Thistles are any of a large number of plants in the family Asteraceae occurring chiefly in Europe and Asia. Many species belong to the botanical groups Carduus, Carlina, Onopordon and Carbenia or Cnicus. Thistles often have prickles on the stems, leaves or flowerheads. They are invasive garden weeds, but some of them have medicinal properties.
Mediterranean Sow Thistle
Mediterranean sow thistle (Sonchus tenerrimus L.), also known as sow thistle or prickly sow thistle, is a common weed throughout Greece and occurs frequently on roadsides, meadows, groves, vineyards, cultivated or neglected lands. Often seen in pastures and crops, sow thistle rarely causes significant problems because cows and other grazing farm animals eat it. But in the garden, it can compete with other plants for sustenance.
Mediterranean sow thistle is a grayish annual measuring up to 6 feet tall. Elliptical to lanceolate leaves are thin with a soft spiny margin. The upper leaves clasp the stem with rounded basal lobes. The flower heads are golden-yellow, measuring up to 1 inch across.
Wind aids in pollinating the sow thistle by blowing its seeds. Each seed measures approximately one-10th of an inch and has a small parachute of hairs that can carry it over large distances in strong winds. Moving water transports seeds lying on the ground or floating on water.
Milk thistle (Silybum marianum), is an annual, winter annual and biennial herb indigenous to the Mediterranean region of Europe. It is a tall herb with spiky leaves and contains milk-like liquid. Milk thistle also grows naturally in California as well as some eastern regions of the United States.
The main stem is stout, ridged and branching. The overall plant size can range from 2 to 6 feet tall. The broad, lobed leaves measures 20 inches long and 10 inches wide. White patches or marbling found along the veins of the dark green leaves are characteristics. The leaves have sharp prickles (woody spines) on the margins. Prickles often occur all over the plant, including the stem and other flat parts of leaves. They help protect the plant against grazing animals.
The flower head of milk thistle has broad leathery bracts with stiff spines at the tip measuring 3/4ths to 2 inches long. Heavy, flat, smooth, shiny seeds measure 1/4th inch long, with color ranging from black to mottled brown. They have a tuft of minute barbed bristles, which fall off in a ring when the seeds mature.
Blessed thistle (Cnicus benedictus L.), also known as St. Benedict thistle, Holy thistle, spotted thistle, cardin and Carduus benedictus, is an annual that originated in southern Europe and has the reputation of curing many ills, including the plague.
The stem of the Blessed thistle is reddish, slender and branched measuring about 2 feet high. It can barely hold the weight of its leaves and flowerheads. The long, narrow leaves clasp the stem. Prominent pale veins appear on the leaves with irregular teeth of the wavy margin ending in spines. The flowers are pale yellow with green prickly heads. Each scale of the involucres (covering of the head) ends in a long, brown bristle. A thin down covers the whole plant.