Thompson's desert parsley, also known as Thompson's lomatium, is a perennial wildflower with several to numerous stems arising 50-100 cm high from a branched base atop a woody taproot. The stems are often covered with numerous small, coarse, stiff hairs while the leaves are often somewhat roughened in texture to somewhat glabrous. The large leaves are all basal or found low on the stems. They are ternate-pinnately divided into numerous small, linear segments from 1-2 cm long and up to 1-2 mm wide.
The inflorescence is an umbel of 5-14 rays elongating unequally from 5-10 cm long in fruit. The base of each umbellet is subtended by several bracts while bracts at the base of the umbel are generally lacking. The pedicels in fruit measure 8-15 mm long and the flowers are yellowish. The young fruits are covered with tiny hairs and become glabrous with maturity. The large fruits are elliptic-oblong in outline with well developed wings up to one-half as wide as the body of the fruit and several narrowly raised ribs running the length of the body. The fruits measure 16-28 mm long. Similar species include Lomatium suksdorfii which is found one hundred or so miles south and Lomatium dissectum.
Thompson's desert parsley may be found on open to wooded slopes in the foothills to moderate elevations in the mountains.
Thompson's desert parsley is endemic to the region around Wenatchee,WA.