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Thomas G. Barnes @ USDA-NRCS PLANTS Database |
Larry Allain @ USDA-NRCS PLANTS Database |
| Location | Rank |
| Kentucky | Special Concern |
| Michigan | Probably Extirpated |
| Ohio | Presumed Extirpated |
| Tennessee | Endangered |
| AL | AZ | AR | CO | IL | IN |
| IA | KS | KY | LA | MI | MN |
| MS | MO | MT | NE | NM | NY |
| ND | OH | OK | SD | TN | TX |
| WI | WY |
| AB | BC | MB | ON | SK |
Purple prairie clover also occurs in Nebraska sandhill prairie [7], cedar glade, limestone glade, dolomite glade [8], dry-mesic savanna, dry-mesic prairie, wet-mesic alluvial floodplain [19], quaking aspen (Populus tremuloides)-prairie ecotone [18], and mesic bur oak (Quercus macrocarpa), black oak (Quercus velutina), white oak (Quercus alba), and northern pin oak (Quercus ellipsoidalis) savanna communities [17]. In Illinois, it occurs in dolomite-hill prairie and "barren" communities [2,19].
In northern Arizona's Grand Canyon National Park, purple prairie clover is considered an "exotic species," and is found in riparian areas of the canyon [95].Purple prairie clover is a perennial forb, 8 to 35 inches (20-90 cm) tall, with a woody stem. The numerous leaves are 0.4-1.6 inches (1-4 cm) long, with 3 to 7 leaflets. The inflorescence is a 0.4- to 2.6-inch (1-7 cm) spike located at the ends of the branches. Branches are numerous, usually 3 per stem, but sometimes as many as 10 to 12. The mature purple prairie clover has a coarse, nonfibrous root system with a strong woody taproot that is 5.5 to 6.5 feet (1.7-2.0 m) deep. The taproot gives rise to several minutely branched lateral roots. The fruit is a 1- to-2-seeded pod enclosed in bracts [6,48,62,100,112].
RAUNKIAER [86] LIFE FORM:Breeding system: Purple prairie clover is cross pollinated [16]. Mating system is primarily xenogamous, but self-pollination also occurs. In a Wisconsin prairie study, 45% of hand-pollinated, outcrossed flowers produced large, viable seeds, and 19% of selfed flowers produced seeds. Native bees and honeybees were pollinators [27].
Pollination is insect-mediated [16,27].
Seed production is highest with favorable soil moisture and nutrient conditions. A survey of native plant horticulturists in Minnesota indicated that purple prairie clover frequently produces low seed yields [15]. Another Minnesota study compared the phenological development of purple prairie clover in cultivated fields to noncultivated managed prairie. Cultivated fields produced 3 times as much seed as noncultivated prairie. Seed and inflorescence production on cultivated and noncultivated native prairie were [16]:
| Item | Cultivated Fields | Noncultivated Prairie |
| Number of inflorescences initiated/plant | 35.0 | 29.1 |
| Weight/inflorescence (g) | 0.36 | 0.25 |
| Seed weight/inflorescence | 0.043 | 0.015 |
| Number of seeds/inflorescence | 33.5 | 11.5 |
| Seed weight/plant (g) | 0.49 | 0.22 |
| Number of seeds/plant | 379.8 | 173.4 |
Cultivated fields were devoid of any other competing plants, fertilized, and only contained evenly spaced transplanted purple prairie clover plants from native prairie lands. The noncultivated, native prairie had a variety of other forb and grass species. The noncultivated prairie was under a regimen of prescribed fire every 2 to 3 years. Season of burning was not described [16]. Stevens [96] found that a single purple prairie clover plant may produce 368 seeds per plant (many of which may not mature), with seeds weighing 1.5 g/1,000 seeds.
Seed dispersal: Neither fruits nor seeds have specialized means of dispersal; thus, most seed falls near the parent plant [110]. A seed dispersal study using purple prairie clover and other seed in cattle feed showed that following ingestion, cattle were inefficient vectors for dispersing viable purple prairie clover seed [37].
Seed banking: Purple prairie clover has soil-stored seed [110], but further studies are needed on the relative importance of seed banking to purple prairie clover regeneration. A study on native Kansas prairie found low numbers of buried viable purple prairie clover seed [1].
Germination: Purple prairie clover germinates at soil temperatures ranging from 59 to 86 °F (15-30 °C) [9] while temperatures as low as 41 °F (5 °C) have broken dormancy [14]. A survey of native plant horticulture in Minnesota indicated low rates of germination of purple prairie clover [16]. Germination of purple prairie clover is enhanced by scarification, disturbing the litter and duff layers to expose soil, and stratification [14,100].
Seedling establishment/growth: Bjugstand and Whitman [14] used several varieties of forbs for reclamation of strip-mined land and found that purple prairie clover showed "excellent" germination and subsequent "vigorous" growth in the greenhouse. Purple prairie clover transplanted to reclamation areas continued to show excellent vigor and growth [14].
Asexual regeneration: The ability of purple prairie clover to regenerate vegetatively is unclear. Meier and Weaver [72] state that purple prairie clover does not reproduce asexually. However, Towne and Knapp [104] suggest that purple prairie clover sprouts from the root crown following top-kill by fire. Further research is needed on the ability of purple prairie clover to regenerate asexually.
SITE CHARACTERISTICS:Soils: Purple prairie clover can be found in most soil types throughout the Great Plains [31]. In the northern Great Plains purple prairie clover is found in sandy to silty loams [17,19]. Some specific soil characteristics have been identified with purple prairie clover in the Nebraska Sandhill prairie region. Here purple prairie clover occurs most frequently in sandy soils that contain medium to coarse grains. It is thought that the coarse sands intercept precipitation with minimal runoff, allowing most of the moisture to reach far below the surface. Due to its taproot morphology, purple prairie clover is able to access moisture from deep in the soil profile and thus is able to persist in areas where other shallow-rooted species cannot [7]. Soils in Minnesota where purple prairie clover is present were found to have pH values of 6.4 to 6.7 [16], with soil depths ranging from 6 to 24 inches (15-6.1 cm) [22].
Elevation: In eastern Colorado purple prairie clover occurs at elevations from 3,500 to 7,500 feet (1,067-2,286 m) [53].
SUCCESSIONAL STATUS:On the mixed-grass prairies of the southern Great Plains, purple prairie clover is part of a group of forbs found in late successional seres. A common pattern of succession in disturbed prairie regions begins with the dominance of native prairie annuals, nonnative annual weeds, and ragweeds (Ambrosia spp.), which may persist for 1 to 3 years. Soon following, a collection of nonnative and native grasses and perennial forbs, including purple prairie clover, create a mosaic of species that may take 15 to 40 years to develop, depending on environmental conditions and "competitive" factors. Common species that coexist with purple prairie clover in the tallgrass prairie during the later stages of succession include lead plant and prairie dropseed [91].
In its eastern range in forest openings where fires and other natural disturbances are suppressed, purple prairie clover can be shaded out by encroaching woody species [55]. Purple prairie clover is thought to be an indicator of prairie in its later successional sere and may be an indicator of pristine prairie ecosystems [91].
SEASONAL DEVELOPMENT:|
Earliest 1st bloom |
Latest 1st bloom |
Median date of full flowering |
Median date when 95% of flowering complete |
Flowering period (days) |
| June 17th | July 13th | July 15th | August 15th | 35 |
Fire regimes: Historically fire has been an important natural component of grassland communities where purple prairie clover occurs [35]. Frequent, stand-replacement surface fires in plains grasslands and prairies affect species composition and vegetation dynamics [79]. Across the Great Plains, lightning-caused and human-caused fires may have occurred as frequently as every 1 to 10 years for thousands of years prior to European settlement [67,117]. The implications of the cessation of historical fire regimes in the last century on purple prairie clover are unknown. Purple prairie clover has responded favorably to burning in several prescribed fire studies [12,13,39,64] using various annual intervals and seasons (see Plant Response to Fire).
In some habitats fire is necessary to maintain purple prairie clover. For example, along woodland-grassland ecotones in purple prairie clover's eastern range, the cessation of fire has caused encroachment of woody species that shade out purple prairie clover and reduce its abundance [45].
The following list provides fire return intervals for plant communities and ecosystems where purple prairie clover is important. It may not be inclusive. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes".
| Community or Ecosystem | Dominant Species | Fire Return Interval Range (years) |
| sugar maple-basswood | Acer saccharum-Tilia americana | > 1,000 [109] |
| bluestem prairie | Andropogon gerardii var. gerardii-Schizachyrium scoparium | < 10 [67,79] |
| Nebraska sandhills prairie | A. gerardii var. paucipilus-S. scoparium | < 10 |
| bluestem-Sacahuista prairie | A. littoralis-Spartina spartinae | < 10 [79] |
| silver sagebrush steppe | Artemisia cana | 5-45 [56,85,117] |
| sagebrush steppe | A. tridentata/Pseudoroegneria spicata | 20-70 [79] |
| basin big sagebrush | A. tridentata var. tridentata | 12-43 [89] |
| mountain big sagebrush | A. tridentata var. vaseyana | 15-40 [5,23,75] |
| Wyoming big sagebrush | A. tridentata var. wyomingensis | 10-70 (40**) [107,118] |
| plains grasslands | Bouteloua spp. | < 35 [79,117] |
| blue grama-needle-and-thread grass-western wheatgrass | B. gracilis-Hesperostipa comata-Pascopyrum smithii | < 35 [79,88,117] |
| blue grama-buffalo grass | B. gracilis-Buchloe dactyloides | < 35 [79,117] |
| blue grama-tobosa prairie | Bouteloua gracilis-Pleuraphis mutica | < 35 to < 100 [79] |
| cheatgrass | Bromus tectorum | < 10 [81,114] |
| sugarberry-America elm-green ash | Celtis laevigata-Ulmus americana-Fraxinus pennsylvanica | < 35 to 200 [109] |
| northern cordgrass prairie | Distichlis spicata-Spartina spp. | 1-3 [79] |
| black ash | Fraxinus nigra | < 35 to 200 [109] |
| Ashe juniper | Juniperus ashei | < 35 [79] |
| cedar glades | J. virginiana | 3-22 [52,79] |
| yellow-poplar | Liriodendron tulipifera | < 35 [109] |
| wheatgrass plains grasslands | Pascopyrum smithii | < 5-47+ [79,85,117] |
| pine-cypress forest | Pinus-Cupressus spp. | < 35 to 200 [4] |
| sycamore-sweetgum-American elm | Platanus occidentalis-Liquidambar styraciflua-Ulmus americana | < 35 to 200 [109] |
| eastern cottonwood | Populus deltoides | < 35 to 200 [79] |
| aspen-birch | P. tremuloides-Betula papyrifera | 35-200 [38,109] |
| quaking aspen (west of the Great Plains) | P. tremuloides | 7-120 [4,50,73] |
| mesquite | Prosopis glandulosa | < 35 to < 100 [71,79] |
| Texas savanna | P. glandulosa var. glandulosa | < 10 [79] |
| black cherry-sugar maple | Prunus serotina-Acer saccharum | > 1,000 [109] |
| mountain grasslands | Pseudoroegneria spicata | 3-40 (10**) [3,4] |
| bur oak | Quercus macrocarpa | < 10 [109] |
| oak savanna | Q. macrocarpa/Andropogon gerardii-S. scoparium | 2-14 [79,109] |
| shinnery | Q. mohriana | < 35 [79] |
| Fayette prairie | S. scoparium-Buchloe dactyloides | < 10 [109] |
| little bluestem-grama prairie | S. scoparium-Bouteloua spp. | < 35 [79] |
| elm-ash-cottonwood | Ulmus-Fraxinus-Populus spp. | < 35 to 200 [38,109] |
Towne and Knapp [104] noted that purple prairie clover that was top-killed by fire showed great capacity to sprout after fire. There is no specific information available on how quickly purple prairie clover recovers after burning. Further research is needed on this topic.
Productivity: Several studies have focused on how the frequency of burning relates to productivity of purple prairie clover. Generally, annual burning favors annual grasses and reduces the abundance of perennial forbs including purple prairie clover [66]. Studies conducted in Minnesota in 1984 [101] and Missouri in 1964 [66] suggest that, compared to annual burning, biennial burning increases frequency and basal areas of legume species [66] including purple prairie clover [101]. In Wisconsin prairie restoration projects where purple prairie clover has prospered, managers recommend a 5-year burning interval [21].
Burning can enhance flower productivity in several prairie forbs including purple prairie clover. Purple prairie clover produced a greater abundance of inflorescences after a single spring burn on a Minnesota prairie than prior to burning [80]. The effects of this burn are attributed to the removal of litter and standing dead stems by the fire. Removal of litter allows for increased light intensities near the soil and thus higher soil temperatures, which enhance plant productivity. Litter reduces the presence and productivity of many forbs including purple prairie clover [39]. For more on the effects of litter on purple prairie clover, see Management Considerations.
The Great Plains region where purple prairie clover commonly occurs is typical of prairie and savanna ecosystems that require fire to maintain historical ranges of species composition and species richness [30]. Most of the available information has been based on short-term research [58]. Long-term effects (beyond the scope of current research; >20 years) of various fire regimes are not well known.
Season of burning: Interactions between season of burn and purple prairie clover phenology are not well known [60]. While spring burning generally decreases the immediate abundance of forb species that are actively growing [2], legume species including purple prairie clover in Kansas have shown increased growth and vigor 3 years following spring burns, nearly doubling stem biomass on upland sites and quadrupling stem biomass on lowland sites [104].
While most studies find that forb production is compromised after late spring burning, Bidwell and others [12,13] found that late spring backburning increased the productivity of purple prairie clover and other forb species (see Fire Management Considerations). Testing seasonal differences in annual prescribed annual burning on a Kansas prairie for 8 years, Towne and Kemp [103] found that legume species including purple prairie clover increased in cover in response to burning at any season. Greatest increases occurred 6 years after fire treatments, on autumn and winter prescribed burn plots. Others have found that most forbs including purple prairie clover decrease in abundance after being top-killed by late spring burning, while purple prairie clover increases after autumn and early spring burning [46,87].
The effects of mid-summer burning are not available in current literature (2005). For the purposes of restoration ecology, dormant-season fires probably do not resemble historical disturbance regimes found before European settlement. It is suspected that varied burn seasons, and intervals brought by natural ignitions from lightning prior to the European settlement era, produced greater levels of biodiversity and species assemblages than any single management method for native prairie lands [60]. Unfortunately, information on purple prairie clover frequency and abundance prior to European settlement is not available.
DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:Palatability/nutritional value:
In North Dakota, crude protein levels of purple prairie clover ranged from 12% in June to 8%
in August [84]. Due to its high palatability and high concentrations of nutritional protein,
purple prairie clover is generally considered one of the most important legumes
in native grasslands on the Great Plains, although some rare instances of bloat have
been reported in livestock [62,100]. Crude and digestible protein content
of purple prairie clover are as follows [76]:
| Crude Protein | 14.1% |
| Digestible Protein | |
| cattle | 9.9 % |
| domestic goats | 9.7 % |
| domestic rabbits | 9.6 % |
| domestic sheep | 10.1 % |
| horses | 9.5 % |
Cover value: No information is available on this topic.
VALUE FOR REHABILITATION OF DISTURBED SITES:Propagation: Purple prairie clover germination is enhanced in scarified soils [100]. Purple prairie clover in its natural habitat is often found in disturbed locations such as black-tailed prairie dog towns [98] and on dug mounds created by American badgers [83]. Stratification [100] and inoculation with rhizobium [65] have increased germination success of purple prairie clover in the laboratory. Purple prairie clover has been successfully used in several roadside vegetation projects throughout the Great Plains [28,33]. In a strip-mine reclamation project, purple prairie clover demonstrated excellent success as a colonizer, exhibiting high rates of germination and subsequent vigorous seedling growth in the greenhouse and afterwards during transplanting [14].
Purple prairie clover is highly dependent upon mycorrhizal fungi. A mycorrhizal inoculation study found prairie species uptake and transport of soil nutrients such as phosphorus and zinc was enhanced by mycorrhizae, but the study did not show any substantial effects on purple prairie clover seedling emergence [54]. Conversely, a study that used a benomyl (a fungicide specifically for the removal of mycorrhizae in soils) considerably lowered survivorship of purple prairie clover [116].
Purple prairie clover is susceptible to interference from with exotic species during establishment due to its relatively slow rate of seedling growth compared to that of nonnative invasive species. In North Dakota some populations of purple prairie clover have been completely eliminated by infestations of leafy spurge (Euphorbia esula) [25]. Reducing weed interference using herbicide applications (imazethapyr + imazapic) has been successful in improving establishment of purple prairie clover in Nebraska [10].
OTHER USES:In prairies near forest lands, encroachment of forest species into grasslands changes vegetation structure and composition. In eastern Nebraska, eastern redcedar encroachment into prairies has been linked to the decline of many prairie species, including purple prairie clover, due to shading [45].
The accumulation of litter on prairies affects purple prairie clover populations. In Kansas, purple prairie clover decreased during a 50-year study on tallgrass prairie that has seen a shift from summer haying to spring biannual burning. The author speculates that these decreases resulted from the cessation of mid-summer mowing. In this study mowing was thought to be responsible for the removal of biomass during the summer months, altering microhabitat conditions that had supported purple prairie clover. The increase of mulch thickness may explain decreases in purple prairie clover and other native forbs [36]. In a study of the effects of cessation of mowing and introduction of prescribed fire, forbs including purple prairie clover increased in abundance due to the reduction of mulch [29]. For more on the effects of litter on purple prairie clover, see Fire Effects
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