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Dyer's woad is typically a biennial but may exhibit a winter annual or short-lived perennial life history under some conditions (see General Botanical Characteristics). Dyer's woad plants require portions of 2 growing seasons to produce seed. In the Intermountain West, dyer's woad seeds germinate in fall or early spring (review by [19]). In northern California annual grasslands dominated by medusahead and cheatgrass, dyer's woad seedlings established sometime between August and February [87]. In the first growing season after germination, roots develop and rosettes form. Bolting, flowering, and fruiting occur the following growing season [12]. Seedlings arising in spring bolt and flower the following spring; fall-germinating seedlings overwinter as small rosettes and usually require the following growing season to develop sufficient belowground support and reserves to sustain flowering plants (review by [19]). Dyer's woad flowers, unripe fruits, and ripe fruits (top to bottom).
©Steve Dewey, Utah State University, Bugwood.org
Dyer's woad is characterized by rapid vegetative growth during spring that typically enables it to produce seed by late spring or early summer on midelevation sites. The period of rapid growth by dyer's woad may overlap with the period of peak water extraction by bluebunch wheatgrass on some sites in some years, suggesting there may be belowground interference between these co-occurring species [21] (see Successional Status). Dyer's woad plants were studied on northern Utah foothill sites at 4,850 to 5,000 feet (1,480-1,525 m) elevation during 2 studies: one from May 1982 to November 1983, and the other during the 1984 growing season. See Seedling establishment and plant growth for similar information from an experimentally established dyer's woad population in the same area. Young dyer's woad plants were marked and phenologically categorized between May 1982 and November 1983. Phenological stages were as follows: dormant, leaf growth, stem growth, floral buds developing, flowering, seed development, seed ripening, seed dissemination, and dead. Leaf growth occurred in both fall and spring, and flowering occurred in late spring. Time between stem growth and seed development was about 8 weeks. Mean stem growth was about 4 inches (10 cm) per week from mid-April until the end of May. Plants were dormant in both summer and winter, corresponding with hot, dry conditions or cold temperatures, respectively. Sixty-five percent of marked plants died and 1% flowered during the 1st growing season. Of the 35% that survived to the 2nd year, about half flowered and produced fruit. All plants that set seeds died; about 12% remained vegetative and may have produced fruit in the 3rd year [21].
Dyer's woad plants observed on Utah foothill sites during the 1984 growing season started vegetative growth by 16 April 1984, less than 1 week after snowmelt. Basal diameter increased between 16 April and 7 May and thereafter remained fairly constant. Likewise, rosette diameter increased during the same period, leveled off by 23 May, and then declined as basal leaves withered and flowering stems developed. Stem growth began during the last week of April, and flowering began the second week of May, reaching its peak about 23 May. Height of flowering stalks increased rapidly between 7 May and 11 June. Seed developed between 9 June and 15 June. By the end of June, most of the seeds had ripened [20].
Root crown buds on dyer's woad plants that have flowered sometimes survive, allowing plants to persist and flower again. The growth of the flowering shoot reduces carbohydrates stored in the taproot during the previous season (review by [12]).
Typical flowering dates by geographic area are given in the following table:
Dyer's woad flowering dates by geographic area Area Flowering dates California April to June [57] Illinois May to June [55] Nevada April to July [40] Utah midspring [59] Utah (Uinta Basin) May to July [28] Virginia May to June [85] Intermountain West May to June [36] Northeast and adjacent Canada May to July [27] Pacific Northwest April to August [32]
Dyer's woad fruits ripen between June and October throughout its range [23]. Dyer's woad seeds become viable relatively early during seed production [36].
A survey in Idaho in 1983 found that timing of flowering and seed dispersal were related to elevation. Flowering and dispersal dates observed in that survey were as follows [12]:
Phenology of dyer's woad in several counties at different elevations in Idaho [12] County Elevation (m) Phenological stage Dates Northern Bannock 1,829 Rosette and bolting 3 June Northern Bannock up to 1,402 Flowering 23 May Jefferson and Bonneville 1,341-1,463 Flowering 26 May Caribou 2,073 Flowering 23 June Caribou 2,079 Flowering 5 July Caribou 2,316 Flowering 12 July Bear Lake 2,256 Flowering as late as 14 July Caribou below 1,981 Full bloom 17 June Bear Lake 2,256 Full bloom 29 June Central and southern Bannock and Franklin -- Full bloom 7 June Franklin county 1,585 Full to late bloom 10 June Eastern Oneida 1,067 Late bloom to seed set 15 June Clark 1,707-2,012 Late bloom to ripe fruit 22 July Adams 899 Dispersing ripe fruit 26-28 July Blaine 1,295 Dispersing ripe fruit 26-28 July Southwestern Oneida 1,492-1,463 Dispersing ripe fruit 20 July
REGENERATION PROCESSES:
Dyer's woad reproduces by seed. It may sprout following damage to aboveground parts, and sometimes after flowering (see Vegetative regeneration); however, persistence and spread of dyer's woad populations is dependent on viable seed production.
Pollination and breeding system: Results from laboratory studies in Italy showed an outcrossing breeding system in dyer's woad. The effects of selfing and crossing on seed production, germinability, and progeny growth were assessed. Self-pollinated plants produced fewer siliques (7.1 g/plant) with lower weight (6.0 mg) and lower seed germinability (8.2%) than outcrossed plants (44.1 g, 8.0 mg, and 46% for each character, respectively). Self-pollinated progenies generally showed lower height growth than outcrossed progenies [72].
Flower and seed production: Dyer's woad requires a cold vernalization period to induce flowering. A greenhouse study in Utah found that both 1-year old dyer's woad plants that had previously flowered (crown rosettes) and 4-month old seedling rosettes required exposure to cold temperatures (39 °F (4 °C) or less) for a minimum of 23 to 47 days to induce flowering [3,4]. The 2 types of rosette responded differently to cold treatments, which ranged from 0 to 93 days at 39 °F (4 °C), suggesting that cold tolerance is dependent not only on length of cold exposure but also on plant age. No seedling rosettes died during any length of cold exposure, while 50% of crown rosettes died after 93 days of cold exposure, and 30% died after 47 days of cold exposure. There was no difference in survival of crown rosettes after 23 days of cold exposure and that of controls [4]. Continual disturbance, such as defoliation, delays flowering of dyer's woad [20] (see Physical or mechanical control).
Reviews describe "prolific" or "abundant" seed production in dyer's woad [12,19,54]. A review by McConnell and others [54] suggests that some plants produced more than 10,000 seeds in 1 year, although the source of this information is not given. Dyer's woad plants studied on Utah rangelands produced about 350 to 500 seeds each [20,21].
Seed production may vary among plants established in different seasons and on different microsites. A field study in Utah found that dyer's woad plants that established in fall had slightly larger rosettes, taller flowering stalks, and produced more fruit (563 fruits/plant) than those that established in spring (345 fruits/plant). Mean fruit production of plants established in spring was similar among plants growing near sagebrush (293 fruits/plant) and those growing in interspace microsites (317 fruits/plant). Fruit weights were similar among all groups (3.9 mg/fruit) [20,21]. In a related study in the same area, average fruit production was 383 fruits/plant [21].
Seed dispersal: Dyer's woad fruits do not release the seed at maturity, but fall to the ground intact [19]. The majority of dyer's woad fruits disperse within a few meters of parent plants. Long-distance dispersal may occur with the aid of humans, livestock, wildlife, and water [20].
Most dyer's woad fruits shed soon after reaching maturity, although some remain on the plants until winter. Fruits are firmly attached to plants, and some abrasive force such as wind or rain is needed to detach them. A field study in Utah recorded daily dyer's woad fruit dispersal from 25 June 1985 until 27 August 1985. Most of the fruits were shed in the first 10 days of the study; thereafter, the dispersal rate declined substantially, leveling off after 4.5 weeks. Ninety-five percent of all trapped fruits fell within 21 inches (54 cm) of parent plants, and mean dispersal distance was positively correlated with the height at which seeds were released (r²=0.85). The greatest distance that fruits traveled via wind was about 8 feet (2.4 m). The relationship between windspeed and number of fruits dispersed was "poor"; however, most fruits scattered in the direction of prevailing winds. Dyer's woad fruits remaining on plants until winter may disperse much greater distances when blown over the surface of crusted snow [20,21]. Fruits may be further transported by ants, as was observed during studies on Utah rangelands [20].
Long-distance spread of dyer's woad fruits and seeds must be aided by vectors such as humans, livestock, wildlife, and water. Humans may disperse fruits in their clothing, vehicles, tools or machinery [20,21,80]. Roadsides and railways are effective avenues of seed dispersal [19]. Long-distance dispersal is likely when dyer's woad seed is a contaminant in alfalfa or other crop seed (review by [12]); or when mature, seed-bearing dyer's woad plants are cut and baled with alfalfa in infested fields, and this baled hay is shipped to where it is used as livestock feed [19,20,21]. Contaminated hay is one of the major causes of dyer's woad spread [36].
Livestock and wildlife may carry fruits in mud on their hooves or in their fur [36]. The curved pedicel of dyer's woad fruits may act as a hook to aid in dispersal by animals. Dyer's woad fruits remaining on plants past the first snowfall may be dispersed by herds of deer and elk in the winter months, when herd use of foothill sites is highest [20]. Farah [20] speculates that a high incidence of dyer's woad infestations on south-facing slopes on Utah rangeland may be related to deer and elk use of these sites in winter. Birds and rodents may also contribute to long-range dispersal of dyer's woad [20].
Downhill and downstream dispersal of dyer's woad fruits may be aided by water; flattened wings facilitate this mode of dispersal. Dyer's woad populations along the banks of drainage systems in Utah may have established after this type of dispersal [20,21].
Seed banking: Information on seed banking in dyer's woad was lacking, and it had not been determined how long seeds are viable in the soil, as of 2009. Anecdotal accounts from Europe suggest that dyer's woad sometimes appears after grasslands are tilled; authors contend that these are sites of former woad crops where the seeds have remained dormant in the soil, presumably for many years (King 1966 as cited by [87]).
While dyer's woad seeds may have no dormancy, they are contained in fruits that have water soluble germination inhibitors such that few seeds germinate immediately in the field, presumably until the inhibitors are leached from the fruit [87]. The inhibitors in the fruit may allow dyer's woad seed to persist in the soil seed bank [19]. Because the inhibitors are removed by leaching, they do not seem likely to contribute to long-term persistence of seed in the soil, because they would be leached by precipitation, allowing germination under favorable conditions [87].
Evidence from field studies indicates that some dyer's woad seeds remain viable in the soil for at least 10 to 12 months. Dyer's woad fruits (1,200 total) were harvested from a Utah study site on 8 July 1982 and buried under about 0.4 inch (1 cm) of soil. Each month, 120 fruits were removed from the field, and seeds were removed from fruits and tested for germination and viability. Germination tests were conducted at 77 °F (25 °C) with 12 hours each of alternating light and darkness, and germinated and viable seeds were counted after 14 days. Germination rates of dyer's woad seed stored in the field ranged from 99% in September 1982 (after 1 month of burial) to 44% in May 1983 (after 9 months of burial). Seed viability remained high, fluctuating between 73% and 100%, and did not decrease over time. Whether dyer's woad seed can germinate after being stored in the soil longer than 10 months is not known. In a related study on the same site, <1% of dyer's woad seeds from fruits sown in September 1984 germinated in fall of 1985, and none germinated after that time. Based on these observations, the authors suggest that either dyer's woad has limited seed banking capability, seeds undergo induced dormancy over time, or seeds experience substantial predation or pathogen attack [20,21]. For more details of this study, see Seedling establishment and plant growth.
Germination: Dyer's woad seeds separated from the fruits do not exhibit dormancy and readily germinate under a variety of conditions, though they do not readily germinate when they remain intact within the fruit. Dyer's woad seeds do not usually dehisce from the fruits under field conditions; thus, the intact fruit imposes dormancy [87]. The majority of dyer's woad seeds collected in August 1969 and separated from the fruits germinated after incubation for 168 hours at temperatures from 37 to 77 °F (3-25 °C). Conversely, when intact fruits were incubated, germination was low and erratic. Seedlings elongated much more slowly from fruits than from seeds [87].
Dyer's woad germination rates and seedling lengths after 168 hours incubation at a range of temperatures [87] Temperature
(°C) Seeds in fruits Seeds free from fruits Germination (%) Seedling length (mm) Germination (%) Seedling length (mm) 3 0 0 78 5 5 4 3 92 15 10 8 5 100 25 15 0 0 100 45 20 12 5 88 45 25 10 5 100 45
Reduced germination and seedling elongation from intact fruits were likely due to chemistry rather than due to a physical obstruction. In a laboratory study, not only were germination and seedling emergence reduced from intact dyer's woad fruits, but the presence of intact fruits or fruit leachate also reduced germination and seedling emergence in both threshed dyer's woad seed and in seeds of several other species (see Successional Status for details). Washing dyer's woad fruits in tap water for 48 hours increased germination, and washing fruits for 96 hours almost eliminated germination inhibition. In the field, some dyer's woad seedlings established from fruits that overwintered [87].
Germination inhibitors present in freshly sown seed are likely leached over winter, thereby allowing greater germination of overwintered seeds. In a field study in Utah, germination of dyer's woad seeds sown in October 1984 was 10 times higher in spring 1985 than fall 1984. The author speculates that seeds that germinated shortly after being sown may have been in damaged fruits [20].
Dyer's woad seed germination is likely inhibited by shade. High percentages (>85%) of dyer's woad seed germinated under red, yellow, and white light within 4 days. Significantly lower percentages germinated under far red and blue light (15% and 37%, respectively) (P<0.05), and germination time was longer. Far red and blue light simulate light conditions under a dense canopy [75].
Seedling establishment and plant growth: Seedling establishment, survivorship, growth, and eventual reproductive output (see Seed production) may vary among dyer's woad seedlings established in the fall versus those established in spring, and among microsites. Dyer's woad population demographics were studied over a 2-year period on a Utah rangeland where 100,000 dyer's woad fruits were collected during the summer of 1984 and sown on 8 September 1984 in a "well-vegetated" area lacking dyer's woad. During the study period precipitation was 18% above the estimated long-term average, and mean monthly temperatures were slightly below the long-term average [20,21]. The following information comes primarily from this single study and is therefore limited in scope; dyer's woad may display different population dynamics on other sites. See Seasonal development for more precise phenological information from dyer's woad populations in the same area.
Seedling establishment: For freshly shed seeds, establishment rates were lower 1 month after sowing in the fall (0.3%) than during the following spring (2.7%) [20,21], which is consistent with findings of Young and Evans [87] that dyer's woad fruits contain water-soluble germination-inhibiting substances that would have leached over winter. Germination in fall 1985 was twice that in fall 1984; these differences were not associated with differences in either precipitation or mean monthly temperatures. Germination from the original seed input ceased after fall 1985 [20,21] (see Seed banking).
Microsites near sagebrush plants seem to provide a more favorable microenvironment for dyer's woad seedling establishment than interspace microsites. Seedling densities were 170 and 26 dyer's woad plants/m² on sagebrush and interspace microsites, respectively [20,21].
Survival: Survivorship patterns were similar in fall- and spring-established dyer's woad populations, with peak mortality in summer. Cohorts of dyer's woad that established in October 1984 (n=285) experienced little mortality during the following winter, slight mortality in early spring 1985, and peak mortality during the summer. Thirty-six of these plants survived the summer drought, overwintered again, flowered, and set seed in spring of 1986. None of the dyer's woad seedlings that established during the spring of 1985 (n=2,664) flowered in the same year. Of the spring-established cohort, 371 individuals survived the summer drought and overwintered. Eighty-seven percent of these plants flowered and produced seeds in spring of 1986, and the other 13% remained vegetative. Peak mortality in both dyer's woad populations occurred during a period with high temperatures and negligible precipitation, suggesting that the main source of mortality was water stress; there was no evidence of predation or pathogens. The authors note that the developing roots of young rosettes of dyer's woad are unlikely to access soil moisture from deep soil layers, where moisture occurs during hot and dry conditions above ground; but they caution that a causal relationship between seedling mortality and soil moisture deficit was not established because soil water content was not measured [20,21].
A life table analysis for dyer's woad showed constriction of population growth at 2 transitions: seed to seedling (establishment) and young rosettes to mature rosettes. The establishment rate was 3%; and only 23% of young rosettes survived to mature rosettes. Once plants became mature rosettes, the probability of surviving to reproduce was 81%. All flowering individuals set seed, with an average fruit production of 496 fruits/plant [20,21].
Neither microsite characteristics nor seedling density appeared to impact mortality rates in dyer's woad populations. Mortality of dyer's woad plants growing near sagebrush and those in the interspaces were similar (73% and 74% respectively), despite a 7-fold difference in seedling density [20,21].
Growth and reproductive output: Fall germination of dyer's woad favors both vegetative growth and reproductive output (see Seed production); however, spring germination was more important than fall germination in terms of overall population growth: Higher germination rates in spring resulted in more individual plants and higher total fruit production from spring-germinated cohorts than fall-germinated cohorts. Fall-germinated individuals had nominally greater rosette sizes than spring-germinated individuals during most of the study period, and differences were most pronounced at the start of the spring 1986 growing season. Stem growth was initiated in both cohorts during the last week of March 1986, and rapid stem growth occurred up to 18 May 1986. By 20 April, the fall cohort was taller. The fall cohort had significantly greater fruit production/plant (P<0.1), but fruit weights were similar and the spring population had more plants [20,21].
Neither microsite characteristics nor plant density in dyer's woad cohorts appeared to translate into better vegetative and reproductive performance: rosette size, height of flowering stalks, and seed production were similar between these 2 groups [20,21].
Vegetative regeneration: Several sources indicate that dyer's woad plants may sprout when the top growth is removed at ground level [19,20,21,36,67]. Sprouting seems to originate from buds on dyer's woad root crowns ([4], review by [12], personal communication [15]). Numerous vague references to vegetative or asexual regeneration in dyer's woad were found in the literature: "Clonal growth has been observed but is not common" [37]; "Asexual reproduction may occur from this underground root system" [80]; "....the weed can spread from underground portions of the root system...." [6]; "It has a large fleshy taproot from which it may reproduce asexually" [19]; and "Damaged plants often resprout from buds located on the root crown and, less frequently, from the roots" [67]. However, vegetative regeneration in dyer's woad seems to be restricted to sprouting from the root crown following aboveground damage.
Dyer's woad is likely to survive and sprout following aboveground damage and defoliation [19,20,21,36,67], depending on timing, frequency, and severity of damage. A review by Evans [19] states that while undisturbed dyer's woad plants typically behave as biennials or winter annuals, perennial behavior can be elicited by mowing, hand-pulling, or breaking the bolting stalk above ground. This is supported by evidence from a field study where plants were clipped at varying intensities, frequencies, and dates: Significant mortality and reduction in reproductive performance occurred when at least 60% of the aboveground phytomass had been removed on or after 23 May (P<0.05) [20] (see Physical or mechanical control for details and methodology). Fuller (1985 as cited by [20]) demonstrated that to substantially reduce flowering capacity and cause adequate mortality before 23 May, dyer's woad had to be clipped 2 inches (5 cm) below ground. "This suggests that regeneration of dyer's woad, following clipping damage, results from activation of crown buds and those located on the roots just beneath ground level". Young rosettes are less likely than older plants to survive defoliation due to the lack of development of the root system in young rosettes [20].
A review by Callihan and others [12], a laboratory study by Asghari [4], and observations by Dewey (personal communication [15]) suggest that buds on dyer's woad root crowns sometimes survive after the plant has flowered, allowing the plants to persist and possibly produce additional seed crops. Callihan and others [12] note, "Frequently, crown buds on plants that have flowered will survive, allowing plants to persist for three or more seasons." Asghari [4] used 1-year-old dyer's woad rosettes that had previously bolted and flowered in a vernalization study: several of these rosettes bolted and produced seed in the greenhouse. Dewey (personal communication [15]) notes repeated observations of established (flowered) dyer's woad plants damaged by tillage, mowing, or fire that have re-emerged and flowered again later in the same summer or in the following season. He suggests that this resprouting is from buds atop the plant's main taproot, not from creeping roots or rhizomes: He has never seen 2 dyer's woad plants connected to each other under ground.
SITE CHARACTERISTICS:
In the western United States, dyer's woad most commonly establishes and persists on rangelands and disturbed sites such as roadsides, rights-of-way, fence rows, uncultivated croplands (e.g., alfalfa and small grain fields, orchards), pastures, old fields, and "waste places" ([17,28,31,32,36,82], reviews by [19,54]). Characteristics of sites supporting dyer's woad in eastern North America were not described in available literature (2009). A Virginia flora describes dyer's woad as infrequent and occurring on disturbed sites [85].
Climate: Dyer's woad is native to parts of Russia, where the climate may be similar to that of the Intermountain West (review by [2]). Few studies of dyer's woad report climate data. On study sites where dyer's woad occurred on coarse, well-drained soils at 2 foothill locations on the western slope of the Wellsville Mountains in northern Utah, mean annual precipitation is 16 inches (400 mm), and mean annual air temperature is °F (9 °C) [21,84]. A review by Parker [59] suggests that dyer's woad has a moisture requirement of 14 to 18 inches (356-457 mm) per year. Specimens of dyer's woad were collected at 40 xeric to mesic sites in Idaho [12].
Elevation: Elevations ranges for dyer's woad were given for the following areas:
Elevation ranges for dyer's woad by geographic area Area Elevation range California <3,280 feet (<1,000 m) [31] Idaho 2,950-8,860 feet (899-2,700 m) [12] Nevada 4,500 to 7,000 feet (1,370-2,130 m) [40] Utah 4,000-7,000 feet (1,220-2,130 m) [59,82] Utah (Uinta Basin) from low elevations up to 8,500 feet (2,590 m) [28] Intermountain West 4,430-8,530 feet (1,350-2,600 m) [36]
Landforms and soils: Western rangelands invaded by dyer's woad typically occur on uplands, foothills, hillsides, and mountain valleys (review by [59]). A survey of dyer's woad in southeastern Idaho found that it occurred primarily on the east side of valleys, extending up canyons, and generally on south-facing, steep to flat slopes in full sun [12]. Infestations are frequently observed on steep hillsides in rugged, inaccessible mountain terrain (review by [19]). Dry foothill sites typically support native bunchgrass, sagebrush, and mountain brush communities [36,45,82] (see Habitat Types and Plant Communities). Dyer's woad is thought to be well suited to the dry, coarse, rocky soils on these sites (reviews by [2,19,59,80]) and is "a weed of dry places" in much of the Pacific Northwest [33]. Dyer's woad occurs on mesic (adequate moisture throughout most of season) and mesic-xeric (abundant moisture early in season, becoming drier later on) valleys in Montana [7,44]. In England, dyer's woad often occurs in old lime pits and chalk quarries (review by [80]) and is said to prefer alkaline soils on western rangelands (reviews by [59,80]).
Although many sources suggest that dyer's woad is well suited to coarse, rocky soils with low water-holding capacity (reviews by [2,19,59,80]), dyer's woad grew larger and had greater nitrate aquisition on a relatively moist site with fine soil textures than on a drier, coarse textured soil in a Utah field study (see table below) [48]. Differences in these variables were not related to proximity, life form, or diversity of neighboring plants (see Successional Status).
Mean values for several response variables in dyer's woad grown at 2 sites in northern Utah [48] Site Millville Hyde Park Soil description coarse-loamy over sandy or sandy-skeletal, mixed, superactive, mesic Calcic Haploxerolls fine, mixed, active, mesic, Aquic Argixerolls Shoot dry mass (g) 31.24* 84.88 Leaf nitrogen (mg/g) 33.38* 42.63 Leaf carbon:nitrogen ratio 11.50* 8.54 Root diameter (mm) 2.25* 2.93 Root dry mass (g) 1.69 2.52 Root length (m/soil core) 1.37 1.12 Specific root length (m/g) 1.07* 0.77 *Indicates a significant difference (P<0.001) between sites for that variable.
SUCCESSIONAL STATUS:
Dyer's woad typically occurs on open sites in full sun (see Site Characteristics). Dyer's woad may establish soon after disturbance and may persist under conditions of continued disturbance such as livestock grazing. Observations by Dewey (personal communication [15]) in northern Utah suggest that dyer's woad persists and spreads in early postfire succession (see Fire adaptations and plant response to fire). Reviews indicate a concern regarding the ability of dyer's woad to establish and persist in relatively undisturbed, dense plant communities as well (e.g., [56]), and suggest its establishment and persistence depend more on initial introduction of seed than on disturbance [63]. Evidence of some shade tolerance in dyer's woad [56] and its ability to establish in well vegetated areas [20,21,87] suggest that it may establish and persist in plant communities in late stages of succession.
Establishment in early succession: In a small-plot (1.5 × 1.5 m) experiment dyer's woad seedling establishment was consistently higher in disturbed than undisturbed plots regardless of growth form composition of plots. Plots were composed of 24 plants of either crested wheatgrass (Agropyron cristatum × A. desertorum), western yarrow (Achillea millefolium) or Wyoming big sagebrush (Artemisia tridentata var. wyomingensis), and were either left intact or disturbed by removing 4 plants from the center and lightly scarifying with a rake. Four hundred dyer's woad seeds were sown in each plot. Dyer's woad seedling density was 52% to 66% higher in disturbed plots than intact plots (P<0.01) [48]. According to Monaco and others [56], the ability of dyer's woad to establish on disturbed sites in early succession may be determined by its "colonizing ability", not its competitive ability for soil nitrogen (see below).
Persistence: Dyer's woad can establish and persist on many types of anthropogenically disturbed sites (see Site Characteristics), and commonly occurs on semi-arid rangelands with a long history of livestock grazing (e.g., [84]). A review by DiTomaso [16] lists dyer's woad among nonnative plants that tend to be avoided by livestock, which can favor a rapid shift in dominant species in grazed rangeland plant communities where these unpalatable plants occur. Another review by Parker [59] classified dyer's woad as an "invader" in terms of its response to grazing. Field studies in northern Utah [20,84] indicate that dyer's woad is readily grazed by domestic sheep prior to flowering; however, little damage is done to the plants (see Biological control).
Competition experiments on old fields in Utah suggest traits in dyer's woad that facilitate its persistence in disturbed, semiarid shrub-steppe ecosystems. In a greenhouse experiment, dyer's woad exhibited low plasticity in response to nitrogen availability, suggesting a low nitrogen requirement, low nitrogen productivity, or both. The authors note that these qualities are associated with the ability of a species to survive and persist under stressed, nutrient-poor conditions [56]. In a similar experiment, nitrate acquisition of dyer's woad was less than that of crested wheatgrass, greater than that of big sagebrush (P<0.01), and similar to that of western yarrow (P<0.01). Dyer's woad was less competitive for nitrate than cheatgrass, and similar to forage kochia (Kochia prostrata). These results suggest that superior competition for soil nitrogen is not the primary mechanism responsible for the dominance and proliferation of dyer's woad [48].
Young and Evans [87] suggest that perennial grasses seem to coexist moderately well with dyer's woad, perhaps due to differences in root systems, although the height, leaf size, and leaf arrangement of dyer's woad may give it an advantage in shading range grasses. Other researchers note that the periods of rapid growth by dyer's woad and peak water extraction by bluebunch wheatgrass may overlap on some sites in some years, suggesting there may be belowground interference between these co-occurring species [21].
Establishment and persistence in late succession: Evidence of dyer's woad's ability to invade established vegetation comes from field studies in Utah [20,21] and California [87]. In a "well-vegetated" area on a Utah rangeland that had not been grazed by livestock for several decades, dyer's woad established from seed sown by researchers [20,21]. In a study in northern California [87], dyer's woad established in annual grass communities considered "ecologically closed" [66]. These annual grasslands, dominated by medusahead or cheatgrass, were thought to represent a culmination of plant succession, and invasion and dominance by dyer's woad prompted an investigation into the mechanism allowing its establishment (see Allelopathy).
Results from small-plot experiments in Utah suggest that sites supporting a diversity of species or life forms may be more resistant to dyer's woad establishment than those dominated by single species or life form. Species used were a combination of native sagebrush-steppe species and nonnative species widely used for revegetation within sagebrush-steppe communities. Dyer's woad seedling establishment was consistently higher in single-species (western yarrow) forb plots than in 4-species forb plots, mixed life form plots (consisting of grasses, forbs and shrubs), or single-species shrub (Wyoming big sagebrush) plots. Dyer's woad establishment was consistently higher in 4-species shrub plots than 4-species forb plots. Dyer's woad establishment in single-species grass plots (crested wheatgrass) and 4-species grass plots was inconsistent between years [48].
Shade tolerance: While dyer's woad tends to occur on open, sunny sites (see Habitat Types and Plant Communities and Site Characteristics), it exhibits some degree of shade tolerance. Callihan and others [12] note dyer's woad occurrence in many types of plant communities in Idaho, including those dominated by trees and large shrubs. In the greenhouse, dyer's woad responded to increased shade through morphological modifications (increased leaf area, specific leaf area, and shoot:root ratio) to improve its light-harvesting ability. These responses may favor the ability to establish and persist on harsh, nutrient-poor sites as well as shaded, undisturbed sites. Dyer's woad also demonstrated morphological plasticity in response to variable water conditions, especially under shaded conditions. The authors suggest that high plasticity in heterogeneous environments may allow dyer's woad to establish and spread into new sites without the lag time required for local adaptation [56]. However, germination of dyer's woad seeds may be inhibited by shade [75].
Allelopathy: Laboratory studies suggest that dyer's woad fruits probably contain allelopathic substances [87], although the allelopathic chemicals have not been identified. In the laboratory, the presence of dyer's woad fruits inhibited germination of dyer's woad, tumble mustard (Sisymbrium altissimum), and alfalfa seeds; reduced root length in seedlings of dyer's woad, tumble mustard, medusahead, cheatgrass, and alfalfa; and reduced shoot length in seedlings of dyer's woad and tumble mustard. Germination and root length were also reduced for several species incubated on substrates treated with dyer's woad fruit leachate, as shown in the table below. Medusahead responded similarly, although data were not provided [87].
Mean percentage germination and root length of species incubated on substrates treated with dyer's woad woad fruit leachate [87] Species Dilution ratio* Germination
(%)** Root length
(mm)** Tumble mustard 1:0 0 -- 1:1 0 -- 1:5 16c 24a 1:10 64b 24a Control 100a 10b Dyers woad seed 1:0 0 -- 1:1 35d 24a 1:5 68c 20a 1:10 84b 25a Control 100a 25a Alfalfa 1:0 56b 3c 1:1 88a 15b 1:5 94a 20a 1:10 90a 35a Control 96a 35a Cheatgrass 1:0 28c 2c 1:1 64b 5c 1:5 88a 20b 1:10 96a 25a Control 92a 30a *The ratio is the volume of water extract of dyer's woad fruit to the volume of water as diluent.
**Means followed by different letters in the same column for the same species are significantly different at P=0.01.
Because dyer's woad produces a large number of fruits, and these fruits seem to suppress germination of associated species, successional trajectories may be altered in communities dominated by dyer's woad, with dyer's woad maintaining dominance by reducing establishment of other species. As a biennial or short-lived perennial, dyer's woad does not have to establish seedlings every year to maintain dominance in annual communities. The researchers noted, however, that some annual grasses established in dyer's woad stands in the field [87].
