Plants of Southern California: Opuntia acanthocarpa var. coloradensis

    The Extreme Northeast San Diego County Voucher
Key and Photographs


Many botanists in southern California became greatly confused when the Jepson Manual did not have a main entry for the most common cholla by far on the extreme western side of the Sonoran Desert, Opuntia ganderi = O. acanthocarpa var. ganderi. Its only mention was an obscure note under O. parryi that claimed it was a hybrid between O. parryi and O. echinocarpa, without citing any supporting evidence.

When O. ganderi was deleted as an entry in the Jepson Manual, many botanists began labeling specimens of O. ganderi as the only variety of O. acanthocarpa given in the Jepson Manual, var. coloradensis. Such determinations of O. ganderi specimens are incorrect; these are two distinct taxa.

This page presents a distribution map for Opuntia acanthocarpa var. coloradensis, discusses the controversial voucher for this taxon from extreme northeast San Diego County, and gives pictures of true specimens of this taxon.

For an overview of chollas in southern California, see Plants of Southern California: Chollas.

For Opuntia ganderi, see Opuntia echinocarpa, O. ganderi, O. parryi, and O. wolfii: Pictorial Identification Guide.


Lyman Benson recognized this taxon in 1969, and named it var. coloradensis since this taxon is confined to the area of the Lower Colorado River. In California, all undisputed specimens are within ~20 miles of the Colorado River. The same concentration to the area of the Colorado River is seen in Arizona, but there a number of specimens extend to ~70 miles east of the Colorado River, with one specimen ~150 miles east.

The following map gives California voucher locations with positions from the Consortium of California Herbaria, augmented with California locations from Benson (1969), and Arizona voucher locations from the Flora of Arizona (1999). It also shows a slightly-modified smoothed Distribution Map from Flora of North America:

The smooth blue contours delineate the range of all vouchers that are undisputed Opuntia acanthocarpa var. coloradensis.

There are two vouchers far to the west of that contour. The one on the west side of the Salton Sea is the voucher discussed below from extreme northeast San Diego County. I have not examined the other voucher from just west of Ridgecrest (UC1121720 from the year 1934), but specimens in the field there should be examined before that voucher is accepted as being Opuntia acanthocarpa var. coloradensis.

The Extreme Northeast San Diego County Voucher

In 1937, Frank Gander collected a cholla from extreme northeast San Diego County that has been the source of much interest and debate. This location is disjunct by some 70 miles west of the nearest known population in California, a total of 80 miles from the Colorado River.

Gander's voucher consists of only a single stem segment and one flower, mounted upside down, so confidence in its determination is not absolute. His voucher was probably originally determined simply as Opuntia acanthocarpa, since varieties ganderi and coloradensis were not defined until 1938 and 1969, respectively. The latest (1986) determination of his voucher is Opuntia acanthocarpa var. coloradensis (henceforth O.a.c.), from the reddish filaments of the then-49 year old flower and the characteristics of that single stem segment.

Because of uncertainty in the determination of this voucher, Jon Rebman has long been encouraging people to look for the plants that were vouchered by Gander. Bill Sullivan has searched three times for this taxon in 2005-2006 without success, finding only infrequent specimens of O. ganderi. Wayne Armstrong and I joined Bill to search for those plants in the field on 21 January 2008 and found only very clear O. ganderi in a single location.

Of course, extreme northeast San Diego County is a very large area, and it is therefore impossible to claim definitively that O.a.c. does not exist someplace here without extensive further searches. But there are three good reasons to believe that O.a.c. does not occur here:

Of course, plants sometimes disregard good reasons similar to the ones above, and further surveys in this area are planned. But I would not bet that any true O.a.c. will ever be found here.

Key and Photographs

Opuntia acanthocarpa var. coloradensis can be distinguished from O. ganderi using the following key:

1. Filaments red; inner tepals bright yellow; trees or sometimes shrubs, with few branches, usually at acute angles, without strongly ascending branches; stem segments 15-30 cm long; stem tubercles (1.5-)2-3 cm; spines 10-15 per areole .... O. acanthocarpa var. coloradensis

1'. Filaments greenish-white to yellow to green, sometimes suffused with bronze or rose, but not red; inner tepals greenish yellow, often with tips reddish abaxially; shrubs or sometimes trees, with many branches, with tips of major branches strict, ascending; stem segments usually 10-26 cm long; stem tubercles usually 1.3-2.6 cm; spines 15-25 per areole ... O. ganderi

Fortunately, since flowers are not frequently seen in the field, the other characteristics work well to separate these two taxa.

Photographs of specimens from Ogilby Road at the base of the Cargo Muchacho Mountains 20 miles west of the Colorado River are given below. The Cargo Muchacho Mountains are at the southeast tip of the Chocolate Mountains labeled in the above map.

The photographs were taken in harsh noon-time sunlight in summer, and hence the plants do not show up as well as photography taken under more optimum conditions.

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Copyright © 2008 by Tom Chester
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Last update: 27 January 2008