• Thistle is a fast-growing, invasive weed.thistle image by Laur from Fotolia.com
  
  Thistle plants are aggressive perennial and biennial plants native to Europe, Asia and Africa. Plants belonging to the thistle family are considered weeds due to their rapid growth habit and ability to easily invade and overrun disturbed and overused land. Thistle competes with desired plant species for nutrients and turns forests and fertile pastures into non-usable areas. Thistle seed production and seedling establishment is greatest in cool and wet weather.
  
  

Bull Thistle

• Bull thistle (Cirsium vulgare) is a biennial thistle variety with long foliage. The lobe and leaf terminal is tipped with stiff, elongated spines and the upper and lower leaf surface is covered with fine hair. The plant blooms with pink to purple flowers consisting of spine tipped bracts. The flowers are up to 2 inches in diameter and have web-like hair at their base. Bull thistle is very common in roadsides and pastures and occurs occasionally in cultivated fields as well.
  
  

Canada Thistle

• Canada thistle (Cirsium arvense) is a perennial thistle that grows both from seed and rhizomes. This dual spreading method of the plant leads to the rapid establishment of large plant colonies. Canada thistle is characterized by wavy, lobed foliage, with the lobes getting more triangular in shape as the plants mature. The upper leaf surface is smooth with occasionally hairy undersides. Of all the thistle varieties, the smallest flowers are found on Canada thistle. The pink, purple or white flowers grow in terminal clusters.
  
  

Musk Thistle

• Musk thistle (Carduus nutans) is a biennial thistle also referred to as the nodding thistle due to the large, bending, single flowers of the plant. Younger musk thistle plants have light green, waxy foliage with shallow lobed margins and irregular prickles. As plants mature, the leaves develop deep cuts and sharp, stiff spines. The leaf tissue is glossy with hair on the main veins. The flowers are up to 2 inches in diameter, purple to pink in color and consist of a large number of small spine-tipped bracts. Musk thistle is a noxious weed found most often in pastures and along roadsides.
  
  

Tall Thistle

• Tall thistle (Cirsium altissimum) is a biennial thistle with a variety of foliage ranging from lobed to unlobed with small, forward pointing spines on the leaf margins. The top leaf surface is dark green and the lower sides have dense, wool-like hair. Tall thistle blooms with small, 1 inch, pink to purple flowers. Another subspecies of tall thistle has white flowers too. Tall thistle is commonly seen in ditches, on roadsides and in waste areas.