Why are thistles illegal

Canada thistle requires a to hour photoperiod to bolt and flower, and growth ceases when temperatures exceed 85 degrees for extended periods. Canada thistle is considered to be naturalized in the northern Great Plains.

Canada thistle usually grows 2 to 3 feet tall and bears alternate, dark green leaves that vary in size Figure 4A. The leaves are oblong, usually deeply cut, and have spiny, toothed edges.

Canada thistle has been classified into several varieties. Within these varieties are many ecotypes that differ in growth characteristics, response to day length, and susceptibility to herbicides and cultivation.

For example, leaf shape, head structure, and the number and size of spines can differ with ecotype. Stem color also can differ from green to lavender. Flowering occurs from June to September. Male and female flowers are produced on different plants, so cross-pollination is necessary for seed production. Flowers produce from 40 to 80 seeds per head. The smooth, light brown seeds have a conical point and are loosely attached to a tannish pappus at the tip, which aids in seed dispersed by wind.

Seeds mature rapidly and are able to germinate within eight to 10 days after pollination. Canada thistle overwinters in the rosette growth stage Figure 4B. Canada thistle generally spreads from field to field in contaminated crop seed or forage.

Within a Canada thistle population, some seed remains in the flower head or falls at the base of the plant. The wind can disperse the remaining seed, either attached to the cottony pappus in late summer or later in the winter, when wind-driven snow moves it across fields. 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. However, ground litter is reduced, which results in uniform thistle regrowth and improved coverage with herbicides Figure 7B.

Herbicides applied to Canada thistle following a burn generally provide better long-term control, compared to treatments applied in very dense stands. Also, prescribed burns may stimulate growth of native species and discourage growth of invasive species, such as Canada thistle.

In spite of its reputation as a spreader, the thistle in my garden volunteers far too little for my liking. The thistles Matt propagates with his parents, Earth Sangha cofounders Lisa and Chris Bright, are among the exceptional natives he promotes. But these descriptions should be restricted to the invasive non-native thistles, like bull thistle Cirsium vulgare and Canada thistle Cirsium arvense ; they are the bane of agricultural growers and anyone with ecologically sensitive areas to maintain.

Our native thistles can be much more refined and are exponentially more valuable to wildlife. They certainly are no less beautiful than any of the other innumerable native plants we already covet. Of the plus ecotype species Earth Sangha grows, two are biennial thistles common to Northern Virginia and much of the Mid-Atlantic: the above-mentioned pasture thistle Cirsium pumilum and field thistle Cirsium discolor.

Butterfly and moth larvae, along with a host of other insect herbivores, feed on the pollen, nectar, leaves and seeds, and pollinators and other flower visitors are plentiful as well. Come fall and winter, thistle seeds are great forage for goldfinches and other seed-eating birds. So, which Cirsium species is Matt most fond of?

Thistles are members of the aster and sunflower family Asteraceae. Of the nine genera of thistles from the tribe Cardueae— Carduus, Carthamus , Carlina , Centaure , Cirsium , Cynara , Echinop , Onopordum and Silybum —the Cirsium genus comprises the true thistles, the most diverse and widespread of our native thistles. Thistles can be biennial, short-lived perennial or annual. Many monocarpic species are biennial, flowering in the second year, but other species can flower in their first year or take two to eight years to flower.

Both C. The introduced Canada thistle, Matt pointed out, can choke out all sorts of native vegetation as well as native thistles. Unlike most of our thistles, he said, Canada thistle is a perennial that forms clonal root networks.

Areas with high invasions can be reduced to dense monocultures that are difficult to control. Growing and observing thistles in your own garden will help you to identify them out in the field. Pasture thistle C. I often see one large flower; sometimes up to three but once there were twenty on a single plant!

Since thistles readily reseed in disturbed soils, a well-established garden may not be conducive to heavy self-sowing. Pasture thistle should be handled with thick gloves; even on a vertically-challenged thistle, the spines can be ruthless. The four who replied all said they loved the plant for its floral display and wildlife value.

I was delighted to learn that the front garden was where it is planted most. All three are identified by the bright purple flowering head, each with a different inflorescence.

Please remember to read and follow herbicide label instructions. For specific recommendations not covered by the label, contact your distributor or manufacturer representative. Managing thistle can be difficult.