Prickly lettuce (Lactuca serriola)

Synonyms: Lactuca scariola, Lactuca serriola f. integrifolia, Lactuca serriola var. integrata

Common Names: China lettuce, compass plant, milk thistle

Description:

Prickly lettuce (Lactuca serriola) is a tall, weedy relative of garden lettuce that came out of the Mediterranean Basin and now grows across nearly all of the United States and southern Canada [1][2][5]. It reached North America in the late 1800s and spread as a contaminant of crop and grass seed [5]. MISIN tracks it because a single plant can make an enormous amount of wind-blown seed and it colonizes any bare, disturbed ground ? roadsides, field edges, gardens, and construction scrapes [5].

Habit:

Prickly lettuce is a knee-to-head-high plant, usually 1 to 3 feet tall but reaching 6 1/2 feet (2 m) in good conditions, with a single stem that branches near the top into an airy spray of small pale-yellow flower heads that look like miniature dandelions [2][6]. Break any part of it and it bleeds milky white sap [5]. That sap, plus one other trait, is most of the identification.

The other trait is the giveaway: turn a leaf over and run a finger along the midrib. Prickly lettuce carries a row of stiff prickles down the underside of the midrib, and they are much stiffer than anything on our other wild lettuces [2][6]. The leaf edges are prickly too, and the leaf bases clasp the stem [6]. There is one more habit worth knowing ? the leaves twist to stand on edge, lining themselves up roughly north-to-south, which is why the plant is also called compass plant [5].

Timing shapes what you will see. Most plants sprout in autumn and spend winter as a flat rosette (a low ring of leaves at ground level), then shoot up a stem in spring [5]. Flowering runs from roughly July into September, sometimes starting as early as May and running to October [2]. Once the heads dry, the seeds go up on white parachutes like a dandelion's [2][6]. The rosettes are easiest to find in fall and early spring; the plant is most obvious in midsummer when it is tall and flowering.

At-a-glance cues:

1. A row of stiff prickles down the underside of the leaf midrib ? stiffer than on any other wild lettuce here [2][6]
2. Milky white sap from any broken stem or leaf [5]
3. Leaves twist to stand on edge, roughly aligned north-to-south [5]
4. Small pale-yellow heads like miniature dandelions, in an open spray at the top; seeds carry white parachutes [2][6]

Leaves:

1. Arrangement: alternate along the stem, carried on the lower half to three-quarters of it, and clasping the stem at the base [2][6]
2. Shape: roughly oblong or paddle-shaped in outline, less than 4 times as long as wide, either deeply lobed or unlobed [2][3][6]
3. Margin: finely toothed and usually prickly along the edge [2][6]
4. Midrib: a row of stiff prickles runs down the underside ? the key character, though rare plants lack it [2][3]
5. Orientation: leaves twist to hold vertical, roughly north-to-south, which cuts water loss [5]
6. Size: from a few inches up to about 14 inches long, getting smaller up the stem [6]
7. Sap: cut or broken leaves release milky white latex [5]

Stems:

1. Habit: herbaceous (soft and green, never woody), erect, single from the base [2][6]
2. Height: usually 1 to 3 feet (30?70 cm), occasionally to 6 1/2 feet (2 m) [2][6]
3. Branching: unbranched below, branching near the top into an open spray of flower heads [2][6]
4. Surface: lower stem smooth or with bristly hairs; often prickly toward the base [6]
5. Sap: releases milky white latex when cut [5]
6. Root: a taproot ? no creeping runners or spreading roots [5]

Flowers:

1. Color: pale yellow, the strap-shaped florets drying bluish [2][3]
2. Size: heads small ? the green cup around each is only about 9 to 10 mm long [2]
3. Florets: about 12 to 20 per head, so each head reads as a miniature dandelion [2][6]
4. Arrangement: many heads in an open, branching spray at the top of the stem [2][6]
5. Bracts: the green bracts under each head usually bend backward as the fruit ripens [2]
6. Bloom time: mainly July through September, sometimes from May and into October [2]

Fruit and seeds:

1. Fruit type: a small dry one-seeded fruit, flattened and narrowly paddle-shaped [2]
2. Color: pale gray to tan [2]
3. Size: body about 2.5 to 3.5 mm long, with 5 to 9 fine ribs on each face and minutely barbed edges [2][6]
4. Beak: a slender thread-like stalk 2.5 to 4 mm long carries the tuft ? about as long as the seed body itself [2][3]
5. Tuft: white parachute hairs about 4 to 5 mm long, so seeds travel on the wind like a dandelion's [2][6]
6. Output: up to about 100,000 seeds on a single plant, which stay viable in soil for up to three years [5]
7. Ripening: through summer and into fall, shortly after each head finishes flowering [2][5]

Habitat:

Prickly lettuce came out of the Mediterranean Basin and is native across Europe, northern Africa, and western and central Asia [1][5]. It is introduced in North America and now grows in nearly every US state and across southern Canada, including Michigan [2][4].

It is a plant of open, sunny, broken ground. Look for it on roadsides, ditch banks, field edges, croplands, orchards, vineyards, landscaped beds, construction scrapes, and any other disturbed site, from near sea level up to about 7,500 feet (2,300 m) [2][5][6]. It also turns up in grasslands and seasonal wetlands [6].

Bare soil is what it wants. Its seeds have no built-in dormancy and germinate readily when they land somewhere open, which is why it appears so fast on freshly disturbed ground and so rarely inside closed, established vegetation [5].

Reproduction:

Prickly lettuce spreads one way only: seed on the wind [5]. There are no runners and no creeping roots ? it grows from a taproot, so a patch does not creep outward from a neighbor's yard the way many weeds do [5]. What it does instead is produce a staggering number of parachute seeds, up to about 100,000 from one vigorous plant, and let the wind take them [5]. They also hitch rides in crop and grass seed, which is how the plant crossed the Atlantic in the first place [5].

The timing is the useful part. Most seeds sprout in late autumn and early winter, with a smaller flush in spring [5]. Autumn seedlings spend the winter as flat rosettes at ground level, then send up a flowering stem the following spring [5]. Flowering runs mainly July into September, and seed follows within weeks [2].

Seeds have no primary dormancy, so they germinate as soon as conditions suit rather than waiting in the soil ? but they stay viable in the seedbank for up to three years [5]. That sets the control expectation honestly: pulling this year's plants does not finish the job, because three years of seed are already in the ground. Getting on top of a patch takes a dedicated effort over three to four years with no new seed allowed to drop [5].

The control window is before the heads dry. Once the parachutes open, the seed is gone and the wind decides where it lands [5].

Impact and Damage:

Prickly lettuce is not a poisoning risk to people or pets in the yard, but it is worth handling with a little care. The leaf edges and midrib prickles are stiff enough to scratch, and the plant bleeds milky sap when broken [5][6]. Gloves are sensible.

For a homeowner, this is mostly a bare-ground weed. It appears fast on any disturbed patch ? a new bed, a construction scrape, a thin spot in the lawn ? and a single plant left to flower can seed the whole neighborhood, since the parachutes travel on the wind [5].

The documented damage is agricultural rather than ecological, and it is substantial where it gets dense. In crops, prickly lettuce causes yield losses of up to 10% at low densities and up to 80% where it exceeds 50 plants per square meter [5]. Its milky sap fouls harvesting equipment and contaminates harvested grain, raising the grain's moisture content [5].

It has also become genuinely hard to kill with chemicals in agricultural settings. Populations resistant to ALS-inhibiting herbicides appeared in Idaho as early as 1987, and by 2007 populations in Washington were reported resistant to 2,4-D, dicamba, and MCPA [5]. That history is the reason the non-chemical tactics below matter, and the reason repeat visits over several years are the realistic plan.

For natural area stewards, prickly lettuce is a lower-tier concern than aggressive habitat invaders. It colonizes disturbance and does not readily push into closed, intact plant communities, because its seed needs open ground to establish [5]. The management value is in catching it on freshly disturbed ground ? trail work, fire lines, staging areas ? before it seeds, and in closing bare soil back over.

Similar species:

Several yellow-flowered, milky-sapped weeds get called wild lettuce. The row of stiff prickles under the leaf midrib separates prickly lettuce from all of them [3][6].

Willow-leaved lettuce (Lactuca saligna) is the closest match in the key and keys out right beside prickly lettuce ? both are non-native, both have pale-yellow heads that dry blue, and both have a long thread-like beak on the seed [3]. Two things separate them. Its midrib is smooth, with at most a few prickles near the base, and its leaves are very narrow and unlobed ? roughly 12 to 30 times longer than wide, where prickly lettuce's are less than 4 times longer than wide and usually lobed [3].

Wild lettuce (Lactuca canadensis), also called tall lettuce, is the one worth knowing because it is native and should be left alone [3]. It is a tall yellow-flowered lettuce with milky sap, so it reads as the same plant at a glance. Its leaves are not prickly on the midrib at all ? that check alone settles it. On ripe seed, it has at most one strong nerve on each face, where prickly lettuce has five to nine [2][3].

Prickly sow-thistle (Sonchus asper) is the sow-thistle most often mixed up with prickly lettuce, since it shares the prickly leaf edges, the small yellow dandelion-like heads, and the milky sap [3]. Check two places. Its midrib underside has no row of prickles, and its leaf bases wrap the stem in big rounded lobes that curl around far enough to suggest a nautilus shell [3].

Common sow-thistle (Sonchus oleraceus) is the other annual sow-thistle in the same weedy places, with the same yellow heads and milky sap but softer, barely prickly foliage [3]. Its leaf bases have pointed rather than rounded lobes, and its ripe seeds are covered in fine bumps ? again, no prickles down the midrib [3].

Monitoring and rapid response:

Signs to watch for:

1. In fall and early spring, flat rosettes of lobed leaves hugging the ground on any bare or disturbed patch [5]
2. From July through September, a tall single stem branching at the top into an open spray of small pale-yellow dandelion-like heads [2][6]
3. Leaves standing on edge rather than flat, roughly lined up north-to-south [5]
4. A row of stiff prickles under the leaf midrib, and milky sap from any break ? check both before pulling anything [5][6]
5. Newly disturbed soil anywhere near a known plant: fresh beds, scrapes, trail work, and thin spots in turf are where it lands first [5]

What you can do:

1. Wear gloves. The leaf edges and midrib prickles scratch, and the plant bleeds sticky milky sap [5][6].
2. Pull or dig it, and get the taproot. There are no runners, so removing the root removes the plant ? this is one of the more satisfying weeds to hand-pull [5]. Rosettes in moist fall or spring soil come up easily and are the best target.
3. Beat the seed. Get plants out before the heads dry and the parachutes open, because one plant can release around 100,000 wind-borne seeds [5]. If you only have time for one pass, make it before the fluff shows.
4. Do not rely on mowing. Cut plants regrow from side branches and go on to flower lower down, so mowing is not a long-term fix [5].
5. Bag the flowering and seeding heads and put them in the trash rather than the compost pile ? heads cut and left on the ground can still ripen seed [5].
6. Close the bare ground. Seed needs open soil to establish, so reseeding or letting existing plants fill in is what stops the next wave [5].
7. Plan on three to four years. Seed already in the soil stays viable for up to three years, so a dedicated effort with no new seed allowed to drop is what actually clears a patch [5].
8. Natural area crews can work the same way at scale: hit disturbed ground first ? fire lines, staging areas, trail work ? since that is where it establishes, prioritize small outlying patches before the main population, and time visits to precede seed set rather than chasing plants after the fluff has flown [5].

When to call a professional:

1. The infestation covers cropland, pasture, or a field at a scale where yield loss is the real cost rather than a yard-sized patch [5].
2. You are managing farmland ? populations of this species are documented as resistant to several herbicide groups, so a advisor can build a plan that does not lean on a product the plants may already tolerate [5].
3. You cannot confirm which lettuce you have. Native wild lettuce looks similar and is worth keeping, so get an identification rather than clearing everything yellow-flowered [3].
4. 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. Plants of the World Online. Lactuca serriola L. Royal Botanic Gardens, Kew. powo.science.kew.org
2. Flora of North America Editorial Committee. Lactuca serriola Linnaeus. Flora of North America Vol. 19, pp. 260, 262, 263. efloras.org
3. Reznicek AA, Voss EG, Walters BS (2011). Lactuca and Sonchus. Michigan Flora Online, University of Michigan. michiganflora.net
4. USDA NRCS Plants Database. Lactuca serriola L. (prickly lettuce), symbol LASE. plants.usda.gov
5. Chadha A, Florentine S (2021). "Biology, ecology, distribution and control of the invasive weed, Lactuca serriola L. (wild lettuce): A global review." Plants 10(10):2157. semanticscholar.org
6. University of California Statewide IPM Program. Prickly Lettuce ? Weeds Identification Gallery. ipm.ucanr.edu

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