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

GENERAL BOTANICAL CHARACTERISTICS:

Spotted knapweed has a deep, stout taproot and basal rosette leaves that grow up to 8 inches (20 cm) long and 2 inches (5 cm) wide. Beginning usually the 2nd year, each spotted knapweed plant produces 1 to 6 flowering stems (15 or more on older plants) that stand 8 inches to 4 feet (0.2 - 1.2 m) tall. Leaves are alternate and grow smaller near the tops of the stems. Stems branch in the upper half and bear terminal flowerheads that are 0.25 inch (6 mm) wide and 0.5 inch (12 mm) long. Flowerheads are solitary or borne in clusters of 2 or 3, with 25-30 flowers per head, and up to 60 flowering heads per plant [221]. Achenes are oval, 1/8 inch (3 mm) long, bearing a pappus of simple bristles which are less than the length of the seed (~1-2 mm) and persistent [176]. Most stems remain erect after drying, with leaves and flowerhead bracts attached [87].

Early and deep fine root development, and the colonization of spotted knapweed roots by arbuscular mycorrhizal fungi, may contribute to its competitive dominance over native grasses [118,119] by allowing for greater resource acquisition. Chemical allelopathy of spotted and diffuse knapweeds has been suggested as a mechanism of interference of the growth of other herbaceous species [19,88,113].

RAUNKIAER [150] LIFE FORM:

REGENERATION PROCESSES:

Flowering and pollination: Spotted knapweed plants may remain in the rosette stage for 1 to 4 years, producing flowering stems the 2nd year [221] or later [16,207]. Flowering during the year of seedling emergence is rare [168]. Boggs and Story [16] observed the percentage of flowering plants increasing with age up to 5 to 7 years, with little or no flowering in the 1st and 2nd years, in Montana. Flowers are pollinated by insects [221], and spotted knapweed is heavily visited by several species of bees [35,64,221]. Large pollen counts in late July and early August in the Missoula Valley, Montana, suggest that spotted knapweed is also wind pollinated [44]. Fertilization in spotted knapweed requires cross-pollination between flowers on different plants (obligately xenogamous). This can limit the reproductive success of isolated individuals, but it also promotes genetic diversity, and may thereby improve competitive ability [64].

Seed production: The number of seeds produced by an individual knapweed plant or a population of plants is highly variable between plants, sites, and years. Reported averages of seed production per plant range from about 65 seeds per plant in Montana [73] to 400-900 seeds under range conditions in British Columbia [221], and about 2,000 seeds per plant, averaged across diverse sites in Washington and Idaho [168]. A population of plants may produce about 5,000 to 40,000 seeds/m²/year [176,207]. Schirman [168] reported an average of 23.8 to 61.1 flowerstems per m², 11.2 to 16.8 seedheads per stem, and 24.3 to 33 seeds per head, producing 11,300 to 29,600 seeds per m² in Washington and Idaho. Similar stem and seed densities were observed in Glacier National Park, Montana [207]. Only about 0.1% of the seed produced under these conditions would be needed to maintain the size of the stands observed [168].

The following data were collected on Idaho fescue-bluebunch wheatgrass habitat types in Montana [73]:

	adults/m2	seeds/plant	% viability	total # seeds/m2	viable seeds/m2
Site 1-1994	99	21	60%	2062	1237
Site 1-1995	10	140	71%	1405	998
Site 1-1996	54	61	79%	3312	2617
Site 2-1994	177	62	71%	11,007	7815
Site 2-1995	151	55	82%	8519	6986
Site 2-1996	89	51	78%	4524	3529

The number of seeds produced may vary with site conditions (available moisture, nutrient availability and competition), herbivory and seed predation, and age of plants. Site conditions and precipitation during the growing season probably have the greatest effect on the number of seeds produced each year, with more seeds produced during wet years [168,176] and on wetter sites [73]. Schirman [168] reported a reduction in the number of viable seeds per head in dry years and an increase in the number of flowerheads per stem in wet years. On an irrigated site, spotted knapweed produced an average of 25,260 seeds per plant, compared to about 680 seeds per plant under range conditions in British Columbia [221]. Adult density also appears to be sensitive to spring precipitation [73]. Maternal treatment (control, herbivory, herbivory + nutrient shortage, and herbivory +nutrient shortage + grass competition) affected physiology, morphology, growth, and size of maternal plants and the numbers of seeds produced, but did not affect the mass or quality of seeds and offspring produced [223]. In Glacier National Park, seed production is 3 to 4 times higher in plants immediately adjacent to the road than in plants on adjacent grassland [207].

Seed predation may be an important factor in seed production. The seed production numbers reported by Schirman [168] in 1973 through 1976 were measured before the introduction of Urophora affinis and U. quadrifasciata. Jacobs and Sheley [73] reported in 1998 that 90% of the seedheads collected in their study were infested with larva of these insects. Shirman [168] consistently found 2 to 10 times the seed production in his study, compared with Jacobs and Sheley [73]. Differences in seed production may be attributable to differences in precipitation and/or seed predation between sites and years [73]. Harris [62] reports similar findings and notes that prior to the release of biocontrol agents in British Columbia, Watson and Renney [221] reported an average of 26.6 seeds per head, whereas in 1986 Harris [62] observed an average of 15.4 seeds/head.

Seed dispersal: As soon as bracts open, any movement of the stem (e.g. by wind or passing animals) expels the loosely held seeds from the head with a flicking action. The seeds usually land within 3 to 4 feet (0.9-1.2 m) of the parent plant [132,175,221]. In this way, spotted knapweed populations spread outward and downwind from the perimeter of existing stands [161,176,221]. Dispersal of achenes over long distances is facilitated by animals and birds. Wallander and others [220] show that both domestic sheep and mule deer excrete viable seeds of spotted knapweed in their feces for 7 to 10 days after consumption, respectively. Seeds mixed with soil and mud may be carried by vehicles or other equipment that, in turn, create an ideal seedbed for spotted knapweed establishment [161,176,221]. Spread of seeds on logging trucks, off-road vehicles, and trail bikes has contributed greatly to the spread of knapweed into new areas in British Columbia [196]. Spotted knapweed seeds can also be transported in rivers and other watercourses, and in crop seed and hay [175].

Seed banking: Spotted knapweed seeds are known for their longevity and durability. They have a thick, durable pericarp that protects the seeds but does not restrict water imbibition or water loss [30]. Nolan and Upadhyaya [132] determined that a portion of the spotted knapweed seed population requires exposure to red light to germinate. This light requirement may permit buried seeds to remain dormant for an extended period of time [29,30]. Experimental evidence indicates that spotted knapweed seeds can remain viable but dormant after 5 years of burial [29]. Perez and others [147] observed evidence of seed banking of spotted knapweed in the Nebraska sandhills prairie.

The following data were collected on 2 Idaho fescue-bluebunch wheatgrass habitat types near Bozeman, Montana [73]:

When seed production was controlled with herbicide treatments at 2 other heavily infested spotted knapweed sites in Montana, the spotted knapweed seed bank decreased by 72 to 81% after 15 months [23]. After 7 years, only 5% of the original seed bank remained, leaving about 160,000 viable seeds per acre (400,000 seeds/ha, or about 4 viable seeds per 0.09 m²) [29,30]. Seedlings showed a decline in vigor associated with length of burial [23].

Germination: Spotted knapweed seeds have the potential for germination shortly after maturity, and approximately 90% are viable upon dispersal [34,168]. However, many studies indicate a dormancy period for some portion of the annual seed crop. For example, Watson and Renney [221] observed an increase in germination from 40 to 80% after 25 days of dry storage. Similarly, reports of low germination and emergence under field conditions range from 65 to 85% of the seed crop ungerminated but viable the year following dispersal [23,34,168]. This period of dormancy may be released by seed aging, cool-moist stratification, freezing [34], or exposure to red light [132]. Nolan and Upadyaya [132] describe 3 distinct types of germination behavior in spotted knapweed seed, a phenomenon that distributes seed germination over time by facilitating the incorporation of seeds into a seed bank.

Spotted knapweed seeds germinate whenever moisture and temperature are suitable, and both fall and spring seedling emergence is common. Optimal temperatures for germination range from 45 to 93 degrees Fahrenheit (7-34 °C), and germination is best at 66 degrees Fahrenheit (19 °C) [221]. Spotted knapweed seeds required at least 55% soil moisture to initiate emergence. Germination increases with increased soil moisture, and 65-70% soil moisture content is optimum for germination [183]. Dormancy may prevent germination at higher temperatures when soil moisture status is fluctuating, and at lower temperatures when germination in late fall may make seedlings susceptible to winter kill. Germination after cold stratification provides a strategy for spring emergence and avoidance of environmental extremes [34]. Canopy cover had no effect on emergence rate, with spotted knapweed germinating equally well over a range from 0-100% canopy cover, as simulated in a laboratory experiment [183]. Spotted knapweed seeds can germinate in light and dark, with maximum germination in alternating light and dark periods. Optimum germination occurs with the seeds at the soil surface, and decreases with depth, with little germination below 2 inches (5 cm) [183,221]. Because of the bare interspace areas between plants and seasonal periods of drought, bunchgrass rangeland provides favorable microsites for spotted knapweed seed germination [132].

Seedling survival: Spotted knapweed seedling survival depends primarily on environmental conditions at the time of emergence and establishment. Seed weight was positively correlated with initial growth, but its influence decreased over time and disappeared after 8 weeks. Competition (with meadow ryegrass (Lolium pratense)) did not influence growth of spotted knapweed seedlings during early weeks, but strongly suppressed growth after 9 weeks [223]. Spotted knapweed seedling survival is poor when conditions are dry following emergence [221], and survival is enhanced if precipitation coincides with the time of seedling emergence [168]. Seedling mortality averages 12%, but can be as high as 55% under dry conditions [176,221]. Seedlings emerging in April in Idaho and Washington had a high rate of survival, with most plants flowering the following growing season. Seedlings emerging after May 15 had a very low survival rate and almost no flowerstem production the following season [168].

SITE CHARACTERISTICS:

Spotted knapweed is found on soils with a wide range of chemical and physical properties [25,221], and often on poor soils [217]. It does especially well in coarse-textured soils [123,164] that are well-drained with low water holding capacity [57]. Spotted knapweed is well adapted to Montana rangelands with "light-textured" soils that receive summer rainfall. In northeastern Washington, it is usually found on glacial till and outwash soils [161]. Spotted knapweed is poorly adapted to irrigated pastures where saturated soil is common [30,221], and does not compete well with vigorously growing grass in moist sites [60]. However, in central Washington spotted knapweed thrives in irrigated land, although it is also found growing on compacted soil in a 10 inch (254 mm) precipitation zone [161].

Spotted knapweed has been observed at elevations ranging from 1,900 to 9,975 feet (578-3,040 m) and in precipitation zones ranging from 7 to 79 inches (200-2000 mm) [97]. Some general ranges by area are as follows:

In Montana, spotted knapweed is most abundant between 4,000 and 6,000 feet (1,219-1,829 m) in elevation and 10 to 80 inches (250-2,030 mm) precipitation and in areas with 50-120 frost-free days [4,25]. In British Columbia, spotted knapweed is more common below 2,950 feet (900 m) on south-facing slopes [124].

Spotted knapweed not only readily occupies disturbed sites, but it also invades relatively undisturbed perennial native plant communities in the northern Intermountain region [31,102,175,207,208], and invades wilderness areas all over Montana [95]. In Glacier National Park, spotted knapweed colonized undisturbed rough fescue grasslands adjacent to roadside spotted knapweed infestations [207]. The frequent breaks in cover, which provide favorable light conditions for seed germination, and the warm-dry climate of these grassland ecosystems appear to facilitate invasion of spotted knapweed and other alien species [132,207].

SUCCESSIONAL STATUS:

Hironaka [69] presents a replacement series of weedy species in the Intermountain Region in which summer annuals are replaced by winter annuals (e.g. Russian thistle (Salsola kali) is replaced by cheatgrass), and the earlier winter annuals are replaced by the later maturing ones (e.g. cheatgrass is replaced by medusahead (Taeniatherum caput-medusae) or knapweeds (Centaurea spp.)).

SEASONAL DEVELOPMENT:

Spotted knapweed overwinters as rosettes or as seeds. Rosette mortality may occur under extreme winter conditions. In the Intermountain West, rosettes that overwinter bolt in early May [176,221]. Seeds that overwinter germinate in early spring. Early spring growth gives spotted knapweed a competitive advantage over many natives for soil moisture and mineral nutrients [110]. Flowering buds are formed in early June and flowering occurs from July through September. Mature seeds are formed by mid-August. Spotted knapweed flowerheads that are not infested with larvae open after they dry, about 2 to 3 weeks after seeds mature, usually in late summer [161], from mid-August through mid-September [34,176]. Most seeds are shed upon maturity; very few overwinter in seedheads [176]. At 2 sites in Montana, seed rain occurred during August and September, with 30 to 62% of the seeds produced reaching the soil surface by October [73]. Rosettes that do not bolt die back to the root crown, which serves as the perennating part of the plant over winter [175]. 

In Virginia, flowering of spotted knapweed occurs from June through August or even in November, and fruiting occurs from mid-June to November [120]. In the Adirondacks, spotted knapweed flowers 19 July to 20 August [93].

Related categories for SPECIES: Centaurea stoebe ssp. micranthos | Spotted Knapweed