CPC Plant Profile: Thompson's Clover
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Plant Profile

Thompson's Clover (Trifolium thompsonii)

Trifolium thompsonii is an integral part of this striking landscape. Photo Credit: Ed Guerrant
Description
  • Global Rank: G3 - Vulnerable
  • Legal Status: N/A
  • Family: Fabaceae
  • State: WA
  • Nature Serve ID: 148240
  • Date Inducted in National Collection: 04/04/1991

Trifolium thompsonii probably doesn't look like any other clover you've ever seen. It grows up to two feet tall and its large, spherical, reddish-lavender flower heads rise above the swaying grass like a sea of cheerleader pom-poms. Unlike most clovers, the leaves of this plant are comprised of 3 to 8 narrow, pointed leaflets (most clovers have three leaflets, hence the name "trifolium" or "three leaves"). It grows only within a fairly limited geographical distribution (only 20 miles north to south), but Trifolium thompsonii can be found in a wide variety of habitats. It grows in open ponderosa pine woods, grass and herb dominated areas, and sagebrush steppe. Steep slopes, along ridgelines, in alluvial fans and along canyon bottoms anywhere from 140 feet to 3760 feet (40m to 1130 m) in elevation are likely places to find this species.

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Updates
  • 10/01/2020
  • Seed Collection

Germination trials at The Berry Botanic Garden were inconclusive. Seeds were subjected to either 8 weeks of cold stratification or no cold stratification followed by either constant 68F (20C) or alternating 50 /68F (10/20C) temperatures. Without cold stratification, both temperature treatments showed 40% germination. With cold stratification, the constant 68F treatments showed 40% germination while the alternating 50/68F treatment showed 80% germination (BBG File).

  • 10/01/2020
  • Reproductive Research

Comprehensive study of the ecological life history of Trifolium thompsonii. Geology, soil characteristics, phenology and morphology, floral biology, seed biology, predation, effects of fire, interactions with cheat grass (Bromus tectorum), population dynamics and effects of cover on the taxon were all investigated (Canfield 1977).

  • 10/01/2020
  • Demographic Research

Comprehensive study of the ecological life history of Trifolium thompsonii. Geology, soil characteristics, phenology and morphology, floral biology, seed biology, predation, effects of fire, interactions with cheat grass (Bromus tectorum), population dynamics and effects of cover on the taxon were all investigated (Canfield 1977).

  • 10/01/2020
  • Propagation Research

Germination trials at The Berry Botanic Garden were inconclusive. Seeds were subjected to either 8 weeks of cold stratification or no cold stratification followed by either constant 68F (20C) or alternating 50 /68F (10/20C) temperatures. Without cold stratification, both temperature treatments showed 40% germination. With cold stratification, the constant 68F treatments showed 40% germination while the alternating 50/68F treatment showed 80% germination (BBG File).

Nature Serve Biotics
  • 05/02/2017

Endemic to Chelan and adjacent Douglas counties in Washington State. There are 14 occurrences known extant (although the number of reproductively isolated populations may be much smaller). Conversion to orchards, range degradation, and fire suppression leading to conifer invasion of the grasslands are all on-going threats. Invasions of noxious weeds, and subsequent efforts to control them, also pose threats.

Edward Guerrant, Ph.D.
  • 01/01/2010

Threats include (in WNHP 1999): The invasion of exotic species such as knapweed (Centaurea diffusa) and cheatgrass (Bromus tectorum). Agricultural conversion. Livestock grazing. Illegal ORV use in population areas.

Edward Guerrant, Ph.D.
  • 01/01/2010

Approximately 15 sites have been observed since 1986. Numbers range from as low as 30-40 to as high as several thousand per site. 11 of the known sites are on Forest Service Land (WNHP 2000).

Edward Guerrant, Ph.D.
  • 01/01/2010

Comprehensive study of the ecological life history of Trifolium thompsonii. Geology, soil characteristics, phenology and morphology, floral biology, seed biology, predation, effects of fire, interactions with cheat grass (Bromus tectorum), population dynamics and effects of cover on the taxon were all investigated (Canfield 1977). Germination trials at The Berry Botanic Garden were inconclusive. Seeds were subjected to either 8 weeks of cold stratification or no cold stratification followed by either constant 68F (20C) or alternating 50 /68F (10/20C) temperatures. Without cold stratification, both temperature treatments showed 40% germination. With cold stratification, the constant 68F treatments showed 40% germination while the alternating 50/68F treatment showed 80% germination (BBG File).

Edward Guerrant, Ph.D.
  • 01/01/2010

A proposal was made in 1975 to designate an area to help preserve this taxon. This resulted in the establishment of the Thompson Clover Natural Research Area in 1977 by the U.S. Forest Service.

Edward Guerrant, Ph.D.
  • 01/01/2010

Search suitable habitats in Douglas and Chelan County for populations (WNHP 1999). Study potential factors influencing the limited distribution of Trifolium thompsonii.

Edward Guerrant, Ph.D.
  • 01/01/2010

Collect and store seeds from across the known range. Collect from a variety of habitats and elevations. Determine optimum germination conditions. Test conditions corresponding to natural elevation and habitat gradients for this species. Determine propagation and reintroduction protocols.

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Photos
Nomenclature
Taxon Trifolium thompsonii
Authority Morton
Family Fabaceae
CPC Number 4332
ITIS 26323
USDA TRTH2
Common Names Thompson's clover
Associated Scientific Names Trifolium thompsonii
Distribution WA: Columbia Basin, Eastern Cascades. Chelan County and Douglas County, WA.
State Rank
State State Rank
Washington S3
Habitat

Trifolium thompsonii grows in a wide variety to habitats. The majority of populations grow on the fringe of the ponderosa pine (Pinus ponderosa) zone in open ponderosa pine woods or grass and herb dominated areas. It grows along ridgelines, on steep slopes, in alluvial fans, and in canyon bottoms anywhere from 140 feet to 2760 feet (40 m to 1130 m).

Ecological Relationships

Trifolium thompsonii has an unusual growth cycle. Vegetative buds develop in the autumn, when temperatures drop to freezing on a regular basis at night. The buds over-winter and then emerge in the end of March after all the snow melts. Once temperatures rise above freezing (usually in early April), the buds begin to sprout. Flowering begins in mid-May and continues for nearly two months. The first batch of seeds matures by the end of June (WNHP 1999).Trifolium thompsonii is found primarily in microsites that are intermediate between well exposed, dry, south-facing slopes and more shaded, mesic, and highly vegetated sites (WNHP 1999). Occasional fires may have played an important role in maintaining suitable habitat for Thompson's clover. It is not known why this species has such a varied habitat but such a limited distribution.Like many other members of the pea family, Trifolium thompsonii is able to form a symbiotic relationship with nitrogen-fixing bacteria. Thus, this plant has an advantage over competing vegetation in soils that are low in available nitrogen (WNHP 1999).The primary pollinators appear to be a butterfly (Plebejus saepiolus) and several species of bumblebee (Bombus sp.). Long distance pollen transfer may be fairly common between nearby populations. Seed predation can often be very high. In one study, over 14% of observed seeds had some kind of insect boring (Canfield 1977).

Pollinators
Common Name Name in Text Association Type Source InteractionID
Reintroduction
Lead Institution State Reintroduction Type Year of First Outplanting

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