• GENERAL BOTANICAL CHARACTERISTICS
• RAUNKIAER LIFE FORM
• REGENERATION PROCESSES
• SITE CHARACTERISTICS
• SUCCESSIONAL STATUS
• SEASONAL DEVELOPMENT

GENERAL BOTANICAL CHARACTERISTICS:

Western yarrow is a perennial forb 11 to 40 inches (30-100 cm) in height with extensive rhizomes. It has few to numerous erect stems. The basal rosette of leaves may remain green throughout the winter [43]. Plants grow in a somewhat scattered fashion and seldom form pure stands in areas larger than 5 square meters [69]. Typical European Achillea millefolium is hexaploid with flat leaves. Native forms are mostly tetraploid, with narrow leaf-segments disposed in various planes so that the leaf is 3-dimensional [33].

McLean [49] reported that in a Douglas-fir (Pseudotsuga menziesii) forest zone in British Columbia, the fibrous roots and rhizomes of yarrow grew mostly in the duff layer or between it and the mineral soil.

RAUNKIAER LIFE FORM:

Hemicryptophyte

REGENERATION PROCESSES:

Western yarrow regenerates from fragments of rhizomes and from colonization through short-distance (1-2 m) wind dispersal of seeds [15,47,61]. In disturbed soils, fragmented rhizomes regenerate shoots which can emerge from soil depths as great as 12 inches (30 cm). In undisturbed soil the rhizomes remain attached to the parent plant, forming new plants at the rhizome apices [15].

The fruit is a small achenes weighing about 0.17 mg. They are produced in large numbers. Several thousand achenes may be produced per flowering stem. The viability of freshly shed seeds exceeds 90%. Western yarrow seed showed 41% germination after 9 years in dry storage [15].

SITE CHARACTERISTICS:

Western yarrow usually occupies dry, open sites in a variety of habitats across its range including sagebrush (Artemisia spp.)-grassland, canyon bottoms, glades, roadsides, and vacant lots. It is prevalent in brushlands, quaking aspen (Populus tremuloides), open timber, and subalpine zones. It is intolerant of dense shade. It is common on thin soils and sandy gravelly loam on open flats, parks, and dry meadows [69]. The elevational distribution in several western states is as follows [19]:

Colorado: 4,000-12,000 feet (1220-3660 m)
Montana: 2,400-10,000 feet ( 730-3050 m)
Utah: 4,300-10,300 feet (1210-3040 m)
Wyoming: 4,600-11,000 feet (1400-3350 m)

SUCCESSIONAL STATUS:

Western yarrow is a pioneer species everywhere it is found [1]. It is an invader species on disturbed rangeland sites. Western yarrow also appears to be tolerant of competition but not tolerant of excessive shade. It is usually present in the earliest stages of vegetation development and persists throughout succession [42]. It dominates on overgrazed high summer ranges, where the undisturbed climax vegetation would be made up of wheatgrasses (Triticeae) [69].

SEASONAL DEVELOPMENT:

Western yarrow has a long flowering season throughout its range, 
which varies as follows [19]:

    State     Earliest     Most Frequent     Latest
                Month         Month           Month
---------------------------------------------------
     CO          May           May             Jun
     ID          Apr           May             Jun
     MT          May           May             Jun
     UT          Apr           May             Jun
     WY          May           Jun             Aug


Average dates of different growth stages at different elevations in Utah
were recorded as follows [22]:

Elev.   Flower buds  Flowers    Seeds    Seeds         Plant
(ft)    evident      in bloom   ripe     disseminated  dried
---------------------------------------------------------------
7,150   May 30       Jun 29     Sept 28  Sept 19       Oct 10
7,655   Jun 01       Jul 05     Aug 26   Sept 24       Oct 13
8,450   Jun 06       Jul 10     Sept 04  ---           Sept 25
9,000   Jun 18       Jul 15     Sept 08  Sept 29       Oct 01
10,100  Jun 25       Jul 21     Sept 20  Oct 08        Oct 08 


Average heights (cm) of plants at various dates and altitudes from
Ephraim Canyon in Utah were as follows (1925-1934) [22]:

Alt.(ft) May 1  May 15  Jun 1   Jun 15  Jul 1  Jul 15
-----------------------------------------------------
7,150    4.9    9.1     16.2    26.2    30.8   33.9
7,655    3.9    8.0     12.9    20.0    28.4   31.1
8,450    1.4    4.6      7.7    15.8    24.2   29.3
9,000    --     0.5      3.5     7.9    19.0   28.3
10,100   --     --       --      5.1    11.1   19.9


Over a 10-year period in Saskatchewan, Canada, flowering dates
were recorded for yarrow [18]:
                                                         Mean
------------First flowering date-----------  Latest date flwring
earliest date & yr  latest date & yr  mean   in flower   period
------------------  ----------------  -----  --------    ------
May 28/1946         Jun 30/1950       Jun 19 Sept 23     78 days

Related categories for SPECIES: Achillea millefolium | Western Yarrow