Plant Species of San Jacinto Mountain:
Tetradymia comosa, cotton-thorn

Table of Contents

Introduction and Geographic Distribution of Tetradymia species of southern California
Geographic Distribution of Tetradymia comosa at San Jacinto Mountain
Survey Notes From Bruce Watts
Comparison of post-1986 Voucher Collections to pre-1980 Voucher Collections


Fig. 1. Photographs of Tetradymia comosa in flower and in fruit taken by Bruce Watts in Garner Valley in late August 2016, showing how varied the appearance of the plants can be at a single time of year. Click on the pictures for larger versions.

Introduction and Geographic Distribution of Tetradymia species of southern California

The genus Tetradymia consists of ten species that all grow only in western North America. Michael Charters gives the origin of the name of this genus as being from the Greek tetra, "four," and dymos, "together," from the four-flowered heads of the first known species of this genus. The first named species of this genus was T. canescens, which has four disk flowers and four phyllaries. The phyllaries are persistent long after the fruit has dispersed; see Steve Matson's photograph of the four persistent phyllaries of T. canescens, along with the four holes in the receptacle that formerly held the four disk flowers. Our species at San Jacinto Mountain, T. comosa, usually has 6-9 flowers per head, but occasionally 5 flowers per head, with typically five phyllaries per head, occasionally six.

There are six species of Tetradymia found in southern California: T. axillaris, T. argyraea, T. canescens, T. comosa, T. glabrata and T. stenolepis. Their geographic distributions in southern California as given by vouchers are shown in Fig. 2; click on the maps to get their complete geographic distribution in all of California.

T. argyraea

T. axillaris

T. canescens

T. comosa

T. glabrata

T. stenolepis
Fig. 2. The geographic distribution of five Tetradymia species in southern California as given by vouchers. Click on the maps to get the geographic distribution in all of California for each species.

Geographic Distribution of Tetradymia comosa at San Jacinto Mountain

Tetradymia comosa is the only one of the six Tetradymia species in southern California that is found in the coastal areas of southern California, and the only one present at San Jacinto Mountain. In fact, it is endemic to southern California and northern Baja California; see voucher locations for California and Mexico from SEINet queried on 3 September 2016.

Much of the population of Tetradymia comosa grows at lower elevations than the other Tetradymia species, and hence much of its population is geographically separated from the other species. However, since T. comosa spills out into the edge of the desert near Cajon Pass where two other species grow, there is some overlap in its geographic distribution with those species, as well as some overlap in elevation ranges.

The main population of Tetradymia comosa is centered on the Riverside / Temecula basin, extending into the mountain ranges along its edges and into the northern Los Angeles Basin and the Santa Clara River area. There is a disjunct population in the southern edge of San Diego County, from El Cajon to Jacumba, and the northern part of Baja California.

Because the area of the main population of this species is now occupied by dense human development, this species is now mostly restricted to the edges of its former distribution, in what might not be ideal habitat for this species. This may be yet another of the species formerly common that have been made rare by human encroachment on its habitat. Worse, it is possible that the edge populations depend on replenishment from the main population, which is no longer possible. Surveys are needed to establish what the current numbers are for this species. See Comparison of post-1986 Voucher Collections to pre-1980 Voucher Collections for what may have happened to this species.

One of those needed surveys was done by Bruce Watts, who surveyed Garner Valley for Tetradymia comosa on eight days from 22 August to 3 September 2016, and found a total of 109 plants. He found a few more patches on subsequent surveys, for a total of 159 plants as of 26 November 2016. Although this is far more than were previously known from Garner Valley, this is one of the rarest species at San Jacinto Mountain.

There is just a single voucher of it from 1964 taken by L.B. Ziegler. Tom had never found this in Garner Valley from his previous surveys, although Tom found a small number of plants east of Garner Valley on the PCT both north and south of SR74 near the Desert Divide. There is also a single voucher from the nearby Vandeventer Flat.

The population in Garner Valley and on the PCT Desert Divide is at the eastern edge of the main population, and the plants here are at the highest elevation known for this species.

Fig. 3 shows the voucher locations in this area in more detail.

Fig. 3. The geographic locations of vouchers of T. comosa for the San Jacinto Mountain area. The rectangle shows the area displayed in Fig. 4.

Fig. 4 shows a Google Earth view of the locations of the 159 plants found by Bruce. See Fig. 3 for the location of the area displayed in Fig. 4 on a map.

Fig. 4. Locations of the 159 plants of Tetradymia comosa found by Bruce Watts in eight days of targeted surveys and from a few subsequent surveys. See Fig. 3 for the location of this area on a map.

Survey Notes From Bruce Watts

As with most plant species, one has to develop a good search image to become good at spotting a given species. In the case of T. comosa, it is very easy to pass by plants that are not in flower without a second thought since at first glance they look like just another plant of Artemisia tridentata, Great Basin sagebrush, or just some dead shrub. I, as well as Tom, have done that in the past in the spring (this species blooms in late summer and early fall).

Once you know what to look for, plants of Tetradymia comosa are fairly easy to spot from a distance, since they are the whitest plants around when they are not in bloom, with almost a bleached appearance. The most similar other species from a distance is Artemisia tridentata, Great Basin sagebrush, but it has dark branches. Most other aster family plants have green stems or leaves.

Knowing this I could scan the hills from a distance and only have to walk toward them if I spotted something white. Sometimes dried grasses that were in the sun could look similar but I soon learned the difference so I did not have to walk over to inspect them.

The plants were mostly found near roads and on flat ground.

Comparison of post-1986 Voucher Collections to pre-1980 Voucher Collections

There were 212 vouchers of T. comosa as of 2 November 2016, collected from 1875 to 2013. In order to investigate whether this species might now be extirpated from most of its former range, we separated the vouchers into the pre-1980 period (63 years with collections) and the post-1986 period (23 years with collections). This was a natural split in the data, since no vouchers were collected in the years 1980 to 1986.

Fig. 5 shows a geographic map for those two periods of time, and Fig. 6 shows the areas where T. comosa may no longer be present, or present with much-reduced numbers.

These maps should be interpeted with caution, since there are 2.7 times more years represented by the pre-1980 period than by the post-1986 period. But it does appear that some areas in the post-1986 map have far fewer than 1/3 of the number of vouchers that they had in the pre-1980 period, while other areas appear to still have their fair share of vouchers.

Fig. 5. Geographic maps of vouchers of T. comosa excluding the San Diego County portion of its range. Left: Map made using only vouchers taken from 1875 to 1979. Right: Map made using only vouchers taken from 1987 to 2013. Click on the pictures for larger versions that show the full range for this species.

Fig. 6. A map of vouchers of T. comosa taken before 1980, showing areas where there have been no or few vouchers collected after 1986, possibly because T. comosa has been extirpated in those areas by urban development. This map does not show the population found in southeast San Diego County.

Voucher data provided by the participants of the Consortium of California Herbaria (

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Copyright © 2016 by Bruce Watts and Tom Chester.
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Comments and feedback: Tom Chester
Updated 26 November 2016.