Spiny Sow Thistle is an introduced and naturalized erect annual (sometimes biennial) forb, growing 4 inches to 5 feet high on stems which are soft and contain a milky juice. The plant may branch a little in the upper part, the stems are green to reddish green, round and usually smooth, often with ridges, and with hollow bases.
The leaves are alternate, 3x longer than wide, and vary in description from toothed to pinnately divided to sometimes lobed. When lobed, the terminal lobe is usually larger than the lateral lobes. Leaves have heart-shaped bases which have a pair of rounded basal lobes (auricles) which clasp and curve around the stem. The auricles are often recurved or curled. All leaf edges with prickly spines. Lower leaves may be up to 10 inches long, upper leaves will be much shorter and less divided. The leaf surfaces are dark green, shiny, free from hair, free from prickles along the mid-vein underside and leaves tend to fold toward the mid-vein. Leaves also have milky juice.
The floral array is a branched cluster at the top of the stems of several flower heads, usually one open at one time. The base of the cluster and at divisions of the cluster there may be small bracts and stalks will usually have glandular hair.
The flowers are between 1/2 to 1 inch wide, each on a short stalk, and composed of many fertile ray florets with yellow corollas and rays and five yellow stamens with brownish anthers. The stamens tightly surround the pistil and style. The rays are shorter than the corolla tube. The receptacle is mostly flat and epaleate (that is, each floret does not have a subtending bract). There are no disc florets. The flower head involucre has many dull green phyllaries with pointed tips. The flower stalks are glandular, but sometimes smooth, likewise the phyllaries.
Seed: Fertile flowers mature to dry reddish-brown cypselae (seeds). They are ellipsoid and strongly compressed, sometimes winged, hairless, not wrinkled, with 3 to 5 ribs on each side and with a tuft of fine white hair for wind dispersion.
Habitat: Spiny Sow Thistle propagates from re-seeding. It grows in many waste places, accepts various soils, most to slightly dry conditions, but needs full sun. Even though it is usually an annual, it has a taproot. Flowering is this area is usually late summer.
Names: The genus Sonchus, is the old Greek name for 'hollow' and was applied due to the hollow stem of the Sow Thistle and the species asper, means 'rough' - as in the spiny edges. The author name for the plant classification is as follows: The first to classify was '(L.)' which refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy. His work was amended by ‘Hill’ which is for John Hill (c1716-1775), English botanist, author of 76 works including the 26 volume The Vegetable System. The origin of the name 'Sow Thistle' is ancient but obscure.
Comparison: There are 3 species of Sow Thistle in Minnesota. A description is given below in 'notes'. The other plant that may look something like this one is Prickly Lettuce, Lactuca serriola , but there the flower has only 5 to 13 ray flowers, the flower head is smaller, and the leaves have a spiny mid-rib on the underside. Compare also the Field Sow Thistle, Sonchus arvensis.
Above: The floral array of the upper stem. Botanical illustration courtesy USDA Agricultural Research Service.
Below: Flower ray florets have notched tips on the rays, yellow stamens with brownish anthers tightly surrounding the pistil and style.
Below: 1st photo - The phyllaries are dull green, have pointed tips, and may have glandular hairs. 2nd photo - A mid stem leaf showing the rounded heart-shaped clasping lobes.
Below: 1st photo - A lower leaf. These can have terminal lobes resembling those of the Common Sow Thistle, S. oleraceus. 2nd photo - The underside is pale and not prickly.
Below: 1st photo - The floral array atop the stem - not all flowers open at the same time. 2nd photo - Some plants may have stalked glandular hair on the flower stalks or stalks of the cluster and the upper stem. 3rd photo - Upper stem leaf without lobes.
Below: 1st photo - A maturing seed head with the fluffy white pappus attached to the seeds. 2nd photo - This lower leaf is simply toothed, not divided. 3rd photo - The rounded basal leaf lobes which clasp and curve around the stem.
Below: A seed head showing the reddish-brown ribbed seeds with fine white pappus for wind dispersion.
Notes: Spiny Sow Thistle has been in the Garden periodically. It was not present in the original Garden area. On Aug. 11, 1914 Eloise Butler noted planting 3 Sonchus aculeatum. That species is a lost name but the word refers to prickly, so I surmise that it may have been our current species, S. asper. She got the plants in the glen at what is now Minnehaha Park in Minneapolis. Nevertheless, it was listed on Martha Crone's 1951 Garden census and on the 1986 census, but was absent on the 2009 census. The plant is an import from Eurasia and has naturalized throughout North America. Within Minnesota it is found about 1/3 of the counties, widely scattered, including most of the metro area. The most common Sow Thistle is the Field Sow Thistle, S. arvensis, which is perennial, has larger more showy flower heads and the basal lobes of the leaf do not wrap all the way around the stem. There are several subspecies of this plant listed by the DNR as present in the state. The third species in Minnesota is the Common Sow Thistle, S. oleraceus, which, contrary to the name, is not so common in Minnesota. It has basal leaf lobes that are arrow-shaped.
Lore and uses: Sow Thistles came to North America with an extensive background of Old World lore. As potherbs they were used from the times of Pliny. In fact Pliny tells us that before Theseus had his encounter with the bull of Marathon he was lavishly supplied with a dish of Sow Thistles as the ancient races considered then to be very strengthening. More modern peoples have used them in salads, but they are most useful as rabbit and pig food as when they are succulent, rabbits adore them and pigs are quite fond. Horses, however, with their larger brains won't touch them.
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"