the soil for extended periods. As a result, perennial pepperweed reproduces primarily vegetatively from roots and root fragments that can survive desiccation on the soil surface for extended periods and develop into new plants. Root fragments and seeds disperse with flooding, soil movement, and human and animal activities.
Local distribution: Small patches mostly occurring in northern Scott Valley.
Cytisus scoparius L.
Family: Fabaceae | Origin: Central and southern Europe and North Africa. Introduced to the U.S. in the 1850s as an ornamental and for erosion control. | Habitat: Typically in mountain regions and cool coastal areas with dry summers. | Cal-IPC Inventory: High Invasiveness
Impacts: Grows rapidly, forming dense stands that most wildlife find impenetrable and unpalatable. Dense stems limit regeneration of most other plants species, and the accumulation of woody biomass creates a dangerous fire hazard. Broom can fix nitrogen, which increases soil fertility and gives competitive advantage to other non-native weeds.
Scotch broom is a fast-growing deciduous shrub, 5'-10' tall, with yellow, pea-like flowers. Stems are 5-angled or ridged, often star-shaped in cross-section. New twigs are green, erect and covered with wavy hairs, becoming smooth and woody with age. The leaves at branch bases have three leaflets alternatively arranged. Upper leaves are simple, without petioles. Leaflets are < 1"-3" long, widest at the tip and often pointed.
Plants begin flowering from 18 months to 3 years of age. The bright yellow, occasionally maroon, flowers are single or in pairs in leaf axils. Reproduction is by seed. Seeds are in small, flattened pods 0.75"-2" long. Pods are dark brown or black when mature; contain 5 to 9 seeds, and have hairs along the margin. When mature, pods eject the seeds several feet from the plant. Seeds can remain viable in the soil for up to 30 years. Large soil seedbanks often accumulate making long term control difficult. Shrubs may live for up to 30 years.
Local distribution: Small patches occur throughout Scott Valley.
Cirsium arvense (L.) Scop.
Family: Asteraceae | Origin: Native to southeastern Europe and the eastern Mediterranean area. | Habitat: Open, disturbed sites such as roadsides, gardens, pastures, hillsides, rangeland, stream banks, forest openings, and sometimes cropland such as alfalfa or grains. Prefers moist soils but will tolerate a wide range of soil types. | Cal-IPC Inventory: Moderate Invasiveness
Impact: Competes aggressively with native plant species. It causes extensive yield loss in crops competing for nutrients, light and water. It may also have an allelopathic effect. The productivity of pastures is significantly reduced because livestock avoid grazing Canada thistle and surrounding plants due to the spiny nature of the mature foliage. Canada thistle can also be economically damaging to ranchers by causing an increase in infections due to abrasions. Canada thistle is a host species for several agricultural insect and disease pests such as the sod-web worm, bean aphid, stalk borer, and cucumber mosaic virus.
Canada thistle is an erect perennial that grows up to 3'-5' tall and forms patches or clumps that are usually of a single sex. Stems ordinarily die back over winter and new shoots are formed in spring from old stem bases or root buds when the soil moisture permits. Canada thistle has an extensive creeping root system that can reach depths of 6'-15' making eradication difficult. The spiny lobed leaves are 6"-8" long and 1"-1.5" wide. The leaves are alternate, oblong or lance-shaped and the base leaves stalkless and clasping.
Plants are dioecious (separate male and female plants) and flower heads are white to purple, borne in clusters of 1-5 per branch. The purplish involucre is glabrous or has white wooly hairs. Plants develop from seed and from vegetative shoots that generate from adventures root buds. Canada thistle can produce between 1,00-5,000 seeds per stem. Most seeds fall near the parent plants or disperse short distances with wind. Birds and small mammals can consume and disperse some seeds. Seeds have been known to survive in the soil for up to 20 years and longevity is favored by deep burial.
Local Distribution: Widespread in small to medium sized patches throughout Scott Valley.
Centaurea stoebe L. ssp. micranthos (Gugler) Hayek
Family: Asteraceae | Origin: Europe, Asia Minor; introduced into the U.S. in the late 1890s. | Habitat: Fields, roadsides, disturbed open sites, grassland, rangeland, especially degraded rangeland, logged areas. Seldom persists in shaded places. Serious infestations often occur on light, well-drained soils in areas that receive some summer rainfall. | Cal-IPC Inventory: High Invasiveness
Impact: Highly competitive with native vegetation. Forms dense stands that can exclude desirable vegetation and wildlife in natural areas.
Spotted knapweed is a bushy biennial to short-lived perennial, to 3' tall, with a long, sturdy taproot. Plants form basal rosettes during winter and early spring (sometimes persisting as rosettes for several years) and develop erect, highly branched flowering stems in late spring and summer. Its alternate leaves are pinnate-lobed, dotted with resin ducts, and covered with short to medium interwoven grayish hairs.
The 30-40 disk flowers in each flowerhead are white, pink, or purple, and the phyllaries have comb-shaped, dark-colored tips which give the flower-heads a "spotted" appearance. After the flower-heads mature and dry out, the pop open, ejecting achenes near the parent plant. Achenes have a short (1-2 mm) bristly pappus on the top. Spotted knapweed can also reproduce vegetatively from lateral roots just below the soil surface. New rosettes may develop at about 3-cm intervals along lateral roots, expanding populations peripherally. Achenes can remain dormant in the soil for 8 years and have three germination patterns: non-dormant seeds that germinate with or without light exposure, dormant seeds that germinate in response to light, and dormant seeds that are not light sensitive. All germination types occur on each plant. Spotted knapweed has been shown to occasionally hybridize with diffuse knapweed.
Local distribution: Mostly occurring in the French Creek watershed and surrounding areas.
Centaurea diffusa Lam.
Family: Asteraceae | Origin: Native to southeastern Eurasia. | Habitat: Plains, rangelands, and forested benchlands, particularly on rugged terrain not well suited for cultivation; often on well drained soils. Needs less moisture than spotted knapweed; can thrive in semi-arid and arid conditions. Seldom persists in shaded places. Not common on cultivation or excessive moisture. | Cal-IPC Inventory: Moderate Invasiveness
Diffuse Knapweed is a bushy herbaceous tap-rooted biennial or short-lived perennial that grows to about 3' tall. The leaves are alternate and variously covered with short to medium interwoven grayish hairs. The upper leaves are entire and linear; the lower stem leaves are 4"-8" long and deeply pinnate-lobed on to two times. Plants are basal rosettes in fall and winter and bolt to produce erect, highly branched flowering stems in late spring and summer.
The flower-heads consist of spiny or comb-like phyllaries and white or pink to pale purple disk flowers. Unlike squarrose knapweed, the spiny phyllaries (3 mm long) do not reflex downward. The achenes either lack a pappus or have a very short bristly pappus (< 1 mm long). Plants reproduce only by seed. Diffuse knapweed inflorescences detach from the parent plant when stems break off near the ground and tumble along the ground in the wind, dispersing seed to a greater distance than most Centaurea species. Data shows that about 20-50 % of plant inflorescences tumble off site. Seeds remain viable in the soil 2-5 years, with some surviving longer.
Local distribution: Mostly occurring in the McAdams Creek watershed and surrounding areas.
Selected Noxious Weeds of Northeastern California
Weed Control in Natural Areas in the Western United States (Weed Research & Information Center, University of California). 2013.