Seedling emergence also can occur in fall. All seedlings grow into rosettes and overwinter in that stage. Rosettes are usually large and compact with a large, corky taproot that is hollow near the crown Figure 3. Leaves have consistent shape, sometimes expressing a frosted appearance around the leaf margins, and often have a cream-colored midrib Figure 4. Early in spring of the second year, overwintered rosettes resume growth. Shoots begin to elongate bolt in late March through May, depending on weather and elevation Figure 5.
Musk thistle flowers Figure 6 and starts to produce seed 45 to 55 days after it bolts. Musk thistle has very large bracts beneath flowers that are armed with sharp spines and shoots beneath flowers are almost devoid of leaves.
Musk thistle dies after it sets seed. It spends approximately 90 percent of its life cycle in a vegetative growth stage. Musk thistle is a prolific seed producer.
One plant can set up to 20, seeds. However, only one-third of the seeds are viable. Musk thistle produces many heads. The terminal, or tallest, shoots flower first, then lateral shoots develop in leaf axils.
A robust plant may produce or more flowering heads. Musk thistle flowers over a seven- to nine-week period. It begins to disseminate seed from a head about two weeks after it first blooms. It is common to observe musk thistle with heads in several stages of floral development and senescence.
Thus, musk thistle sets seed over an extended time period. Most seed is dispersed within the immediate vicinity of the parent plant. This leads to a clumped pattern of seedling development and results in intraspecific competition and mortality.
Wind and water are good dissemination methods and seeds are also spread by animals, farm machinery and other vehicles. Once the plant becomes established, roots are the most important means of propagation. Canada thistle has an extensive underground root system that may penetrate the soil to a depth of 10 feet or more and grow laterally 12 to 15 feet per year.
Root buds occur randomly along the roots and initiate new shoots whenever environmental conditions are favorable. Root segments as small as 0. Flodman thistle is a deep-rooted perennial that is native to North America and usually grows 3 to 4 feet tall Figure 5A. It occurs from Saskatchewan and Manitoba to Iowa and Colorado. Flodman thistle is more competitive than most other native species and has the potential to infest large areas.
It is tolerant to high salt concentration in soil but not as tolerant as Canada thistle. As with most thistle species, Flodman thistle grows best under moist conditions but it can survive under drought conditions, which gives it a competitive advantage on semiarid rangeland.
The leaves of Flodman thistle are shiny green on top, white and pubescent below, rigid and deeply lobed. The lobes stick out at nearly right angles flipping. A sticky secretion often is found on the small, oval-shaped heads, which attracts and catches insects Figure 5B. Flodman thistle usually flowers from mid-July through September in North Dakota.
The flower color ranges from red to violet and very rarely white. Figure 5B. Flodman thistle flower has a sticky secretion on the oval-shaped heads, which often attracts and catches insects.
Wavyleaf thistle is a perennial native plant that often is confused with Flodman thistle. Wavyleaf thistle tends to flower from July to September, which is a week or two earlier than Flodman thistle. Wavyleaf thistle tends to be more spiny and the leaves less deeply lobed than Flodman thistle Figure 6A.
Also, wavyleaf thistle is found in well-drained soils, generally in drier locations than Flodman thistle occupies. Wavyleaf thistle grows 3 to 4 feet tall and often is associated with sagebrush communities and rangeland, but is less common in moist meadows.
Figure 6A. Wavyleaf thistle is very pubescent and often gray, with leaves less divided than Flodman thistle. Wavyleaf thistle sometimes is called gray thistle because it has a white cast Figure 6B. The leaves are very pubescent, with long, wavy hairs on both upper and lower surfaces.
The large, globe-shaped heads contain glands that are tipped with strong, yellow spines. The flowers are most often pink or purple, but wavyleaf thistle has a white-flowered form, f. Figure 6B. Wavyleaf thistle flowers vary in color from lavender to pink, and stems are often white and very pubescent.
P revention is the best control method for both perennial and biennial thistles. Thistles often invade overused or disturbed land, such as cultivated fields. Plant weed-free seed to help prevent introduction into cropland, and keep field borders thistle-free. The best preventive measure in noncropland is to maintain a thick plant cover and reseed disturbed areas with a desirable species as soon as possible.
Proper grazing management and rotational grazing practices should be established and maintained to prevent thistle establishment in grazing land. Controlled and rotational grazing can prevent thistle establishment because overgrazing weakens desirable species, making the pasture more susceptible to invasion. Pastures protected from overgrazing have little thistle establishment. An adequate fertility program ensures a healthy and vigorous pasture with species competitive to thistle.
Avoid spreading thistle seed to uninfested areas with manure, mowers or other farm equipment. Establishing competitive grasses can reduce the size of rosettes and decrease thistle height, root weight and crown size. Once thistle invades an area, several control options are available, depending on the location and land use. Control options include cultural, mechanical, chemical, and biological methods.
Combining two or more control options in an integrated management program generally is better than relying on a single control method. Repeated mowing will reduce thistle infestations, especially if the plants are biennial.
Mow whenever the plants are in the early bud growth stage to prevent seed-set. Several mowings a year are needed because plant populations vary in maturity. Mow as close to the surface as possible. If plants are cut above the terminal bud before the stems elongate, they likely will regrow.
Mowing before the flowers start showing color is important because plants mowed after that likely will produce some viable seed. Mowing for several years will reduce the root vitality of the perennial species and will prevent seed production, reducing the seed reserve.
Mowing should be combined with a chemical control program for best results. Tillage is an effective method for perennial thistle control and will control biennial species completely. Rotations out of forage crops to annual crops will eliminate biennial thistle. Cultivate fields before the perennial species are 3 inches tall and repeat before regrowth reaches 3 inches tall until freeze-up.
Cultivation depletes the energy reserves of the root system and eventually will control an established stand. Persistence and proper timing are important for control. Fallowing and repeated cultivation for one or more seasons prevents crop production and may expose fields to serious soil erosion.
Integrating cultural, mechanical and chemical control practices into a single system is the preferred approach for perennial thistle control. Fire often is used for management of plant communities in North America, including management of invasive weeds such as Canada thistle Figure 7A. Canada thistle response to fire has been erratic, with infestations sometimes reduced or occasionally enhanced by fire. Bull thistle: a biennial that spends its first year as a basal rosette, followed by the second year reproductive phase.
Reproduces via wind-blown seed dispersal. Musk thistle : a biennial sometimes a winter annual that begins as a basal rosette followed by reproduction in the second year.
Figure 6. Bull thistle flowers. Pink to purple flowers begin in June and can last until October Fig. Bull thistle characteristics:. Cultivation is effective at controlling musk and bull thistle, but can actually increase the infestation of Canada thistle.
Mulching can help in reducing the germination percentage of wind-blown seeds. Preventing Canada thistle roots and rhizomes when bringing soil into a property is effective in preventing an infestation. Prevent importation and spread of root fragment and seeds.
Root fragments may get moved on cultivation equipment and seeds in contaminated crop seed, hay, straw, manure and irrigation water. Spear thistle does not usually persist in arable rotations or routinely cultivated soils, but is encouraged by fallows, grass breaks and perennial crops.
In contrast, the sow-thistle species are intolerant of grazing so do not persist in grassland but are more commonly encountered as arable weeds. Heavy infestations of creeping thistle in grassland is an indication of unbalanced grazing pressure, involving under-utilisation of the herbage during the growing period of the thistles combined with over-grazing during the winter and early spring.
The resulting sward tends to become open and start growth late, offering little competition to the emerging thistle shoots which then tend to be avoided by grazing stock. This situation is common on sheep farms.
Close stocking, preferably with cattle, or cutting for conservation at a young stage, combined with at least moderate fertiliser use, can reduce infestations progressively. Cutting, mowing or grazing thistles in grassland can progressively weaken plants, but cutting needs to be done as low as possible to remove all the leaves. The most effective timing is at the early flower bud stage. It must be repeated at least twice during the growing season for several years to have a permanent effect.
Pulling up plants is more effective than cutting. In grassland, minimising sward damage from trampling, poaching and uneven slurry application will prevent thistle seedlings establishing in the disturbed patches. Control of thistles is largely dependent on herbicides on most farms, so maintaining the availability and efficacy of a wide range of herbicides is essential.
Herbicides and resistance Chemical control of creeping thistle should be part of an integrated management approach which include, where feasible, cultivations, mowing or grazing, and competitive crops.
Creeping thistles generally have a varied response to herbicide treatment, as there is much variation within the species including the rooting system and growth habit. Clopyralid is the most effective selective herbicide available for controlling thistles and is approved for controlling creeping thistle and smooth sow-thistle in a wide range of arable and perennial crops and also in grassland.
It also controls mayweeds, but otherwise has a narrow weed control spectrum, so is often used in mixture with other herbicides to broaden the range of weeds controlled.
Herbicide application should occur when creeping thistle plants are actively growing, ideally in April or May, but recommendations vary with the specific crop. Efficacy will be reduced if cultivations are carried out soon after spraying. Clopyralid has been detected in water at levels that regularly exceed the EU Drinking Water Directive limit for individual active ingredients 0. This puts the UK at risk of non-compliance with Water Framework Directive objectives for drinking water catchments.
Appropriate measures should be taken to minimise the risk of clopyralid leaching to water. Although control of leaves and shoots is usually good, translocation of the herbicides to the roots of creeping thistle is less effective than with clopyralid, so overall control tends to be less effective and greater regrowth is likely after synthetic auxin treatment.
MCPB can be used for control of thistles in peas. Glyphosate can be used to control thistles non-selectively but will kill or severely damage other plants exposed to the spray.
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