Fool's Parsley is an introduced, naturalized annual plant growing 1 to 2 feet high on slender smooth green stems (not spotted) that branch in the upper part. Fine, darker green lines may be visible, especially at stem branches which may also have traces of purple color.
The leaves are dark green, 2 to 3 times pinnate with each leaf segment finely divided like those of true parsley. Lower leaves are on slender stalks, the upper sessile or nearly so. Leaves have a very disagreeable odor when crushed.
The inflorescence is a long-stalked somewhat flat topped umbel, composed of a number of umbellets, each on a stalk of unequal length. Each umbellet has 8 to 12 stalked flowers. Unlike the umbel of parsley, the flower umbel here does not have an involucre (or bracts). Instead, the umbellets have involucels (An involucre that forms at the base of a flower cluster within a compound inflorescence) of 2 to 4 long narrow down-ward turned green bracts.
The flowers have 5 white petals with notched tips and much narrowed bases. The ovary is 2-parted. Five stamens with white filaments rise from a central yellow-green disc. These are arranged opposite (in-between) the petals. The ovary has a paired style also rising from the disc. Flowers also have a fetid odor when crushed and true parsley flowers are yellow.
Seed. Mature flowers produce two seeds that are somewhat triangular in shape, almost as broad as long, but with 5 distinct straight ridges. These usually split into two sections when dry.
Toxic: The plant is poisonous - see below.
Habitat: Fool's Parsley grows in fields, waste places and sunny moist areas. The root is a radish shape, 3 to 6 inches long.
Names: The genus, Aethusa, is derived from old words, thought to be Greek or Arabic, meaning 'burning' and referring to the effect of the plants toxin (see below). The species, cynapium, has as it's root 'apium' which is the classical Latin name for parsley and celery and thus the name can mean "resembling" those plants. The author name for the plant classification - 'L.' refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy.
Comparisons: The flowers are similar to those of the more poisonous Hemlocks - Water Hemlock, Cicuta maculata, and Bulblet Water Hemlock, Cicuta bulbifera, but the leaves are quite different. And as to true parsley, that plant has yellow flowers, not white. An invasive plant that has a similar umbel, but flowers with unequal length petals and different leaves is Japanese Hedge Parsley, Torilis japonica.
Above: The upper stem section with two leaves. Illustration courtesy Kurt Stüber's Online Library.
Below: 1st photo - The inflorescence is a long-stalked somewhat flat topped umbel, composed of a number of umbellets. 2nd photo - The flowers have 5 white petals with notched tips and much narrowed bases.
Below: 1st photo - The umbellets have involucels of 2 to 4 long narrow down-ward turned green bracts. Unlike true parsley the umbel here does not have an involucre. 2nd photo - The leaf is 2 to 3 times pinnate with each leaf segment finely divided like those of true parsley.
Below: 1st photo - The stems are smooth with some darker green lines and some purple color at the branch nodes. 2nd photo - This compound leaf is 3-pinnate with the entire leaf in 3 branching divisions with the terminal branch much larger, and each branch divided twice: First sub-division is the paired laterals (plus a terminal section) and then each sub-division is further divided into leaflets.
Notes: Fool's Parsley is a naturalized plant that arrived from Europe. It is found in North America from Minnesota eastward to the coast, south as far as Illinois and Kentucky and in Canada from Ontario eastward. Within Minnesota it is known mainly in the SE part of the state. This is the only species of Aethusa found in Minnesota and is the only species assigned to the genus world-wide.
Toxicity: While less poisonous than the Hemlocks of the Cicuta genus, this plant will still cause severe suffering from burning sensations. Symptoms of the poisoning are heat in the mouth and throat and redness in the windpipe, gullet and stomach. The active principal is the alkaloid 'Cynopine'.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"