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 ⅛ 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).