Xanthium (cocklebur) in Monterey County, California

UPDATE 7-March-2024. Some time in the last few months, Jepson was updated to recognize X. orientale and eliminate X. strumarium. Great!

ORIGINAL POST 21-October-2023 (and maybe some edits after that):

As far as I can tell from the literature, all the plants formerly known as Xanthium strumarium should now be recognized as Xanthium orientale.

The primary reference is Tomasello (2018) which includes the key that I have reproduced at the bottom of this post. Californian concurrence with Tomasello can be seen in Keil & Hoover's (2022) "Vascular Plants of San Luis Obispo County, California", which is the most recent flora published in California. Note that one of the authors (Keil) is also an editor of the Jepson Manual. All the other major Xanthium references applicable to California pre-date Tomasello. These pre-Tomasello authorities include Baldwin (2012) in "The Jepson Manual: Vascular Plants of California, 2nd ed." which is in turn "adapted from" Strother (2006) in "Flora of North America: Vol 21", and Matthews & Mitchell (2015) "Plants of Monterey County". Styer's (2019) "Flora of Fort Ord" post-dates Tomasello, but only by a few months. So it is easy to imagine how Tomasello's revision might not have influenced Styer. Styer's book also does not have a key or species descriptions; instead, it has maps of occurrence and notes on collections.

In Tomasello's key, Xanthium orientale burs are large, generally hirsute, with uncinate prickles, and beaks strait or curved. By contrast, Xanthium strumarium burs are small, glabrous, sparse-prickled, and straight beaked.

So among the non-spined Xanthium, if you have a large hirsute bur with a curved beak, it's a Xanthium orientale.

Keil & Hoover (2022) specifically note that the epithet "strumarium" has in the past been "misapplied" to orientale. They also include X. spinosum (which is very spiny and looks completely different). They don't include X. strumarium as occurring in San Luis Obispo County, other than as a "misaplied" name to what is actually X. orientale.

Since I learned of Tomasello in 2023, I've been checking every non-spined cocklebur I find in Monterey County, and so far they have all easily keyed to orientale. I've checked numerous previous "strumarium" in the field and on iNaturalist and so far there's been no evidence that would support strumarium, in light of Tomasello. I think strumarium simply doesn't exist in Monterey County, and that as Keil & Hoover noted, the term is simply a "misapplication".

Links:

• Jepson (out of date): https://ucjeps.berkeley.edu/eflora/eflora_display.php?tid=661
• FNA (out of date): http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=135017
• Tomasello: https://www.sciencedirect.com/science/article/pii/S1055790317309363
• A post about Xanthium in northeast US by @peakaytea : https://inaturalist.ca/journal/peakaytea/75415-xanthium-in-the-northeastern-us-what-i-ve-learned-so-far

Key to Xanthium (worldwide) from Tomasello (2018):

"
1 Stems with spines. Leaf blade lanceolate to ovate. Burrs bearing 0-1 apical beak (if 2, then unequal)........................ 2

1’ Stems without spines. Leaf blade cordate to orbicular. Burrs bearing 2 equal beaks ............... 3

2 Erect to decumbent annual reaching 50-100 cm in height. Leafs 3-4 times longer than broader, burrs only lightly hairy................... Xanthium spinosum L.

2’ Decumbent annual reaching 20-30 cm in height; leaves pinnatifid, twice as long as broad. Burrs woolly-hairy .......................... Xanthium ambrosioides Hook & Arn.

3 Burrs glabrous and small, rarely exceeding 1 cm in length, with few, thin prickles and strait beaks. Leaves to a large extent cordate................Xanthium strumarium L.

3’ Burrs glabrous or hirsute, 1 to 3 cm long; densely covered by prickles (if not, then burrs longer than 1 cm and with strong prickles and beaks), with strait or curved beaks. Leaves mostly cuneate-ovate .................................... 4

4 Burrs generally hirsute, up to 3 cm long (including beaks); prickles uncinate; beaks strait or curved .................. Xanthium orientale L.

4’ Burrs generally glabrous, (also subhirsute), seldom exceeding 2 cm in length (including beaks); prickles mostly strait, hooked just in the terminal part; beaks strait ............... Xanthium chinense Mill.
"

Posted on October 21, 2023 12:13 PM by fredwatson