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Species
Centaurea diffusa Lam.
IUCN
NCBI
EOL Text
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More info for the terms: cover, root crown, top-kill, vernalization
Diffuse knapweed reproduces by seed and is generally biennial. The plants begin as low rosettes and may remain in this form for 1 to several years, until they reach a critical size (e.g., root crown diameter [80]), or stage of development (e.g., leaf number). At this point they respond to vernalization (low temperatures) by bolting, flowering, setting seed, and dying. Thus, they may behave as annuals, biennials, or short-lived semelparous perennials [116].
Pollination: Diffuse knapweed is primarily insect pollinated [39,129]. Honeybees, bumble bees, and digger bees are frequent diffuse knapweed flower visitors [39]. Fertilization in diffuse knapweed requires cross-pollination between flowers on different plants. This can limit the reproductive success of isolated individuals, but it also promotes genetic diversity and may thereby improve competitive ability. Watson and Renney [128] reported that diffuse knapweed is self-compatible, but the results of Harrod and Taylor [39] refute this assertion.
Seed production: Average seed production by diffuse knapweed is reported by area as follows:
Location | Flowerheads per plant | Seeds per flowerhead | Seeds per plant | Seeds per m² | Reference |
British Columbia - rangeland | 74 | 12 | 925 | not reported | |
British Columbia - irrigated | 1,404 | 13 | 18,248 | not reported | [128] |
Washington state - averaged across diverse sites and years | 89 | 13 | 1,157 | 26,400 | [93] |
Considerable variation in seed production was observed between sites and years in northeastern Washington; nonetheless, seed production was estimated to be 1,000 times what would be necessary to maintain observed levels of infestation [93].
Seed dispersal: Dispersal of diffuse knapweed seed is mainly by wind. Seeds usually remain in the flowerheads after they mature and break away from the receptacle. Dispersal in the vicinity of the parent plant is facilitated by horizontally placed seedheads that open at the top and release seeds as dehydration occurs and plants sway in the wind. Dispersal over longer distances occurs when plants are broken off at ground level and tumble in the wind, dispensing seeds individually from the small opening at the top of the seedheads [128]. This technique adapts well to "hitchhiking" on the frames of vehicles and colonizing the bare shoulders of roads. Seeds may also be transported in mud adhering to vehicles or shoes [129]. Plants bearing seeds may also be carried in rivers and irrigation systems, thus colonizing the banks of waterways [85]. In British Columbia, logging trucks, off-road vehicles, and trail bikes have greatly contributed to the spread of both spotted and diffuse knapweed [112].
Seed banking: Evidence suggests that diffuse knapweed seed germination is distributed over time [69]. This may be considered as evidence of seed banking, although the length of time that diffuse knapweed seeds remain viable in the soil seed bank is undetermined.
Viability: Seed viability information for diffuse knapweed is scarce. In British Columbia, diffuse knapweed seed removed from seedheads at maturity exhibited 40% germination; 25-day-old seed stored under dry conditions exhibited 68% germination; and seeds that overwintered in the seedhead under field conditions exhibited 88% germination [128]. Watson [129] reports successful (93-95%) laboratory germination of 20-month-old seed stored at room temperature [138]. Another laboratory germination test showed greater than 95% viability of diffuse knapweed seeds, although germination values observed in the field were <70% [93].
Germination: Diffuse knapweed seeds germinate in spring (May/June) or late summer/early fall (August/September), and develop into low-lying, tap-rooted rosettes given sufficient moisture [11,68,128]. Diffuse knapweed seeds germinated under a wide range of environmental conditions simulated in the laboratory. Germination of over 80% occurred between 55 and 82 °F (13-28 °C) at optimum moisture levels [128]. Diffuse knapweed seeds require more than 55% soil moisture to initiate germination, with optimum emergence between 65% and 70% [107]. Diffuse knapweed seeds germinate best on the soil surface, with emergence rate decreasing as seeding depth increases, and little to no emergence below 1 inch (2.5 cm) [107,128]. Spears and others [107] found diffuse knapweed seeds germinated equally well over the range of 0 to 100% canopy cover. Nolan and Upadyaya [69] observed 3 types of germination behavior in diffuse knapweed with respect to light conditions: nondormant seeds that can germinate in the dark; light-sensitive seeds that germinate after exposure to red light; and light-insensitive dormant seeds that do not respond to exposure to red light. They concluded that bunchgrass rangeland and other open canopy conditions provide favorable light conditions for diffuse knapweed seed germination.
Seedling establishment: Stannard [108] reports high seedling vigor in diffuse knapweed, while Myers and Berube [68] indicate that the greatest mortality in diffuse knapweed occurs between the seedling and rosette stages. Seedling mortality is highest during the summer and is largely related to moisture availability [68,80,93,128]. Once diffuse knapweed seedlings establish as rosettes, they become very drought resistant [11]. Crowding of plants is also a factor in seedling and juvenile mortality, with mortality rates highest among the smallest rosettes, and declining with size under crowded conditions. The proportion of diffuse knapweed plants that flower and produce seed each year (and subsequently die) increases with available growing space [80]. Additionally, Powell [80] observed that the majority of diffuse knapweed rosettes that died during the summer at 1 site in British Columbia were infested with the larvae of the introduced biocontrol beetle Sphenoptera jugoslavica, and suggested that the damage caused by the larvae increased the susceptibility of rosettes to interference-related mortality. Amount and pattern of rainfall are important for diffuse knapweed survival. Wetting increases germination, but when followed by drying (e.g., the wet-dry cycle occurring with the summer convective storms in British Columbia ranges) seedling survival is diminished [11]. High spring precipitation appears to favor diffuse knapweed seedling establishment [101].
Asexual regeneration: Diffuse knapweed can sprout from the root crown after top-kill [85,128]. Other methods of asexual regeneration are not known to occur in diffuse knapweed [128].
The currently accepted scientific name for diffuse knapweed is Centaurea diffusa
Lam. (Asteraceae) [18,30,42,51]. Hybridization with spotted knapweed (Centaurea stoebe ssp. micranthos) is
reported in at least 7 U.S. states [70], including Michigan [126]. The
hybrid is named Centaurea × psammogena Gayer [70]. For more information, see the Centaurea website.
More info for the terms: allelopathy, forb, interference
Diffuse knapweed is a nonnative, annual, biennial, or short-lived perennial forb [22]. During the juvenile stage, diffuse knapweed is a rosette with deeply divided basal leaves borne on short stalks on a central crown with a taproot. At maturity diffuse knapweed produces 1 upright stem, rarely 2. Stems are 1 to 3 feet (0.3-1 m) tall, with numerous, spreading branches that give the plant a ball-shaped appearance and tumble-weed mobility when broken off. Stem leaves on diffuse knapweed are stalkless, getting smaller and less divided higher up the stem. Flowerheads are solitary or borne in clusters of 2 or 3 at the ends of branches. Diffuse knapweed flowerheads are 3 to 6 mm in diameter and 8 to 11 mm long, excluding spines and flowers. Bracts are edged with a fringe of spines, with a longer 1.5 to 4 mm, erect spine at the tip. Seeds are achenes, 2 to 3 mm long with a plume of bristle-like hairs that vary from scalelike to 1/8 the length of the seed [18,85].
Once it is established diffuse knapweed can form monotypic stands. The competitive ability of diffuse knapweed has been attributed to its being adept at depleting soil moisture [108], to allelopathy [14,15,26], and to other competitive or interference mechanisms [14,15].
More info on this topic.
More info for the term: hemicryptophyte
RAUNKIAER [82] LIFE FORM:
Hemicryptophyte
diffuse knapweed
white knapweed
tumble knapweed
Annuals or perennials, 20–80 cm. Stems 1–several, much-branched throughout, puberulent and ± gray tomentose. Leaves hispidulous and ± short-tomentose; basal and proximal cauline petiolate, often absent at anthesis, blades 10–20 cm, margins bipinnately dissected into narrow lobes; mid cauline sessile, bipinnately dissected; distal much smaller, entire or pinnately lobed. Heads disciform, in open paniculiform arrays. Involucres narrowly ovoid or cylindric, 10–13 × 3–5 mm. Principal phyllaries: bodies pale green, ovate to lanceolate, glabrous or finely tomentose, with a few prominent parallel veins, margins and erect appendages fringed with slender stramineous spines, each phyllary tipped by spine 1–3 mm. Inner phyllaries lanceolate, ± acute, appendage lacerate or spine-tipped. Florets 25–35; corollas cream white (rarely pink or pale purple), those of sterile florets 12–13 mm, slender, inconspicuous, those of fertile florets 12–13 mm. Cypselae dark brown, ca. 2–3 mm; pappi 0 or less than 0.5 mm, only rudimentary. 2n = 18, 36.
License | http://creativecommons.org/licenses/by-nc-sa/3.0/ |
Rights holder/Author | eFloras.org Copyright © Missouri Botanical Garden |
Source | http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=200023635 |
More info for the term: forb
Forb
Acosta diffusa (Lamarck) Soják
License | http://creativecommons.org/licenses/by-nc-sa/3.0/ |
Rights holder/Author | eFloras.org Copyright © Missouri Botanical Garden |
Source | http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=200023635 |
More info on this topic.
More info for the terms: density, phenology
The phenology of individual diffuse knapweed plants is highly variable and dependent on climatic and site conditions such as moisture, temperature, and plant density. Diffuse knapweed seedlings usually emerge under favorable conditions in spring and/or fall and develop into rosettes. Most root development occurs during this stage [128]. Rosettes respond to a period of cold temperature (vernalization) and bolt when a critical size or stage of development is reached [80,116]. Diffuse knapweed plants that complete their juvenile growth by fall and overwinter as rosettes usually bolt in early May. Plants that do not complete the juvenile stage by the end of fall remain as rosettes through the second year and may bolt during the third year. Flower buds are formed in early June. Flowering occurs July through September [18,128] or later as permitted by adequate moisture and mild temperatures [85]. Mature seeds are usually formed by mid-August, followed by death of the plant. Dead plants break off at ground level in the spring and tumble with the wind, spreading seed as they roll [128].
Powell [80] found that under conditions of severe crowding, a plant might not flower for 5 or more years. Other exceptions include plants that germinate in early spring that may flower the first year (<1% observed) [93], and plants that continue to grow after producing seed to flower again the following year, behaving as short-lived perennials [85].