Observations of the Month: Sow-Thistles (Asteraceae)
Common Sow-Thistle (Sonchus oleraceus) https://www.inaturalist.org/observations/76017233 by jrebman
Prickly Sow-Thistle (Sonchus asper ssp. asper) https://www.inaturalist.org/observations/47041630 by jrebman
Common Sow-Thistle (Sonchus oleraceus) and Prickly Sow-Thistle (Sonchus asper ssp. asper) are widespread annual weeds in San Diego County. They are most common in the coastal lowlands inland to the foothills and less common in the mountains and desert slopes. Prickly Sow-Thistle is not as common as Common Sow-Thistle.
The plants share many characteristics. Both have yellow or light-yellow flower heads composed of only strap-shaped flowers (“ligulate”). Before the flower heads open, the swollen bases of their involucres are very apparent. Both have stout stems with milky sap. Both have leaves at the base of the plant and also distributed along the stem. But if you look carefully at the base of the leaves where they clasp the stem, you can determine whether the plant is S. oleraceus or S. asper ssp. asper.
Sonchus asper ssp. asper (Top), Sonchus oleraceus (Bottom), Photos by Jon Rebman
If the base of the leaf where it clasps the stem is rounded or coiled, the plant is S. asper ssp. asper. If the base of the leaf where it clasps the stem is pointed and not rounded or coiled against the stem, the plant is S. oleraceus. If you find a Sow-Thistle, always take a photo of the base of a leaf where it meets the stem. Consider posting this photo as the first photo of your observation to bring this important characteristic to the attention of identifiers.
You can find more information about these plants and others at the San Diego County Plant Atlas website.
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