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

GENERAL BOTANICAL CHARACTERISTICS:

Once it is established, the highly competitive 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].

RAUNKIAER [82] LIFE FORM:

REGENERATION PROCESSES:

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:

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 degrees Fahrenheit (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].

SITE CHARACTERISTICS:

Diffuse knapweed is tolerant of a wide range of total precipitation and temperature conditions but does best in semiarid and arid conditions, and is most competitive in areas receiving between 12 and 17 inches (305-432 mm) of annual precipitation [11,36,87,128]. Diffuse knapweed is susceptible to flooded or waterlogged conditions, and infestations stop abruptly with an increase in soil moisture near temporary and permanent streams. Irrigation can also eliminate diffuse knapweed [68]. Diffuse knapweed is not competitive in moist microsites such as gullies, depressions, and poorly drained soils [11,85]. Diffuse knapweed seeds require conditions near field capacity for at least 4 days to begin germination. Seedling root growth in diffuse knapweed may be sensitive to saline conditions (electrical conductivity of 4 dS m^-1). Seedling root growth may also be sensitive to and germination delayed by water stress (osmotic potential of -0.5 mP or less) [45].

In eastern Washington, diffuse knapweed grows on all aspects and slope positions, from flat to over 60% [85,87,115]. In the Gilpin range, British Columbia, knapweed is commonly found on south-facing slopes below 3,000 feet (900 m) [63]. The following table provides some elevation and precipitation ranges as reported by state or province:

Location	Elevation range	Annual precipitation range	Mean annual temperature range	References
Arizona	up to 7,000 ft (2,134 m)	----	----	[138]
Washington	0-5,000 ft (0-1,500 m)	6-35 inches (150-900 mm)	----	[85,115]
British Columbia	500-3,000 ft (150-900 m)	----	45-49 oF (7-9 °C)	[128,129]

Diffuse knapweed is commonly found on well-drained soils such as sandy or gravelly loams or loamy fine sands, with coarse fragments from 0 to over 80% [86,87]. It is less competitive on shallow soils (< 15 inches (38 cm) deep) and very coarse textured soils such as sand or loamy coarse sand, although it may thrive on these sites when disturbance removes other vegetation [87]. It grows best on fertile, well-watered Cryoborolls, mesic Argiudolls, and mesic Hapludolls in open and uncultivated sites with summer drought [36,99].

SUCCESSIONAL STATUS:

SEASONAL DEVELOPMENT:

Related categories for SPECIES: Centaurea diffusa | Diffuse Knapweed