Determine whether you have perennial or annual sow thistle in your yard. Scot’s thistle (Onopordum acanthium) Not known to exist in Clackamas County; Biennial (sometimes annual or short-lived perennial) The whole plant is very branched, strongly spiny, with a bluish gray appearance; The stem is woody and very spiny-winged (flat leaf projections extend down stem) Flower heads are purple and 1-3 inches wide Light infestations of perennial sow thistle cause yield losses in field … Threshold/Yield Loss. Recognizable features: thistle and bull thistle have been listed as noxious weeds only in New Mexico. Classification: Noxious Weed. As for the sow thistle’s botanical name: Sonchus is the ancient Greek name for the plant and means “hollow” referring to the plant’s hollow stem, a point of identification. Although grazing animals (and butterflies) actually prefer the Sonchus to grass farmers rant about the plant because it’s a weed amongst their crop. Annual sow thistle is very similar but reproduces only by seed and its flowers are smaller. Common sow thistle is in the Compositae (Asteraceae) family. Perennial sow thistle typically occurs in patches. Native to west-ern Asia and Europe and was probably intro-duced as a seed contaminant. Perennial Sow Thistle is a perennial that is commonly confused with Annual Sow-Thistle and Prickly Lettuce. A perennial plant grows for more than one season. This field guide serves as the U.S. Forest Service’s recommendations for management of annual and biennial invasive thistles in forests, woodlands, and rangelands associated with its Southwestern Region. Sonchus oleraceus, with many common names including common sowthistle, sow thistle, smooth sow thistle, annual sow thistle, hare's colwort, hare's thistle, milky tassel, milk thistle, soft thistle, or swinies, is a plant in the dandelion tribe within the daisy family.It is native to Europe and western Asia.. Its specific epithet oleraceus means "vegetable/herbal". This is a nutritious plant that contains several minerals (calcium, magnesium, iron, phosphorus, sodium, potassium and zinc) and vitamins ( A, B1, B2, B3, B6, & C). Smooth Sow-thistle has pointed leaf auricles and Rough Sow-thistle has prickly leaves. Perennial weeds grow from root stalks year after year. Examine the leaves of the plant: perennial sow thistle leaves are alternate and have toothed margins, while annual sow thistle leaves fully clasp the main stem, with either rounded or flat spiny lobes. Both Rough Sow-thistle and Smooth Sow-thistle are similar, but Perennial Sow-thistle has larger, more showy and deeper yellow flowers. Perennial weeds spread by seeds and roots, and therefore the entire plant must be destroyed. Some perennials are considered “tender perennials’ and won’t survive a cold winter; those are the vegetables most of us plant fresh each year, treating them as an annual. 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