Thistles have a very recognizable shape and texture. They generally begin as rosettes in the first season and produce a flowering stem the next year. The leaves are pointed and tipped with spines, and the stems have a hairy, prickly construction. Thistles produce flowers that are actually clusters of florets and are flanked by barbed bracts. The tenacious plants were mostly introduced from Europe and are not native to North America. They have successfully naturalized and are now found in most parts of the United States. Thistles may be perennial or annual.
Among the many species of thistle there are several that produce yellow flowers. Yellow thistle is a native plant that is endangered in Connecticut. The plant may grow from 1 to 3 feet tall with a large 3-inch-wide bloom. The flower is a dull yellow with green, hairy bracts that cover much of the bloom. Yellow Star thistle is a pest plant that is toxic to horses. It goes by several other names such as yellow cockspur and geeldissel. Yellow Star thistle is native to Eurasia and was introduced to the United States in the 1800s.
Wild lettuce resembles spiny sowthistle. It is a native plant that exudes white sap and has spines under the midribs of the leaves. Grounsel is an annual weed with serrated leaves that resemble thistle foliage. The yellow flower has soft multiple petals and is more daisylike in form. Knapweed is another yellow thistlelike plant that can grow 5 feet tall. The leaves are long and wavy edged without serration, but the flower looks like a bull thistle bloom, except it is a sunny yellow color.
Spiny sowthistle is a common weed in fields, pastures and disturbed soils. It has several styles of leaves, from oval mature leaves to very small lobed, lower leaves. The yellow flowers are slightly flatter than the usual globe- or oval-shaped thistle form. There is also a similar prickly sowthistle that looks like a cross between a dandelion and a thistle. The plants start as rosettes that look like the thistle form and become 2 feet tall. They are annuals without significant taproots, which makes them easy to remove.