Spiny Sowthistle: Sonchus asper
||Weed Description: An annual that may
reach as much as 6 feet in height with bluish-green leaves and stems that emit a milky sap
when cut. Spiny sowthistle is primarily a weed of landscapes, winter small grains,
pastures, hay fields, orchards, and roadsides. It is found throughout the United
|Seedlings: Cotyledons are egg-shaped,
approximately 3 to 8 mm long, and occur on petioles. Young leaves form a basal rosette.
Both young leaves and
cotyledons have a whitish coating.
||Stems: Unbranched, erect, reaching as
much as 6 feet in height but generally around 2-3 feet in height. Stems emit a milky
sap when broken.
|Leaves: Leaves initially
develop as a basal rosette and for this reason are often confused with thistles. All
of the leaves are deeply cut with two rows of lateral appendages (pinnatifid), and range
from 2 1/2 to 12 inches in length and 1/2 to 6 inches in width. Leaves are generally
hairless (glabrous), egg-shaped in outline and have prickly margins. Leaves that
occur on the flowering stem are are alternate and have rounded lobes that clasp the stem.
Leaves emit a milky sap when broken.
||Roots: A taproot.
Flowers: Occur in clusters at the ends of stems.
Individual flowers range from 1/2 to 1 inch in diameter and are yellow in color.
Fruit: A brown, wrinkled achene approximately 4 mm
long. Mature seed have a white feathery pappus that collectively form a white
"puff ball" similar to that of dandelion.
|Identifying Characteristics: The
leaves with very prickly margins that initially develop as a basal rosette and then occur
alternately along the flowering stem, the bluish-green color of the leaves, and the yellow
flowers with a 'puff-ball' seedhead are all characteristics that help in the
identification of spiny sowthistle. Annual
Sowthistle (Sonchus oleraceus) is very similar in appearance, however this
species has distinctly pointed lobes that clasp the stem whereas spiny sowthistle has
rounded lobes. Also, the leaf margins of annual sowthistle are much less spiny or
prickly than those of spiny sowthistle. Both of the sowthistles can be distinguished
from 'true' thistles by the milky sap they emit when broken. The sowthistles might
also be confused with Prickly Lettuce (Lactuca
serriola), however the midvein on the leaf undersides of this weed has distinct
spines or prickles, whereas those of the sowthistles do not.