Sonchus arvensis

• INTRODUCTORY
• DISTRIBUTION AND OCCURRENCE
• BOTANICAL AND ECOLOGICAL CHARACTERISTICS
• FIRE ECOLOGY
• FIRE EFFECTS
• MANAGEMENT CONSIDERATIONS
• REFERENCES

INTRODUCTORY

AUTHORSHIP AND CITATION:
McWilliams, Jack 2004. Sonchus arvensis. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [].

FEIS ABBREVIATION:
SONARV:

SYNONYMS:
For Sonchus arvensis spp. arvensis:
    Sonchus arvensis var. arvensis [26,42,64,125].

For Sonchus arvensis spp. uliginosus:
    Sonchus uliginosus [34,58,71,127]
    Sonchus arvensis var. glabrescens [26,42,64,114,125].

NRCS PLANT CODE [122]:
SOAR2

COMMON NAMES: 
perennial sowthistle
perennial sow thistle
field sowthistle

TAXONOMY:
The currently accepted scientific name for perennial sowthistle is Sonchus arvensis L. (Asteraceae) [26,34,42,45,57,58,62,64,71,72,81,114,125,127]. There are 2 recognized subspecies:

S. arvensis ssp. arvensis
S. arvensis ssp. uliginosus (Bieb.) Nyman [45,72]

In this summary, perennial sowthistle will be used when discussing Sonchus arvensis, and the subspecies will be referred to by their scientific names when information pertaining to them individually is available.

Naturally occurring hybrids produced by the 2 subspecies have been detected in areas where both subspecies occur [74].

LIFE FORM:
Forb

FEDERAL LEGAL STATUS:
No special status

OTHER STATUS:
As of this writing (2004), perennial sowthistle is listed as a noxious weed in 13 states. More information is available through the Plants database.

DISTRIBUTION AND OCCURRENCE

• GENERAL DISTRIBUTION
• ECOSYSTEMS
• STATES/PROVINCES
• BLM PHYSIOGRAPHIC REGIONS
• KUCHLER PLANT ASSOCIATIONS
• SAF COVER TYPES
• SRM (RANGELAND) COVER TYPES
• HABITAT TYPES AND PLANT COMMUNITIES

Perennial sowthistle is reported throughout most of the United States, with the exception of Hawaii, Arizona, Oklahoma, Arkansas, Alabama, Georgia, South Carolina, and Florida. It occurs throughout Canada. Sonchus arvensis spp. arvensis has the same distribution as perennial sowthistle, but it is not recorded in Nebraska, Kansas, Virginia, West Virginia, North Carolina, or Alaska. Sonchus arvensis spp. uliginosus occurs across the northern portion of North America, from Alaska south to Oregon and Utah, and east to Virginia and North Carolina; but it is not reported in New Hampshire, Kentucky, British Columbia or the far northern territories of Canada [65].

No specific mention of perennial sowthistle in Mexico occurs in the literature. Since it occurs in Texas and New Mexico, it is reasonable to assume it may also occur in northern Mexico.

Plants database provides a state distribution map of perennial sowthistle and its infrataxa.

The following lists include North American ecosystems, habitat types, and forest and range cover types in which perennial sowthistle may occur. Perennial sowthistle grows well in wet and even saturated soils. Consequently, perennial sowthistle may occur in riparian areas or wetlands within these habitats. Additionally, perennial sowthistle often occurs in cultivated areas, especially small grain and row crops, so it may occur in cultivated areas within these communities, with the potential to spread into adjacent, undisturbed areas.

These lists are not necessarily inclusive or exhaustive. More information is needed to determine particular ecosystems and plant communities where perennial sowthistle is likely to occur in natural areas.

ECOSYSTEMS [41]:
FRES10 White-red-jack pine
FRES11 Spruce-fir
FRES12 Longleaf-slash pine
FRES13 Loblolly-shortleaf pine
FRES14 Oak-pine
FRES15 Oak-hickory
FRES16 Oak-gum-cypress
FRES17 Elm-ash-cottonwood
FRES18 Maple-beech-birch
FRES19 Aspen-birch
FRES20 Douglas-fir
FRES21 Ponderosa pine
FRES22 Western white pine
FRES23 Fir-spruce
FRES24 Hemlock-Sitka spruce
FRES25 Larch
FRES26 Lodgepole pine
FRES27 Redwood
FRES28 Western hardwoods
FRES29 Sagebrush
FRES30 Desert shrub
FRES31 Shinnery
FRES32 Texas savanna
FRES33 Southwestern shrubsteppe
FRES34 Chaparral-mountain shrub
FRES35 Pinyon-juniper
FRES36 Mountain grasslands
FRES37 Mountain meadows
FRES38 Plains grasslands
FRES39 Prairie
FRES40 Desert grasslands
FRES41 Wet grasslands
FRES42 Annual grasslands
FRES44 Alpine

STATES/PROVINCES: (key to state/province abbreviations)
UNITED STATES

Perennial sowthistle is found on disturbed sites in saline habitats in Saskatchewan, Manitoba, and Alberta in association with rayless alkali aster (Symphyotrichum ciliatum), spear saltbush (Atriplex patula), curlycup gumweed (Grindelia squarrosa), summer-cypress (Kochia scoparia), Nuttall's alkaligrass (Puccinellia nuttalliana), red swampfire (Salicornia rubra), and Pursh seepweed (Suaeda calceoliformis) [16].

Major species associated with S. a. ssp. uliginosus in halophytic or semihalophytic communities in Saskatchewan near saline depressions include western yarrow (Achillea millefolium), rosy pussytoes (Antennaria microphylla), manyflowered aster (Symphyotrichum ericoides var. pansum), saltgrass (Distichlis spicata), wild licorice (Glycyrrhiza lepidota), foxtail barley (Hordeum jubatum), mat muhly (Muhlenbergia richardsonis), and gray goldenrod (Solidago nemoralis) [33].

Redmann [98] described plant communities along a soil salinity-moisture gradient of an eastern North Dakota prairie. Perennial sowthistle was present in every plant community except the muhly (Muhlenbergia spp.) and bluestem (Andropogon spp.) types. In a prairie cordgrass (Spartina pectinata) community, perennial sowthistle commonly occurs with foxtail barley, slender wheatgrass (Elymus trachycaulus), scratchgrass (M. asperifolia), mat muhly, bluejoint reedgrass (Calamagrostis canadensis), northern bog aster (Symphyotrichum boreale), and marsh hedgenettle (Stachys palustris). In a bluegrass (Poa spp.) community type, perennial sowthistle occurs at lower elevations with foxtail barley, scratchgrass, wild licorice, and Maximilian sunflower (Helianthus maximiliani). Sonchus arvensis ssp. uliginosus is found in a "salt flat" area, or saltgrass community type, with saltgrass, serpentine aster (Symphyotrichum ericoides), curlycup gumweed, alkali cordgrass (Spartina gracilis), foxtail barley, slender wheatgrass, scratchgrass, and plains bluegrass (Poa arida). It is also found in the foxtail barley community type where it occurs with plains bluegrass, scratchgrass, curlycup gumweed, serpentine aster, curly dock (Rumex crispus), prairie wedgescale (Sphenopholis obtusata), and Cuman ragweed (Ambrosia psilostachya) [98].

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

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

Perennial sowthistle is a perennial herb [53,74] that reproduces by seeds, by vertical, thickened roots, and by cylindrical, horizontal, spreading roots [110]. Vertical roots can penetrate 5 to10 feet (1.5-3  m) deep. Horizontal roots, frequently 2.5 to 5 mm in diameter (rarely exceeding 10 mm), are found 2 to 4 inches (5-10 cm) below the soil surface [10]. These horizontal roots can reach 3 to 6 feet (0.9-1.8 m) in length in a single growing season [110]. Fruits are achenes [15,90] with a pappus that generally stays attached to the achene [92].

Stems are erect, 0.1 to 0.4 inches (3-10 mm) in diameter, and most commonly 24 to 59 inches (60-150 cm) tall; although they range from 12 to 71 inches (30-180 cm) tall. Stems are hollow and branched, varying from 2 to many per plant. Leaves are crowded on the lower stems and sparse on the upper stems. The entire plant is filled with milky latex [74].

RAUNKIAER [97] LIFE FORM:
Hemicryptophyte

REGENERATION PROCESSES:
Perennial sowthistle can reproduce by seed and vegetatively [10,30,110].

Breeding system: Perennial sowthistle flowers are perfect [31] and generally self-incompatible [31,110].

Pollination: Perennial sowthistle is pollinated by insects including honeybees and other bees, hover flies, and blister beetles [31,110].

Seed production: Perennial sowthistle can produce large numbers of seeds [31,53,110,112]. Seeds produced by self pollination are generally nonviable and smaller than those produced by cross-pollination [31,110].

Heads contain many fertile flowers but the number of achenes produced varies widely among heads, plants, and locality. Variability likely results from several factors, including environmental conditions and availability of pollinators [110].

Perennial sowthistle can typically produce an average of 30 achenes per head and up to 50,000/yd² [110]. In North Dakota, 1 main stalk, with "relatively little competition", produced 62 heads and 9,750 well-developed achenes. The author collected seeds from the plant for a 30-day period [112]. In South Dakota, artificially cross-pollinated heads from greenhouse- and field-grown plants produced about 50 achenes per head, but number of achenes per head in natural populations varied from about 20 to 40 or from 60 to 80, depending upon the year [31].

Seed dispersal: Seeds of perennial sowthistle are mostly wind dispersed [28,53,110], but other dispersal agents may play a minor role. The pappus, attached to the seed, aids in wind dispersal [92]. Hume and Archibold [63] placed seed traps at varying distances from a "weedy" field in Saskatchewan. Results show wind-blown seeds of perennial sowthistle can disperse at least 110 yards (100 m). They do not report wind speed.

Sheldon and Burrows [105] conducted experiments to determine maximum dispersal distance of perennial sowthistle seeds at differing wind speeds. They used perennial sowthistle plants with a mean height of 3 feet (90 cm). They observed a maximum dispersal distance of 11 yards (10 m).

In addition to wind dispersal, seeds of perennial sowthistle may be dispersed by birds and other animals. Martin and others (as reported in [133], a literature review) state perennial sowthistle is a minor element in the diet of some North American birds, and some seeds may germinate after ingestion and excretion by birds and animals. Hooked cells at the tips of pappus hairs allow the pappus to cling to clothes and animal hairs and aid in seed dispersal [110,133].

Seed banking: While viable perennial sowthistle seeds have been found in the seed banks of marshes and wetlands [60,88], longevity of seeds in the soil seed bank under field conditions of these communities is unknown. A study of perennial sowthistle seed dormancy suggests that some seed may remain viable for 3 or more years in cultivated soils [23].

Seed banking studies in the Delta Marsh, Manitoba, suggest that viable perennial sowthistle seeds occur in marsh habitats. Sowthistles (Sonchus spp.) were dominant in the drier upland areas, so seed was likely dispersed throughout the marsh. Perennial sowthistle seedlings emerged from soil samples taken from the marsh and exposed to "drawdown" conditions (soil surface kept moist), but not from samples exposed to "submersed" conditions (continuously flooded to a depth of 2 to 3 cm above the soil surface) [88].

In an experiment designed to test seedling emergence from boreal wetland soils under changing climatic conditions, perennial sowthistle seedlings emerged from the soil seed bank in willow (Salix spp.) savanna and bluejoint reedgrass vegetation zones of a mid-boreal wetland in Alberta [60].

Although seeds of perennial sowthistle have low viability in cultivated fields, some can remain dormant but viable for more than 3 years in cultivated soil. Chepil [23] conducted 3 separate seed dormancy tests for "weed" species in cultivated soil in Saskatchewan. In the 1st experiment an indefinite number of perennial sowthistle seeds was planted in 3 soil types on 18 September, 1937. Introduction of seeds from other sources was prevented. No seeds were planted greater than 3 inches (7.6 cm) deep. Number of viable seeds remaining in the soil after 3 years was determined by repeated germination tests in the laboratory until no more germination occurred. Results are shown in the table below [23]:

In the 2nd experiment, 50 perennial sowthistle seeds were planted no deeper than 3 inches (7.6 cm) on 14 October, 1938, in 3 soil types. Again, number of viable seeds remaining in the soil after 6 years was determined by repeated germination tests in the laboratory until no more germination occurred. Values given are number of viable seeds [23].

The 3rd experiment utilized 1,000 perennial sowthistle seeds planted no deeper than 3 inches (7.6 cm). Seeds were planted between 1 and 5 November, 1940, in 3 soil types and only seeds germinated in the field were counted. Numbers are actual seeds germinating, not percentages [23].

Clay appears to be most conducive to long-term viability of perennial sowthistle seeds [23] (See Site Characteristics).

Germination: Germination of perennial sowthistle seeds increases with both increasing soil temperature and time since flowering. Perennial sowthistle seed in the field begins to germinate when the soil has "warmed" [74].

Seeds may be capable of germination about 5 days after pollination [74]; however, germination rates increased from low to none 4 days after flowering to a maximum 7 to 9 days after flowering [31,66,110]. In field germination experiments in South Dakota, Derscheid and Schultz [31] noted that percentage of viable seeds produced by perennial sowthistle ranged from 10% 6 days after blooming to 89% 9 days after blooming. If perennial sowthistle plants are pulled or cut and placed in a pile it is possible for viable seeds to be produced if flowers are present when the plants are cut [110].

In laboratory germination tests, perennial sowthistle seed viability is "relatively" high. Kinch and Termunde [66] achieved 95% germination in the laboratory using "well-matured" seed.

Orientation of perennial sowthistle seeds in the soil profile is important to germination, and light may stimulate germination. Bosy and Aarssen [15] conducted seed germination tests on perennial sowthistle using agar as a germinating medium. Agar was used to eliminate any environmental differences at a given depth and enabled the authors to maintain seed orientation. They found surface-lying seeds of perennial sowthistle displayed higher germination than buried seeds [15]. Germination was 50% for seeds germinated in soil and 80% for seeds germinated on moist filter paper, and germination was higher in diffuse laboratory light than in complete darkness [90]. When seeds were buried, seeds oriented with the radicle horizontal had significantly greater (P<0.05) germination than seeds with the radicle oriented either upward or downward.

Studies indicate temperatures from 77 to 86 °F (25-30 °C) are optimal for germination. Seeds germinate poorly (<5%) below 68 °F (20 °C) and above 95 °F (35 °C), but alternating temperatures were more favorable for germination than constant temperatures if temperatures above 77 °F (25 °C) are included in the cycle [52]. Stevens [110] reports seeds exposed to 90 °F (32 °C) for a "few hours daily" germinate "freely" in 4 to 7 days.

Perennial sowthistle seed germination in wetlands could be limited by saturated soils. For example, Hogenbirk and Wein [60] germinated seeds of perennial sowthistle from combined soil and litter samples from a mid-boreal wetland in Alberta. No perennial sowthistle seeds germinated in samples taken from a sedge (Carex spp.) marsh. Perennial sowthistle seeds stored in fresh water were 100% decomposed after 3 months storage [18].

Seedling establishment/growth: Perennial sowthistle seedlings survive best in areas with protective plant cover or litter and high moisture compared with open cultivated soil [110]. Accordingly, seedlings are often only found along pond, ditch, or field margins, or in lawns, meadows, or uncultivated fields [92]. In a series of field germination experiments with perennial sowthistle seeds, Stevens [110] had little success growing seedlings in cultivated field plots. Laboratory germination tests with the same lot of seeds showed 56% germination.

Most perennial sowthistle seedlings do not emerge until mid- to late May in Saskatchewan and the Great Plains of the United States [74]. Seedlings grow slowly for about the first 2 weeks until leaves are about 1.2 inches (3 cm) long [110]. They develop rapidly after that, and reproductive ability of spreading roots is established quickly [52,110]. Stevens [110] noted 10 seedlings on 17 May, 1923. The 10 seedlings grew slowly until 1 June when the largest leaves were 1.2 inches (3 cm) long. After that, they developed "rapidly" and on 5 July, a horizontal root 28 inches long (71 cm) was removed from the largest plant [110].

Most seedlings do not flower the first year, but flowering in late summer is possible from some first-year seedlings in favorable environments [52,110].

Asexual regeneration: Perennial sowthistle reproduces vegetatively from buds on horizontal and vertical roots and on basal portions of aerial stems located just under the soil surface. Thickened roots develop as a result of secondary growth of original fibrous roots [51] and begin to show reproductive capacity when thickened to 1 to 1.5 mm [50]. This occurs on vertical primary roots when seedlings reach the 4-leaf stage and on horizontal roots when seedlings have 6 to 7 photosynthetic leaves. One-month-old seedlings can have 7 to 8 leaves with horizontal roots from 4 to 6 inches (10-15 cm) long and 1.5 mm thick. Horizontal roots from 24 to 39 inches (60-100 cm) and vertical roots penetrating 20 inches (50 cm) can develop from seedlings within 4 months after emergence. Vertical roots can produce vegetative buds as deep as 20 inches (50 cm) below the soil surface, and new aerial growth has been observed from buds as deep as 16 inches (40 cm) below the soil surface [110]. New shoots can develop from buds that overwinter on both vertical and horizontal "spreading" roots, and/or on basal portions of aerial stems [51,89]. In North Dakota, the rate of vegetative spread of perennial sowthistle clones varied from 1.6 to 9 feet (0.5-2.8 m) per year, depending on the clone (personal observation in [74]).

Harris and Shorthouse [53] describe the horizontal roots of perennial sowthistle as "easily broken", and new plants can grow from root fragments and flower within 1 year [50,110]. Of perennial sowthistle root fragments planted on 3 May in a field experiment in North Dakota, approximately 50% of 0.25-inch-long pieces, 75% of 0.5-inch-long pieces, and 85% of 1-inch-long pieces produced plants within 20 to 34 days. Where well developed buds were present on root fragments, plants emerged quickly and were strong; if buds were not present, new plants grew more slowly from the cut surface and were weak. Plants grown from these root fragments reached a height of 3 feet (1 m) and flowered abundantly between 27 July and 6 August. On 29 June the largest of these plants had 2 horizontal roots 42 to 45 inches long (107-114 cm). The 45-inch root had 42 buds and sprouts in various stages of development. By the end of the growing season, horizontal roots from these plants reached about 6 feet (1.8 m) in length [110].

SITE CHARACTERISTICS:
Perennial sowthistle is adapted to moist, sunny locations in temperate regions but is absent from tropical areas [74]. Within temperate regions, perennial sowthistle has a broad tolerance to variable environments and adapts well to wet sites, even with little soil disturbance. In Canada, perennial sowthistle occurs in areas that receive average annual precipitation of 12 to 120 inches (300-3,000 mm) [133]. In a greenhouse study, growth of perennial sowthistle plants was positively correlated with increasing soil water, with greatest growth occurring at complete saturation [132]. However, perennial sowthistle also establishes on dry sites [98]. Neither the climatic conditions required for successful establishment nor conditions, if any, favoring S. a. ssp. arvensis over S. a. ssp. uliginosus have been established [74].

Perennial sowthistle is adapted to many soil types but appears to prefer fine-textured soils and does not thrive on dry, coarse-textured sand. Perennial sowthistle seems to prefer slightly alkaline or neutral soils and does not thrive in acid soils, salt marshes, or highly alkaline areas [110]. However, Zollinger and Kells [132] determined soil pH had little effect on leaf production, plant height, or number of capitula produced.

Perennial sowthistle is present in a variety of community types from those occurring on wet, very strongly saline surface soil and strongly saline subsoil to nonsaline and dry soils [98]. Dodd and Coupland [33] describe perennial sowthistle as occurring in halophytic or semihalophytic communities in Saskatchewan.

SUCCESSIONAL STATUS:
Perennial sowthistle is an early-successional plant. Komarova [67] and Zollinger and Parker [133] describe perennial sowthistle as a pioneer species. In a study of succession after fire in "highland hardwoods" in Wisconsin, it appeared in 6 out of 10 plots in the herbaceous stage of succession [44]. Although infrequent, perennial sowthistle is part of the early successional community on wetlands in the blast zone after the Mount St. Helen's eruption [121].

Perennial sowthistle is most competitive under abundant precipitation and moderate climates [133].

SEASONAL DEVELOPMENT:
Shoots and new roots in established stands begin to develop when the soil starts to warm [51,110]. Small leaves begin to appear from shallow roots about 1 week from initial growth [110], and adventitious root development begins 3 to 4 weeks later. Initial thickening of new roots begins when plants have 5 to 7 leaves [50,52]. Secondary thickening proceeds quickly, and spreading roots 4 mm thick and over 79 inches (200 cm) long can be detected by 3 months after initial growth [110]. Thickening of new roots ceases by mid-summer. New shoots develop from roots 2-3 mm in diameter until late summer [74].

Flowering stems begin to develop when plants have 12 to 15 leaves [50,110]. Flowering begins about 1 July in the northern United States and continues until plants are frosted, although most flowering is complete by late summer [110]. Time required from flowering to fruit maturation is about 10 days [74].

FIRE ECOLOGY

• FIRE ECOLOGY OR ADAPTATIONS
• POSTFIRE REGENERATION STRATEGY

Perennial sowthistle seeds are dispersed by wind (see Seed dispersal), and seedlings may establish on burned areas from offsite seed sources when mature plants occur in the vicinity of the burn. Seedlings established on burned sites in red pine forest in Minnesota [3] and on the Delta Marsh in Manitoba [120], while no perennial sowthistle plants occurred on unburned control plots in either study. Probability of postfire establishment from offsite seed may be related to season of burning (see Plant Response to Fire). Information on seed banking for perennial sowthistle suggests that it is possible for seedlings to establish from the soil seed bank after fire, although this has not been documented in the available literature.

Perennial sowthistle plants are likely to survive and persist on burned areas, even after high-severity fire, and the limited available data on postfire response of perennial sowthistle indicate little difference in abundance between burned and unburned sites [59,86] (see Plant response to fire). Perennial sowthistle shoots develop from numerous underground buds on both vertical and horizontal roots, and on basal portions of aerial stems [51,89] (see Asexual regeneration). Vertical roots can be 5 to 10 feet (1.5-3 m) deep [10] with the potential to produce shoots from buds as deep as 16 inches (40 cm) below the soil surface [110]. These buds would not be affected by fire. Horizontal roots of perennial sowthistle occur 2 to 4 inches (5-10 cm) below the surface [10] and would probably also be protected from all but the most severe fires.

Fire regimes: As of this writing (2004), no information regarding fire regimes in which perennial sowthistle evolved was found in the available literature; nor was information available regarding impacts of perennial sowthistle invasion on fuel characteristics or fire regimes in native North American plant communities. The following table provides fire return intervals for plant communities and ecosystems where perennial sowthistle may occur in North America. Perennial sowthistle may also occur within riparian or wetland areas included in these ecosystems. For additional information on these fire regimes, see the FEIS summary on the dominant species listed below:

FIRE EFFECTS

• IMMEDIATE FIRE EFFECT ON PLANT
• DISCUSSION AND QUALIFICATION OF FIRE EFFECT
• PLANT RESPONSE TO FIRE
• DISCUSSION AND QUALIFICATION OF PLANT RESPONSE
• FIRE MANAGEMENT CONSIDERATIONS

Established perennial sowthistle plants are likely to persist after fire on burned sites, though it is unclear whether its overall abundance will increase or decrease in the postfire environment. Postfire data from studies in Alberta, Canada [59] and North Dakota [86] shows little difference between burned and unburned sites, and no detectable postfire trend in perennial sowthistle abundance 1 to 2 years after fire.

Flowering of perennial sowthistle may increase after fire. Postfire flowering response may be related to postfire moisture availability [91].

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:
Perennial sowthistle may establish on burned sites from wind-dispersed seed. Soil samples were taken from burned and unburned areas of a 270-year-old red pine forest in Minnesota 3 years after wildfire. No perennial sowthistle germinants emerged from soil taken from unburned areas, while the equivalent of 109,000 perennial sowthistle seedlings per hectare emerged from soil taken from burned areas. No perennial sowthistle plants occurred in either burned or unburned plots, and no perennial sowthistle seeds were found in unburned soil samples. The author concluded that perennial sowthistle seedlings probably developed from seeds blown into the burned areas after the fire [3].

Probability of postfire establishment from offsite seed sources may be related to season of burning. Thompson and Shay [120] conducted 3 prescribed burn treatments in 3 different seasons on the Delta Marsh in Manitoba. Perennial sowthistle was absent from unburned plots, but seedlings established on both summer and fall burned plots, with greatest establishment 1 month following summer burns. These seedlings persisted into the following year, resulting in increased nonseedling shoot density and biomass on summer burned plots:

Perennial sowthistle has a long flowering period that begins in early July and continues until plants are frosted, with peak flowering in mid to late summer [110]. Summer burns on the Delta Marsh were conducted at the time of year when the most mature sowthistle seed was likely being dispersed, while timing of fall burns corresponded to a time of year when fewer seeds were likely to be available, and spring burns occurred when no perennial sowthistle seed would likely be available [120]. Although there is evidence that perennial sowthistle seeds occur in the soil seed bank in the Delta Marsh [88] (see seed banking), it is more likely that perennial sowthistle seedlings detected in this study [120] established from wind-dispersed seed, given the correspondence of burn season, perennial sowthistle phenology, and seedling establishment.

While perennial sowthistle is likely to persist after fire, data are insufficient for detecting trends in its postfire abundance. Simulated "light" and "deep" burns using a propane torch in both bluejoint reedgrass and willow savanna habitats in northern Alberta found little difference in perennial sowthistle cover 2 growing seasons after summer burning [59]:

Abundance of perennial sowthistle plants was highly variable on burned and unburned prairie sites in a study to evaluate the effects of prescribed burning on grassland species desired for wildlife habitat on the Tewaukon National Wildlife Refuge in southeastern North Dakota. Cover of perennial sowthistle was mostly the same on burned and unburned sites, but in some years was either significantly higher on or significantly lower on burned versus control plots. Data show both great variation in percent canopy cover and no clear trend of increase or decrease on burned versus control plots, 1 month or 26 months after fires in May or June [86].

In northwestern Minnesota, flowering of perennial sowthistle increased on some sites after prescribed fire was used as part of a prairie restoration project. Burns were conducted in spring 1973, and data on flowering response were collected during the growing season of the same year. Results were based on comparison of burned and unburned transects. Perennial sowthistle was recorded on 3 site types in the study area: a wet-mesic site in "badly" disturbed prairie, a wet swale site in undisturbed prairie, and a gently sloping to nearly level mesic site in undisturbed prairie. Flowering increased after prescribed fire, relative to controls, on both disturbed and undisturbed wet prairie sites, but was not different from controls in the mesic prairie site [91].

FIRE MANAGEMENT CONSIDERATIONS:
Postfire colonization potential: Perennial sowthistle may establish on burned sites after fire [3,120], especially when burning is conducted during the flowering period [120]. Managers should be alert to this possibility when perennial sowthistle occurs in the vicinity of the burn area. Additionally, flowering of perennial sowthistle may be stimulated by prescribed burning [91], resulting in increased seed production and further increasing the likelihood of postfire establishment.

Control of perennial sowthistle using prescribed fire: While no research is available examining the potential for using prescribed fire to control perennial sowthistle, evidence from other fire studies [59,86] suggest that it is likely to persist after fire and may increase in abundance on some sites. Given the abundance of underground buds on both vertical and horizontal roots, and on basal portions of aerial stems [51,89], it is unlikely that prescribed fire would be severe enough to substantially damage established perennial sowthistle plants.

If perennial sowthistle plants are pulled or cut and placed in a pile it is possible for viable seeds to be produced if flowers are present when the plants are cut. These plants could be burned to reduce the possibility of viable seeds being produced [110].

MANAGEMENT CONSIDERATIONS

• IMPORTANCE TO LIVESTOCK AND WILDLIFE
• OTHER USES
• IMPACTS AND CONTROL

Perennial sowthistle is considered "excellent" forage for rabbits [118] and Martin and others (as reported in [133], a literature review) state perennial sowthistle is a minor element in the diet of some North American birds.

Perennial sowthistle is listed as a nonnative plant occurring in critical habitat of the threatened desert tortoise in the Mojave and Colorado deserts. It is of concern because it competes with native plants vital to the tortoises' survival [17].

Palatability/nutritional value: Although perennial sowthistle compares favorably with alfalfa (Medicago sativa) for nutritional value, it is not especially palatable to grazing animals. Dry perennial sowthistle is about 10% protein by weight [19,20]. Palatability of perennial sowthistle to lambs was lower compared to grasses and alfalfa, and infestations of perennial sowthistle in pastures and hayfields may decrease overall forage feeding value [76].

Perennial sowthistle has equal or higher in vitro digestible dry matter, micro- and macromineral content and crude protein and lower neutral detergent fiber compared to alfalfa [76]:

Herbage macromineral and micromineral concentrations for perennial sowthistle are given in the following tables:

Cover value: Cover value of perennial sowthistle for several classes of wildlife for 2 western states is provided by Dittberner and Olson [32] in the following table:

Most of the latex of perennial sowthistle is oil and may be a potential crop for oil or hydrocarbon production [19,20]. Perennial sowthistle is a good source of pentacyclic triterpenes, which may become important in the pharmaceutical industry [61].

IMPACTS AND CONTROL:
Impacts: Information concerning the impacts of perennial sowthistle on natural communities is absent from the literature. Research is needed to determine and document what effects perennial sowthistle may have on wildlands.

Control: Perennial sowthistle is relatively resistant to many common broadleaf herbicides compared to most annual broadleaf weeds. Consequently, the best systems for control often include a combination of cultural and chemical treatments designed to reduce competition from perennial sowthistle, prevent seed production, and reduce the reproductive capacity of its roots (Fryer and Makepeace, 1982, as reported in a literature review [74]).

As of this writing (2004) there is no information available on control of perennial sowthistle in natural areas.

Prevention: The most efficient and effective method of managing invasive species is to prevent their invasion and spread [107]. Since perennial sowthistle seed is so easily disseminated by wind, scouting and detection are keys to preventing plant establishment [133]. It is easier to prevent initial colonization by perennial sowthistle than to eliminate established populations. Seedlings are easily controlled through mechanical and chemical methods. Planting weed-free crop seed and controlling perennial sowthistle on field borders can prevent initial infestations in wildlands adjacent to agricultural settings [133] (See Seedling establishment/growth).

Integrated management: Components of any integrated weed management program are sustained effort, constant evaluation, and the adoption of improved strategies [106]. Factors to be addressed before a management decision is made include inventory and assessment to identify the target weed(s) and determine the size of the infestation(s); assessment of nontarget vegetation, soil types, climatic conditions, and important water resources. An evaluation of the benefits and limitations of each control method also needs to be accomplished [84].

Combinations of tillage plus cultural practices or herbicides applied regularly have controlled perennial sowthistle in agricultural settings [30]. No information is available on integrated control measures for perennial sowthistle in wildlands.

Timing of control measures may increase the effectiveness of integrated management techniques. Schimming and Messersmith [102] conducted artificial freezing experiments with perennial sowthistle. They determined a temperature of 1 ^oF (-17 ^oC) reduced survival of perennial sowthistle roots by 50% and a temperature of 4 ^oF (-15 ^oC) reduced total dry weight of emerging perennial sowthistle shoots by 50%. The authors speculate conditions that tend to minimize hardening, such as lack of photosynthetic material in fall after tillage or chemical treatment, stimulation of fall growth after tillage, or high nitrogen levels may increase injury caused by freezing temperatures in the field.

Physical/mechanical: Tillage generally reduces perennial sowthistle, but its effectiveness depends on plant growth characteristics at time of tillage [10,50,52], type of tillage being utilized [30,51], and frequency of tillage [92]. Intensive tillage is usually not appropriate in wildland settings, so it is not discussed further here.

Studies of mowing as a control method for perennial sowthistle show mixed results. Defoliation was less effective than burial for reducing infestations of perennial sowthistle in a study done in Sweden in 1967 [52], suggesting mowing is not as effective as tillage for control of perennial sowthistle [74]. However, Stevens [110] found defoliation an efficient method to control perennial sowthistle. Plants grown from root cuttings planted 3 May, had their leaves removed by hoe on 23 May when the largest leaves were about 6 inches (15 cm) long. The plants had the leaves removed again on 1 June, when leaves had again grown to about 6 inches (15 cm). After the 1 June defoliation, leaf growth was less vigorous. There was "very little" regrowth of leaves after a 1 July defoliation and none after a 19 July defoliation although weather conditions were favorable for growth. No plants appeared the next spring.

Fire: See the Fire Management Considerations section of this summary.

Biological: There appears to be limited biological agents available to help control perennial sowthistle. A tephritid fly from Europe that transforms the seedhead of perennial sowthistle into a gall has been released into Canada but has not become established [53]. Cystiphora sonchi, another fly native to Europe, was released into Canada and has become established in Alberta, Saskatchewan, Manitoba, and Nova Scotia [93]. Zollinger and Parker [133] report as many as 721 galls were formed on one plant of perennial sowthistle, but Lemna and Messersmith [74] state that no reduction in perennial sowthistle because of Cystiphora sonchi has been observed. A third fly, Liriomyza sonchi, has been authorized for release into Canada (Peschken and Derby 1988, reported in [74]).

Zollinger and Parker [133] provide a literature review of biological control efforts as of 1998.

Chemical: Auxin-type herbicides are the primary chemicals used to control perennial sowthistle. Perennial sowthistle is "moderately susceptible" to auxins such as 2,4-D, 2,4-DB, and MCPA in the seedling stage, and established stands are "moderately resistant" (Fryer and Makepeace, 1982, as reported in a literature review [74]). Growth of aerial portions can be retarded by auxin-type herbicides, and flowering can be completely suppressed if the plant is treated when growth is vigorous (Fryer and Makepeace 1982 as reported in a literature review [74]), and [77]. A more detailed discussion of chemical control of perennial sowthistle is provided by Lemna and Messersmith [74] and by Zollinger and Parker [133].

Cultural: Patches of perennial sowthistle were cut for hay or were pastured as an early control measure [111,129]. An alfalfa or alfalfa-grass mixture, regularly cut for hay, can eliminate 90% of perennial sowthistle in 3 years (Martin and others 1961 in [74]).

"Intensive" grazing by domestic sheep or cattle weakens perennial sowthistle when the animals eat new growth and sometimes roots [133]. Grazing also enhances other control practices. However, perennial sowthistle is classified as an "increaser" under heavy grazing because it increases as more palatable plants are preferentially grazed [70].

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