Plant Species of San Jacinto Mountain

Toxicodendron diversilobum,
poison oak

Fig. 1. Poison oak observed on 7 November 2012 in "Idyllwild Creek" at 5900 feet elevation just below the Deer Springs Trail. Left: View of fallen leaves that are mostly "leaves of 1" and not "leaves of 3". Right: Naked stems that have lost their leaves. Click on pictures for larger versions.


One of the delights of botanizing the higher elevations of San Jacinto Mountain is the complete absence of poison oak - or so we thought until we encountered over 50 plants in one spot in Idyllwild, in the creek that parallels the Deer Springs Trail at 5830 to 5900 feet elevation (called Idyllwild Creek on some maps). Unfortunately, the plants had lost their leaves, leaving only woody sticks with pretty red dead leaves below them that didn't even have the decency to remain intact as "leaves of three". As a result, we were handling the leaflets and hiking through the stems, oblivious to any possibility that these might be poison oak, convinced they must be some benign species which we just couldn't identify in its current state.

When we finally came upon a plant that still had a few leaves attached, making the identification crystal-clear, Tom reacted with complete horror since he now usually needs a cortisone shot to stop his immune system from over-reacting to the oil from this plant. (Tom used to think he was immune to poison oak, since he estimates he got about 200 free exposures to it, with no reaction at all, before his immune system decided to react violently to the 201st exposure.)

This page exists primarily to alert hikers to be aware that poison oak does indeed exist in the Idyllwild area and at lower elevations at San Jacinto Mountain. We present a map showing the distribution of poison oak at San Jacinto Mountain, and a table describing those locations, many of which were reported to us by others. If you know of locations not mentioned below, please let us know.

Our former belief that poison oak was absent from the higher elevations of San Jacinto Mountain was well grounded. Throughout southern California, poison oak is almost entirely found only below 5000 feet elevation. Out of 674 vouchers in the Consortium of California Herbaria, only two have an elevation above 5000 feet, both from San Diego County. One is from Palomar Mountain at 5380 feet, and one is from Cuyamaca Peak at 6300 feet. Our observations at 5830 to 5900 feet are now the second highest record for this species.

Distribution at San Jacinto Mountain

The following map shows all the poison oak locations known to us, plotted using the Berkeley Mapper.

The locations are given in Table 1 in order of declining elevation, along with the observer who reported each location.

Table 1. Locations Of Poison Oak at San Jacinto Mountain

Elevation (feet)LocationObserver
6080Pine CoveMichael Wangler
5940"Idyllwild Creek", the creek east of Lilly Creek, north of the Deer Springs TrailheadTom Chester, Dave Stith
5380a few plants at nw corner of Lolomi Lodge, James Reserve, Hall Canyon (only known location at James Reserve)Michael Hamilton, Andy Sanders
5350Fuller Mill CreekLarrynn Carver
5150Lawler LodgeMichael Duval, Bruce Watts
4550Pine Tree Spring, south of Lake HemetJordan Zylstra
4330Below Lake Hemet DamEric Baecht
4280North Fork, San Jacinto River, at 5S09Norm Johnson
4070South Fork, San Jacinto River, at South Fork TrailJordan Zylstra
3840Peach Tree Spring area below the Vista Grande Guard StationKen Berg
3780Halfway Spring on Old Control RoadNorm Johnson
3550North Fork, San Jacinto River, at bottom of Webster TrailDave Stith
2920SR74 at mouth of Strawberry CreekNorm Johnson
2640Needle's Eye, Indian Creek, above Palm CanyonEric Baecht
2160South Fork, San Jacinto River, 0.4 miles west of jct. with Old Control RoadTom Chester
2090"South Fork, San Jacinto River, east of Cranston RS"Voucher by M. Wall
1900South Fork, San Jacinto River, at Cranston RSJordan Zylstra

Note the complete absence of locations above 6500 feet, which we have extensively surveyed without finding any poison oak plants.

Ken Berg reported that poison oak was:

abundant in the "Soboba burn" area on the west slope below the James Reserve. [Tim] Krantz and I did vegetation mapping for USFS in spring 1978 and thrashed through it regularly on transects... It ... wouldn't surprise me if we had a veg type that was ToDi [poison oak] subdominant.


Mark Reese alerted us to the mention to "watch out for poison oak on the river bed" in the lower section of the North Fork, below the use trail from Fisherman Point, from San Jacinto River (North Fork) Ropewiki. Most of that area is owned by the Lake Hemet Municipal Water District, and is not open to the public.

If you know of locations not mentioned below, please let us know.

Distribution in Southern California

The following histogram of elevations from vouchers from Los Angeles, Orange, Riverside, San Bernardino and San Diego Counties shows that poison oak is most definitely happiest at lower elevations:

Vouchers are most frequent at elevations of sea level to 1000 feet, with the number of vouchers declining almost linearly with elevation to 5000 feet. Above 5000 feet, there are only the two vouchers discussed above.

For a very different histogram, see the plot for Arctostaphylos pungens, Mexican manzanita.

The vouchers were retrieved from the Consortium of California Herbaria on 9 November 2012.

The following map shows poison oak locations from all vouchers:

This plot makes it appear that in southern California poison oak is mostly confined to San Diego County, with a smattering of low elevation distributions to the north. This may, in fact, be correct, but one has to keep in mind voucher distributions can be quite misleading due to various biases. The biggest one in this case might be that few people want to collect this species, due to the "poison" (and we certainly fall into that category, too).

San Diego County probably stands out for two reasons. First is the San Diego County Plant Atlas, which has encouraged the complete collection of specimens in each small grid cell, with a number of cells having plant collections.

Second, San Diego County may well have more of the favored habitat for poison oak, which is moist, shady, low-elevation canyons. (It can live in other low-elevation habitats where it has enough moisture, such as out in the open on the 2000 foot grasslands of the Santa Rosa Plateau.)

The terrain of San Diego County west of the mountain crest contains abundant low-elevation canyons, whereas the terrain to the north has extensive open flattish basins (the Los Angeles Basin and the San Bernardino / Hemet / Perris plains) as well as mountains that rise much more abruptly from the surrounding area.

Note how few vouchers are east of I-215 and north of San Diego County. There is no reason to think that the San Bernardino and San Jacinto Mountains would be this significantly undercollected relative to the San Gabriel Mountains, so it is probably true that poison oak just doesn't find conditions there are suitable as in San Diego County. Moist, shady canyons are mostly absent in the Perris Plain area east of I-15, and ones below 5000 feet elevation are mostly absent at San Jacinto Mountain except in the major drainages such as the North Fork and Strawberry Creek.

Poison oak is a very widespread taxon in California. The voucher location map (first map on the right halfway down the page) shows a dense distribution of locations all the way to the Oregon border, essentially mirroring all voucher locations (the red dots on the map) except for its absence in the desert.

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Copyright © 2012-2021 by Tom Chester, Dave Stith and Norm Johnson.
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Comments and feedback: Tom Chester
Updated 27 February 2021.