Common sowthistle (Sonchus oleraceus)

Common Names: Sowthistle, sow thistle, annual sowthistle, smooth sowthistle, hare's thistle, milky tassel, soft thistle, puha

Description:

Common sowthistle (Sonchus oleraceus), also called annual sowthistle, is a fast-growing annual weed in the aster family, native to Europe, North Africa, and western Asia and now found nearly worldwide [1][3]. It is a familiar garden and field weed with soft, lobed, slightly prickly leaves, milky sap, and small pale-yellow dandelion-like flowers [1][2]. MISIN tracks it as a non-native weed of gardens, crops, and disturbed ground, where its wind-blown seed spreads it quickly [1][2].

Habit:

Common sowthistle is an upright annual, usually 1 to 4 feet tall, that starts as a low rosette and then sends up a hollow, branching stem [1][2]. Break any part of it ? leaf, stem, or root ? and it bleeds a milky white sap, a good first clue [2]. Its leaves are soft and dull bluish-green, deeply cut into triangular lobes, with edges lined by small soft prickles (not stiff spines); at the base, each upper leaf has a pair of pointed lobes that clasp the stem and flare out past it [1][2]. The flowers are small pale-yellow heads, about ? to ? inch across, that look like little dandelions and grow in loose clusters at the branch tips; each one ripens into a puff of fluffy seeds like a small dandelion clock [1][2].

It is easiest to spot from early summer into fall, when the pale-yellow flower heads and fluffy seed puffs are present on the branching, leafy stems [1][2]. Earlier, look for the soft, lobed, bluish-green rosette leaves with small prickly edges on bare or disturbed soil ? and confirm with the milky sap [1][2].

At-a-glance cues:

1. Milky white sap from any broken leaf, stem, or root [2]
2. Soft, bluish-green, deeply lobed leaves with small soft prickles and pointed, stem-clasping basal lobes [1][2]
3. Hollow, branching stem on an upright annual 1?4 feet tall [1][2]
4. Small pale-yellow dandelion-like flower heads in loose clusters, ripening to fluffy seed puffs [1][2]

Leaves:

1. Arrangement: in a basal rosette at first, then alternate up the stem, becoming smaller upward [1][2]
2. Shape: deeply cut (pinnatifid) with triangular lobes, up to about 8 inches long; upper leaves with pointed basal lobes that clasp the stem [1][2]
3. Margin: toothed and lined with small, soft prickles (not stiff spines) [1][2]
4. Surface: soft, smooth, dull bluish-green; exudes milky sap when broken [1][2]

Stems:

1. Habit: an erect annual, 1?4 feet tall, branching mostly near the top [1][2]
2. Texture: hollow, smooth, dull green and sometimes tinged reddish-purple; full of milky sap [1][2]
3. Root: a shallow taproot, which makes plants easy to pull [2]

Flowers:

1. Color: pale yellow [1][2]
2. Type: small heads of strap-shaped ray florets only (no central disk), about ??? inch across ? dandelion-like [1][2]
3. Arrangement: in loose clusters at the tips of the branches; the heads open in the morning [1][2]
4. Bloom time: early summer into fall [1][2]

Fruit and seeds:

1. Fruit type: a small, flat, ribbed seed (achene), brown and a bit wrinkled [1]
2. Tuft: each seed carries a tuft of fine white hairs (pappus) that acts like a parachute [1][2]
3. Output: prolific ? a single plant can produce large numbers of seeds [1][2]
4. Dispersal: by wind, and also in water and soil; the plant spreads only by seed [1][2]

Habitat:

Common sowthistle is native to Europe, North Africa, and western Asia and has spread to nearly every part of the world, including across North America [1][3]. It is a weed of disturbed, open ground rather than a plant of intact natural habitat [1][2].

It grows in full sun on a wide range of soils and strongly favors disturbed sites [1]. Look for it in gardens, crop fields, pastures, nurseries, roadsides, vacant lots, and other waste and recently worked ground [1][2].

Reproduction:

Common sowthistle spreads only by seed, and it makes a lot of it. Each flower head ripens into a puff of seeds, each carried on a fluffy white tuft, so the seeds blow easily on the wind into new ground ? and they also move in soil, in water, and as a contaminant in crop and grass seed [1][2]. That wind-borne seed is the main way it turns up in a fresh garden bed or field [1].

It is a fast annual: seeds sprout, the plant grows up, flowers from early summer into fall, sets seed, and dies, often fitting more than one generation into a season [1][2]. Because it has only a shallow taproot, plants pull up easily ? so the key timing for control is to remove plants before the flower heads turn fluffy and release seed [2].

Impact and Damage:

For homeowners, common sowthistle is a minor nuisance weed rather than a hazard. It is not toxic ? its young leaves are actually eaten as a cooked or salad green in many places ? and the trouble it causes is in the garden: it grows fast on bare soil, competes with vegetables and ornamentals for space and water, and reseeds heavily, so it keeps coming back [1][2]. Its milky sap and prickly leaf edges make it unappealing to handle but not dangerous [1].

For natural area stewards, common sowthistle is a low-level concern compared with aggressive habitat invaders. It is a weed of disturbed and open ground ? field edges, roadsides, and recently cleared soil ? and does not typically take over healthy, closed natural plant communities [1][2]. Its main nuisance value is as a fast colonizer of bare ground and a contaminant of crop and pasture seed, so keeping soil covered and removing plants before they seed limits its spread [1][2].

Similar species:

Common sowthistle is one of several milky-sapped, yellow-flowered weeds and is easily confused with them. Its surest marks are its milky sap, its soft (not stiffly spiny) prickled leaves with pointed clasping basal lobes, and its small dandelion-like heads in clusters [1][2].

Spiny sowthistle (Sonchus asper) is the closest match and grows in the same places. It is told apart by its leaves, whose edges are stiffly spiny and whose rounded basal lobes curl into tight coils against the stem, unlike common sowthistle's softer prickles and flat, pointed basal lobes [1][2].

Perennial sowthistle (Sonchus arvensis) looks similar but is a longer-lived perennial that spreads by creeping underground roots, not just seed, and it has larger, showier yellow flower heads ? making it harder to control than the annual common sowthistle [1].

Prickly lettuce (Lactuca serriola) is another milky-sapped weed with small pale-yellow heads. It is separated by the row of stiff prickles along the underside of the leaf midrib and by its leaves, which often twist to stand vertically, neither of which common sowthistle has [2].

Monitoring and rapid response:

Signs to watch for:

1. Soft, bluish-green, deeply lobed leaves with small prickly edges on bare or disturbed ground; milky sap when broken [1][2]
2. Upright, hollow, branching stems 1?4 feet tall with loose clusters of small pale-yellow flower heads, early summer into fall [1][2]
3. Fluffy white seed puffs forming on the heads ? a sign seed is about to blow [1][2]

What you can do:

1. Hand-pull or hoe plants while young ? the shallow taproot lifts easily, especially in moist soil [2]
2. Remove plants before the flower heads turn fluffy, and bag flowering or seeding plants rather than composting them, since the seed blows readily [1][2]
3. Keep bare garden soil mulched or planted so wind-blown seeds have less open ground to colonize [1]
4. Catch and pull new plants through the season; repeating this before seed-set draws down the seed in the soil [1][2]
5. For natural areas and field edges, crews can focus on disturbed ground and outlying plants, pulling or mowing them before the heads go to seed and revisiting to catch new seedlings [1][2]

When to call a professional:

1. Large or recurring infestations across crop fields or worked ground where hand removal cannot keep up [1][2]
2. Patches that keep returning from wind-blown seed or from a persistent seed bank despite removal [1][2]
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. Hilty J. Illinois Wildflowers. Common Sowthistle (Sonchus oleraceus). illinoiswildflowers.info
2. Penn State Extension. Annual Sowthistle (Sonchus oleraceus). extension.psu.edu
3. Plants of the World Online (POWO), Royal Botanic Gardens, Kew. Sonchus oleraceus L. powo.science.kew.org

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