Field Sow Thistle is an introduced and naturalized erect perennial forb, growing 2 to 5 feet high on stems contain a milky juice. The plant may branch a little in the upper part, the stems are dull green, round and usually smooth, and often with hard or woody hollow bases.
The leaves are alternate, 4x longer than wide, mostly pinnately divided with 2 to 5 lobes with pointed tips to the lobes. These are concentrated in the lower part of the stem. They have heart-shaped bases which have a pair of small rounded basal lobes (auricles) which clasp the stem, but are not united around the stem. All edges with small prickles. The upper leaves will be much shorter and less divided or not divided at all. The leaf surfaces are green, free from hair, free from prickles along the underside mid-vein. Prickles only on the margins. Leaves also have milky juice.
The floral array is an open branched cluster of several flower heads, at the top of the stems, not all open at one time. The cluster is on a long stalk. The base of the cluster and at divisions of the cluster there may be small bracts.
The flowers are between 1-1/4 to 2 inches wide, largest of our Minnesota Sow Thistles, each on a short stalk, and composed of numerous fertile ray florets with whitish corolla tubes with yellow rays, the rays, toothed at the tip, and as long as the tubes. Disc florets are lacking. Stamens (5) and anthers are pale yellow and tightly surround the pistil and style, which is exserted from the corolla during flowering. The flower head base (the involucre) has many green bracts (phyllaries) in 2 or 3 series, the outer series shorter, the inner series longer with pointed slightly recurved tips. All have a darker green mid-vein. There are two varieties (see below at page bottom) one with glandular hair on the phyllaries, and one without. The same condition applies to the flower stalks.
Seed: Fertile flowers mature to a dry reddish-brown cypselae (seed resembling an achene), 2.5 to 3.5 mm long, oblong to ellipsoid, without a beak, hairless, wrinkled, with 4 to 5 ribs on each side and with a tuft of fine white hair for wind dispersion.
Habitat: Field Sow Thistle propagates from re-seeding and from its root system which consists of long, spreading rhizomes. It grows in many waste places, accepts various soils, moist to slightly dry conditions, but needs full sun. Flowering is this area is usually early to mid summer and then into Autumn.
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 arvensis, means 'of planted fields' and accounts for the common name. 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. The origin of the name 'Sow Thistle' is ancient but obscure. 'Corn Sow Thistle' refers to the plants frequent appearance in corn fields.
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 12 to 20 ray flowers and the leaves have a spiny mid-rib on the underside. Compare also the Spiny Sow Thistle, Sonchus asper.
Above: Most leaves of the plant are on the lower stem section. Illustration courtesy Kurt Stüber's Online Library.
Below: 1st photo - Typical floral array of several flower heads per flower stalk. The variety pictured here is without glandular hair. 2nd photo - A seed head.
Below: 1st photo - Flowers are composed of many fertile ray florets with whitish corolla tubes with yellow rays, the rays, toothed at the tip, as long as the tubes. Stamens and anthers are pale yellow and surround the style. 2nd photo - The phyllaries in 2 or 3 series with pointed tips and a darker green mid-vein.
Below: The cypselae is dry, reddish-brown, no beak, hairless, wrinkled, with 4 to 5 ribs on each side and with a tuft of fine white hair for wind dispersion.
Below: 1st photo - The floral array - note the long stalks of the flower clusters and the small leafy bracts at the base of the clusters. 2nd photo - leaves have 2 to 5 lobes with pointed tips to the lobes. 3rd photo - leaves have heart-shaped bases with a pair of small rounded basal lobes which clasp the stem, but do not enclose the stem.
Below: 1st photo - a small leaf of the very upper stem. 2nd photo - a large basal leaf with deeply cut lobes.
Below: Leaves are concentrated in the lower part of the stem, All edges with small prickles.
Below: 1st photo - The underside of the leaf is paler in color, but no surface hair and no prickles on the mid-vein. 2nd photo - The lower stem is hollow and contains a milky juice in its pores, as do the leaf veins.
Notes: Field Sow Thistle was introduced to the Garden by Eloise Butler on Aug. 10, 1915 when she planted 3 roots obtained from the vicinity of Medicine Lake (west Minneapolis metro area) but has not been recorded as present in the Garden in a number of years. The plant is an import from Eurasia and has naturalized throughout North America except some states of the deep south. Within Minnesota it is found in most counties including most of the metro area. It is the most common of the Sow Thistles in Minnesota and is restricted from seed sales.
There are two subspecies of this plant listed by the DNR as present in the state. These are subsp. arvensis where the flower stalks and the phyllaries have stalked glandular hair and subsp. uliginosus where they have glands not stalked or sometimes phyllaries without glands; the latter is by far the most common in Minnesota according to DNR surveys. The second species in the state is the Spiny Sow Thistle, S. asper, which has a smaller flower head and the auricles of the leaf are often recurved or curled around the stem. 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. There are five species of Sonchus in North America.
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"