Western Yarrow (Occidentalis)

The Western Yarrow (Occidentalis) is generally described as a perennial forb/herb. This is native to the U.S. (United States) has its most active growth period in the spring . The Western Yarrow (Occidentalis) has green foliage and inconspicuous white flowers, with a moderate amount of conspicuous brown fruits or seeds. The greatest bloom is usually observed in the late spring, with fruit and seed production starting in the summer and continuing until fall. Leaves are not retained year to year. The Western Yarrow (Occidentalis) has a moderate life span relative to most other plant species and a moderate growth rate. At maturity, the typical Western Yarrow (Occidentalis) will reach up to 3 feet high, with a maximum height at 20 years of 0 inches.

The Western Yarrow (Occidentalis) is easily found in nurseries, garden stores and other plant dealers and distributors. It can be propagated by seed, sprigs. It has a moderate ability to spread through seed production and the seedlings have high vigor. Note that cold stratification is not required for seed germination and the plant cannot survive exposure to temperatures below -38°F. has medium tolerance to drought and restricted water conditions.

Uses of : Landscaping, Medicinal, Culinary, etc.

Conservation: Western yarrow is an early successional species that readily establishes on disturbed sites. It is recommended for adding species diversity in native seed mixtures for rehabilitation of disturbed sites such as rangelands, mined lands, roadsides, park and restoration areas, prairie reconstruction projects, and farm bill program conservation plantings. Secondary use is for ornamental application in pollinator friendly, low maintenance, or naturalized landscapes. Forage: Western yarrow is a food source for bighorn sheep, pronghorn antelope, and deer. Sage-grouse, especially chicks, and other upland birds rely heavily on the foliage of western yarrow as a food source. Sage-grouse chicks also benefit from eating the insects associated with yarrow. In Montana, domestic sheep and goats derive approximately 40 percent of their summer diet from western yarrow, while it constitutes 20 percent of cattle and horse diets (Reitz and Morris, 1939). The leaves and flowers contain volatile oils, alkaloids, and glycosides that are considered toxic, but the plant is seldom overgrazed and eaten in large enough quantities to be harmful to foraging animals. Ethnobotanic: Native Americans used western yarrow for many purposes, such as a tea to cure stomach ailments, a poultice on infected wounds, and as a mosquito repellant.

General Characteristics

General: Western yarrow is a member of the Asteraceae (Sunflower) family that is commonly found in natural and disturbed habitats throughout the western U.S. (Hitchcock and Cronquist, 1973). It is a self-incompatible, insect-pollinated species (Clausen et al., 1958) occurring as native forms that may differ in chromosome number (n=18 or n=27); native races originating in the western U.S. (except the Pacific coast) are mostly tetraploid and introduced races mostly hexaploid (Hitchcock et al 1955). Botanists currently acknowledge genetic and ecological differentiation among native ecotypes at the local and regional level (Lavin, 2002; Lesica, 2001).

Taxonomic: Western yarrow is a moderately rhizomatous, long-lived, native, herbaceous perennial forb. It is 30-100 cm tall with few to many unbranched, erect, lanate stems (Cronquist et al., 2002). Leaves are alternate, sessile, pinnately dissected, semi-evergreen, and aromatic with an anthemideous scent (chamomile or dog fennel-like odor). The basal rosette foliage is up to 25 cm long, and cauline leaves are typically up to 10 cm long and 3 cm broad. Inflorescences are arranged in a compound, flat-topped corymb 6-20 cm wide, consisting of numerous, small flower heads 4-6 mm in diameter. Involucre bracts are usually pubescent and greenish, with papery, straw-colored margins (Robinson and Fernald, 1908). Outside ray flower numbers are 3-12, mostly white to cream-colored, 1-2.5 mm long and encircle the center disk flowers. The disk flowers number 10-75 each, and are yellow, tubular, perfect, and seed-producing. Fruit is a flattened, glabrous achene with compressed margins in a mostly reverse egg-shape with no pappus (USDA FS, 1966).

Required Growing Conditions

For current distribution, please consult the Plant Profile page for this species on the PLANTS Website.

Habitat Western yarrow is one of the most abundant and widely distributed wildflowers in the western United States. It is circumboreal and grows in a variety of plant communities from Alaska across Canada and into northern Mexico (Johnson and Larson, 1999). Western yarrow prefers full sun on roadsides, hills, canyons, pastures, and disturbed areas. It is scattered in sagebrush areas, open timber, and subalpine zones.

Western yarrow thrives in droughty conditions on gravelly loam and on thin or sandy soils. It is a common component of such ecological sites as shallow, silty, shallow to gravel, and silty steep (USDA SCS, 1976). Associated species include western wheatgrass (Pascopyrum smithii), bluebunch wheatgrass (Pseudoroegneria spicata), prairie Junegrass (Koeleria macrantha), Sandberg bluegrass (Poa secunda), common gaillardia (Gaillardia aristata), big sagebrush (Artemisia tridentata), and prairie coneflower (Ratibida columnifera).

Adaptation Western yarrow is highly variable and displays wide ecological amplitude to diurnal temperature, altitude, latitude, and climatic (Hiesey and Nobs, 1970) and edaphic conditions (Higgins and Mack, 1987). It is considered an “environmental specialist” due to the successful evolution of a number of ecotypic races (Taylor, 1992).

Cultivation and Care

Natural: Western yarrow initiates growth in early spring and blooms for an extended period of time from late spring through mid summer (Budd and Campbell, 1959). It reproduces by seed and vegetatively, but in undisturbed habitats the rhizomes remain attached to the parent plant and vegetative spread is relatively low (Bostock and Benton, 1979). In disturbed areas, rhizome fragments do not survive on the soil surface, and bud sprouting success in buried rhizomes is dependant on fragment length and soil depth (Bourdot, 1984).

The growth performance of western yarrow is reduced under conditions of increased competition and shading (Bourdot et al., 1984 and 1985). It is rated as good in maintaining a state of evergreenness (Monsen et al., 2004) and is not highly flammable, although flames can wick up through the hollow, dry flower stalks. Late-spring burning will reduce western yarrow (Anderson et al., 1970), as will heavy fires. In certain environments yarrow populations tend to temporarily increase after less intense fires (Bartos and Mueggler, 1981). Studies conducted on the use of yarrow as a sodding technique in erosion control projects have produced satisfactory results (Airhart, 1988).

Direct Seeding: Viability of fresh western yarrow seed is generally high and seeds germinate under normal test conditions in 2 to 8 days, with 75 percent germination occurring in 5 days (Sorenson and Holden, 1974). Seed should be planted into a firm, weed-free seedbed with a drill that will ensure uniform seed placement to a depth of [! to ¼ inch (3 to 6 mm) or broadcast seeded, then harrowed or raked, and firmed with a packer or roller. Field conditions during seedling emergence must be monitored for impermeable crusts, especially on sites with clayey soils. If crusting is observed, the soil crust can be fractured with a roller or periodic sprinkler irrigation.

There are approximately 4.4 million seeds/lb (9.5 million seeds/kg). The full seeding rate is ¼ to ½ lb/acre (0.3 to 0.6 kg/ha) pure-live-seed (PLS), but western yarrow would seldom be seeded in a pure stand. It is recommended that western yarrow be included as a component of a native seed mixture, where the seeding rate is adjusted to the desired potential of the plant community. Spring seeding is preferred over a dormant fall planting date. Periodic mowing during the establishment year is one option for weed suppression.

Containers: Containers should be sown in fall for outside nursery production and in spring for production in the greenhouse. Pots are filled with a well-drained soilless medium and wetted prior to seeding. Seeds are placed directly on the surface and lightly covered with a thin layer of the medium, perlite, or pea gravel, and then thoroughly irrigated. Containers are kept moist with light irrigation or misting during the establishment phase. Germination occurs in 6 to 14 days at approximately 70° F, followed by rapid root and shoot development over the next 60 days. Supplemental nutrition is not necessary, but may be applied at the recommended rate as a controlled release, encapsulated fertilizer (Luna et al., 2004).

Potted material should be acclimated to natural spring temperatures for at least 1 month prior to lining out. The site should be prepared so that the soil is workable, but not so loose as to resist packing. Transplants are placed in a hole slightly deeper than the length of the root ball, firmly tamped to remove unwanted airspace, and watered until the soil is settled. Additional soil may be required to fill cracks as they occur around the plants. Supplemental irrigation is advised for as long as feasible, or on a regular basis during the active growing season to promote flowering and seed set. Survival is high in increase plantings receiving proper care, but lower on sites with existing vegetation (Skinner, 2003).

General Upkeep and Control

There is a high potential for using this species to revegetate alpine and subalpine disturbances and in other degraded areas of the western U.S (Wasser, 1982). New seedings may need protection from trampling and weeds during establishment. Clipping weeds above the seedlings is a preferred method of weed suppression as there are no herbicides selective for broadleaf plants. Western yarrow vigor, aesthetics, and stand persistence after establishment may be sustained with properly timed grazing or defoliation of associated species. Satisfactory control of western yarrow is achieved through herbicide application mixtures of dicamba and dichlorprop (Robocker, 1977). Always consult product label prior to application and properly follow recommendations.

Pests and Potential Problems Inflorescences that are harvested for seed production often contain small quantities of insect larvae and numerous live insects. Many beneficial and pollinating insects, such as minute pirate bug (Orius), big-eyed bug (Geocoris), hoverflies (Syrphidae), and several tachnid flies (Archytas apicifer, Gymnosoma, Tricopoda pennipes, Cylindromia) are known to frequent yarrow plants (Long, 2001). Pest insects include common leaf bugs (Lygus) and flea beetles (Chrysomelidae). Root rot and mildew may occur in poorly drained soils (Warwick and Black, 1982). Environmental Concerns Western yarrow is a pioneer species and considered an increaser where the forage resource has been overstocked and excessively utilized. It should be considered noninvasive when used within a diverse plant community in its anticipated area of adaptation.

Seed Production Production fields should be seeded at a rate of 25 PLS per linear foot of row (82 per linear meter of row). Between-row spacing is dependent on the type of planting and cultivation equipment, and ranges from 22 to 36 in. (56 to 90 cm). Adequate between-row space should be provided to perform mechanical cultivation. At 24-in. row spacing, the recommended seeding rate is 0.12 PLS lb/acre (0.14 kg/ha), and at 30- and 36-in. row spacing, the seeding rate is 0.09 and 0.08 PLS lb/acre (0.1 and 0.09 kg/ha), respectively. There are presently no herbicides specifically labeled to control weeds in seed production fields. Seed harvest can be accomplished by swathing and combining from the cured windrows, or direct combining. The indeterminate ripening may necessitate periodic mechanical stripping as seed heads mature.

Seed is processed over a two-to three-screen fanning mill (slotted mesh screens), with final cleaning over an indent cylinder or gravity table. An acceptable seed quality testing standard is 95 percent purity and 80 percent viability (Stevens and Meyer, 1990). Seed production of 100 to 150 lb/acre (112 to 170 kg/ha) can be expected under irrigated conditions. Seed longevity is at least 5 years when stored at moderate temperatures and low humidity (USDA NRCS, 2004).

Plant Basics
Growth Rate Moderate
General Type Forb/herb
Growth Period Spring
Growth Duration Perennial
Lifespan Moderate
Plant Nativity Native to U.S.
Commercial Availability Routinely Available
Physical Characteristics
Bloom Period Late Spring
Displays Fall Colors No
Shape/Growth Form Rhizomatous
Drought Tolerance Medium
Shade Tolerance Intermediate
Height When Mature 3
Vegetative Spread Rapid
Flower Color White
Flower Conspicuousness Yes
Fruit/Seed Abundance Medium
Fruit/Seed Seasonality Summer Fall
Seed Spread Rate Moderate
Gardening Characteristics
Propagations (Ways to Grow) Seed, Sprigs
Moisture Requirements Medium
Cold Stratification Required No
Minimum Temperature -38
Soil Depth for Roots 8
Toxic to Nearby Plants No
Toxic to Livestock No
After-Harvest Regrowth Rate Moderate
After-Harvest Resprout Ability No
Responds to Coppicing No
Growth Requirements
pH Range 6–8 pH
Precipitation Range 8–8 inches/yr
Planting Density 4800–19000 indiv./acre
Soil Textures Coarse, Medium
Soil Depth for Roots 8
Minimum Frost-Free Days 100 day(s)
Salinity Tolerance Low
CaCO3 Tolerance Medium
Sustainability & Use
Leaf Retention No
Palatability Low
Fire Resistant No
Causes Livestock Bloating None

Source: USDA, NRCS, PLANTS Database, plants.usda.gov.
National Plant Data Center, Baton Rouge, LA 70874-4490 USA