The following description provides general characteristics that may be relevant to fire ecology, and is not meant for identification. Keys for identification are available (e.g. [6,43,48,49,100,108,112]).
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].
Fire adaptations: Purple prairie clover establishes from soil-stored seed after fire. While not specifically documented, it is implied that purple prairie clover root crowns may survive burning that consumes the aerial portions of the plant, allowing postfire sprouting from the root crowns [104]. The large woody taproot allows for photosynthate and nutrient storage that can support postfire root crown sprouting. Additionally, fire creates favorable conditions (disturbed soil, decreased levels of mulch, reduced interference from forbs) that are favorable for purple prairie clover seedling establishment and growth [12,13,39,64].
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 67,79] Nebraska sandhills prairie A. gerardii var. paucipilus-S. scoparium < 10 bluestem-Sacahuista prairie A. littoralis-Spartina spartinae 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. 79,117] blue grama-needle-and-thread grass-western wheatgrass B. gracilis-Hesperostipa comata-Pascopyrum smithii 79,88,117] blue grama-buffalo grass B. gracilis-Buchloe dactyloides 79,117] blue grama-tobosa prairie Bouteloua gracilis-Pleuraphis mutica 79] cheatgrass Bromus tectorum 81,114] sugarberry-America elm-green ash Celtis laevigata-Ulmus americana-Fraxinus pennsylvanica 109] northern cordgrass prairie Distichlis spicata-Spartina spp. 1-3 [79] black ash Fraxinus nigra 109] Ashe juniper Juniperus ashei 79] cedar glades J. virginiana 3-22 [52,79] yellow-poplar Liriodendron tulipifera 109] wheatgrass plains grasslands Pascopyrum smithii 79,85,117] pine-cypress forest Pinus-Cupressus spp. 4] sycamore-sweetgum-American elm Platanus occidentalis-Liquidambar styraciflua-Ulmus americana 109] eastern cottonwood Populus deltoides 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 71,79] Texas savanna P. glandulosa var. glandulosa 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 109] oak savanna Q. macrocarpa/Andropogon gerardii-S. scoparium 2-14 [79,109] shinnery Q. mohriana 79] Fayette prairie S. scoparium-Buchloe dactyloides 109] little bluestem-grama prairie S. scoparium-Bouteloua spp. 79] elm-ash-cottonwood Ulmus-Fraxinus-Populus spp. 38,109] **meanPurple prairie clover grows on a variety of sites throughout the Great Plains including dry plains, prairies, hillsides, open woodlands, shaded ravines, sandhills, and roadsides. It occurs on mesic and xeric sites in mixed-grass and tallgrass prairies of the northern and central Great Plains and the shortgrass prairie of the southern Great Plains [24,49,80]. It is most common on marginal sites where soil is exposed and grasses have not formed dense stands [82]. Mean annual precipitation for regions where purple prairie clover subsists ranges from 11 inches (280 mm) in southeast Alberta [32] to 32 inches (810 mm) in Kansas [105] and Oklahoma [13] to 53 inches (1,350 mm) in Mississippi [115].
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].
Purple prairie clover produces excellent forage for livestock and wildlife. When abundant on pasture lands it may be an important component in hay [62,100]. Purple prairie clover is recommended for use in restoration seed mixtures. It produces forage with high yields, extended grazing periods, and increased nutritional values [84,90]. Pronghorn graze purple prairie clover on summer ranges of Montana [113]. A 2-year study in Minnesota found that white-tailed deer did not browse purple prairie clover [40], although this does not imply that deer and other ungulates never graze this species.
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]:
Cover value: No information is available on this topic.
Purple prairie clover is an important component of Great Plains grassland
communities. It is considered "common" in most
grassland habitat types of the midwestern United States and southern Canada. Graminoids dominate these regions, comprising 80%
to 90% to the total plant population. Purple prairie clover and other forbs generally make up 10% or less
of total plant population in the Midwest [119]. Purple prairie clover is found in tallgrass, shortgrass, or mixed-grass prairies.
Common associates in tallgrass prairies include little bluestem (Schizachyrium scoparium), big bluestem
(Andropogon gerardii), prairie Junegrass (Koeleria macrantha),
prairie dropseed (Sporobolus heterolepis), lead plant (Amorpha canescens),
and silky aster (Aster sericeus). Associates in mid-grass
prairie include silver bluestem (Bothriochloa saccharoides),
purple threeawn (Aristida purpurea), sideoats grama (Bouteloua curtipendula), and sand dropseed
(Sporobolus cryptandrus). Grass associates in shortgrass prairie include blue grama (Bouteloua gracilis),
hairy grama (B. hirsuta), and buffalo grass (Buchloe
dactyloides). Forb associates include wavyleaf thistle (Cirsium undulatum),
gayfeather (Liatris punctata), and scarlet globemallow (Sphaeralcea
coccinea). Forbs may be interspersed with several shrubs including American hazelnut (Corylus
americana), smooth sumac (Rhus glabra), creeping juniper (Juniperus
horizontalis) and/or trees including green ash (Fraxinus pennsylvanica),
eastern cottonwood (Populus deltoides), eastern redcedar (Juniperus
virginiana), white oak (Quercus alba), bur oak (Q. macrocarpa),
and shagbark hickory (Carya ovata). Woody associates are especially
common near riparian areas and on grassland/forest ecotones in the northern and
eastern fringes of purple prairie clover's distribution [7,8,17,18,19,32,62].
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].
Low- to moderate grazing pressure may enhance purple prairie clover production
by removing vegetative cover [18], but overgrazing can decrease coverage and frequency of purple prairie
clover [49]. In Illinois, populations of purple prairie clover have recovered in areas where
they had been removed
under high grazing pressure [19]. In Iowa, purple
prairie clover
was 1 species in a large group of native forbs that decreased or disappeared
under unspecified grazing pressure [111].
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
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 35Plant growth: Purple prairie clover may recover from fire by establishing from soil-stored seed and sprouting from the root crown. Purple prairie clover has responded favorably to prescribed fire in various studies [12,13,39,64]. Due to the hard seedcoat of legumes like purple prairie clover, these species' germination rates maybe enhanced by burning [70].
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.
Purple prairie clover reproduces by seed [16].
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 379.8 173.4Cultivated 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.
Generally purple prairie clover is considered a mid- to late successional species [102]. Purple prairie clover can also fill a pioneer role, as seen in roadsides and disturbed locations [91]. The following is a general description of the successional pathways on prairie lands. Many details of succession in these associations remain unknown.
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].
Purple prairie clover is commonly found in seed mixtures recommended for revegetation, reclamation [34,93], and native prairie restoration projects [65]. Gustafson and others [51] found that by using several local seed sources of purple prairie clover for restoration projects, local gene pools were maintained and regional genetic diversity was enhanced, promoting persistence and vigor in restored purple prairie clover populations. Purple prairie clover is frequently used in seed mixes for erosion control due to its ability to establish on disturbed sites and its capability to condition soil with nitrogen [92]. Legumes such as purple prairie clover fix atmospheric nitrogen in the soil. They may have an advantage over forbs and some grasses in degraded prairie or pasture sites [24,62].
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].
Dalea purpurea is a species of flowering plant in the legume family known as purple prairie clover.[3] Native to central North America, purple prairie clover is a relatively common member of the Great Plains and prairie ecosystems. It blooms in the summer with dense spikes of bright purple flowers that attract many species of insects.
It is native to central North America, where it occurs from central Canada to the southeastern and southwestern United States, except for the east and west coasts.[4][5][1] It is a common and widespread plant within its range, especially on the Great Plains.[6] In 1804, Meriwether Lewis collected a specimen in Nebraska.[7]
Dalea purpurea is a perennial herb growing 20 to 90 cm (8 to 35 in) tall. The mature plant has a large taproot 5.5 to 6.5 feet (1.7 to 2.0 m) deep. The stem is woody with several branches. The leaves are a few centimeters long and are divided into 3 to 7 narrow leaflets. The inflorescence atop each stem branch is a spike up to 7 cm (2+3⁄4 in) long containing many purple flowers. The fruit is a legume pod containing 1 or 2 seeds.[6] The Latin specific epithet purpurea means purple.[8]
This plant is adapted to a habitat with periodic wildfires. In some areas, it depends on fire to clear encroaching woody vegetation, as it cannot tolerate shade.[6]
Dalea purpurea is a common member of the flora on the plains of central North America, occurring in a variety of habitat types, including several types of grassland. It occurs in glades, riverbanks and floodplains, oak woodlands, pinyon-juniper woodlands, shrubsteppe, many types of forests, and the Sand Hills of Nebraska. It occurs in a variety of prairie ecosystems. On tallgrass prairie it is associated with plants such as little bluestem (Schizachyrium scoparium), big bluestem (Andropogon gerardi), prairie Junegrass (Koeleria macrantha), prairie dropseed (Sporobolus heterolepis), lead plant (Amorpha canescens), and silky aster (Symphyotrichum sericeum). On midgrass prairie it grows alongside several grasses such as silver bluestem (Bothriochloa saccharoides), purple threeawn (Aristida purpurea), sideoats grama (Bouteloua curtipendula), and sand dropseed (Sporobolus cryptandrus). On shortgrass prairie it is associated with grasses such as blue grama (Bouteloua gracilis), hairy grama (B. hirsuta), and buffalo grass (B. dactyloides). This species may be considered an indicator of pristine prairie.[6]
The nectar and pollen of Dalea purpurea attract many bees, wasps, flies, butterflies, and skippers. Several plasterer bees (genus Colletes) are specialist pollinators of Dalea species, and other insects eat the seeds and leaves.[9] It is a larval host to the southern dogface (Zerene cesonia).[10]
This species is used for revegetation efforts on reclaimed land, such as land that has been strip mined. It is good for preventing erosion and for fixing nitrogen in soil. Though it is often found in mid- to late-successional stages of ecological succession, it may also be a pioneer species, taking hold in bare and disturbed habitat, such as roadsides.[6]
Purple prairie clover provides food for a number of animals, such as pronghorn. It also grows in cultivated fields and becomes included in hay for livestock. It is nutritious and is "considered one of the most important legumes in native grasslands on the Great Plains." It also had a number of uses for Native Americans. The leaves are edible and good for making tea and medicines, and the roots are palatable when chewed. The stems were used as brooms by the Pawnee people.[6]
Dalea purpurea has been found to contain several active constituents, including pawhuskin A, pawhuskin B, pawhuskin C, and petalostemumol.[11][12][13] The pawhuskins possess affinity for the opioid receptors, and pawhuskin A, by far the most potent of the group, acts as a non-selective antagonist of all three opioid receptors, with preference for the κ- and μ-opioid receptors over the δ-opioid receptor.[11][13]
Dalea purpurea is a species of flowering plant in the legume family known as purple prairie clover. Native to central North America, purple prairie clover is a relatively common member of the Great Plains and prairie ecosystems. It blooms in the summer with dense spikes of bright purple flowers that attract many species of insects.
