Flora of Sweeney Pass and Volcanic Hills Area, Anza-Borrego Desert State Park

Botanical Highlights
Procedure For Compiling The Checklist
Floral Regions and Coverage Maps
Species Checklist

Fig. 1. The Sweeney Pass / Volcanic Hills Area. Sweeney Pass itself is inside the red ellipse, where S2 switchbacks up from the Carrizo Creek lowlands to the tableland of the Volcanic Hills Area. The Volcanic Hills are capped by lava flows resulting from the opening of the Salton Basin 15 to 22 million years ago.


Sweeney Pass and the Volcanic Hills (Fig. 1) are in the southernmost part of Anza-Borrego Desert State Park. Sweeney Pass itself is a sweeping omega-shaped switchback of highway S2 in the area dividing the Jacumba Mountains from the Coyote Mountains, roughly 14 miles north of I-8. Even in dry years, one typically picks up some blooms along the highway from a good stand of Lupinus arizonicus, Arizona lupine, in the pass, along with blooms of one notable plant of Asclepias albicans, white-stemmed milkweed, just before one emerges from the top of the pass. In good years, the bloom is stupendous!

Driving southeast, one switchbacks up from about 700 feet elevation at the Carrizo Creek crossing to 1200 feet elevation in the flattish alluvial plain leading to the Border Patrol checkpoint at the San Diego / Imperial County border, and then the town of Ocotillo (not to be confused with the off-road vehicle area of Ocotillo Wells 28 miles to the north). (See a one minute panoramic video from the top of the Pass.)

300 feet of that relief is seen on the north side of the top of Sweeney Pass at the Carrizo Badlands overview, looking down in Canyon Sin Nombre. Less visible are the ~200 feet deep canyons of Sweeney Creek to the south, which are an impressive place to botanize; see Fig. 2.

Past Jojoba Wash, S2 enters the Volcanic Hills and even passes through a water gap eroded into the lava flow. The Volcanic Hills are capped by lava flows resulting from the opening of the Salton Basin 15 to 22 million years ago. Very little of the lava itself is left except for thin caps on the top of the mesas and scattered remnant rocks below the mesas.

But the influence of the lava is still seen in the flora. Despite the barren-looking landscape in places, the diversity of the flora is high, and this area contains plant species not found elsewhere in ABDSP, such as Linanthus filiformis and Leptosiphon aureus ssp. decorus. On 14 April 2020, Fred Melgert and Carla Hoegen found 135 species in bloom on a 7.5 mile loop in the Volcanic Hills, which is their all-time record for the number of species in bloom on a single hike. They wrote: "No way! We just broke our all time bloom record by 3 plants and in the most unlikely place".

The ~200 foot high east bank of Sweeney Canyon that is just below the top of Sweeney Pass, but is unseen by motorists on S2. A short stretch of the S2 guardrail is labeled at left. One can walk to an unmarked, unsigned overlook area (labeled in the photograph) by parking along the roadside just after the last curve at the top of Sweeney Pass. Photo taken on 16 December 2012. Click on the picture for a larger version.

Botanical Highlights

There are a number of botanical highlights here, some of which have been mentioned in the Introduction. One additional major highlight is that Sweeney Canyon has the largest population of Carlowrightia arizonica, Arizona carlowrightia, in California. This species was discovered and vouchered here on 12 February 2011 by Jon P. Rebman, Mike Rood & Samantha Rebman-Rood. This was an amazing find, extending the range of this species in California by 15 miles to the south. They reported that the species was "rare with about 8 plants seen".

Tom Chester and Kate Harper specifically went to their location on 16 December 2012 to see this population, and were amazed to find a large population of at least 50 plants (we counted them!) that were unbrowsed. Nearly every other plant of this species that we've seen from five of the six other locations in California has been severely browsed. Subsequently, further surveys by Walt have found over 99 plants of this species here.

Procedure For Compiling The Checklist

The Checklist was compiled from extensive fieldwork by the authors, mostly by the first author Walt Fidler, augmented by voucher records, iNat observations, and Calflora records.



Voucher records were obtained from three separate searches of the Consortium of California Herbaria on 14 March 2020 by Tom Chester:

Duplicate vouchers were removed, and the localities were reviewed manually to remove ones not in the target area. That left 235 vouchers for the Sweeney Pass Floral Area and 316 vouchers for the Volcanic Hills Floral Area.

The top collector in this area by far was Joe Barth, with 190 vouchers. Larry Hendrickson was next, with 85 vouchers, followed by Jon Rebman with 60 vouchers. No other collector had more than 30 vouchers.

iNat observations

There were a whopping 2,425 iNat observations from this area. The top two observers are authors of this flora: the team of Fred Melgert and Carla Hoegen contributed 1,292 observations, and Don Rideout contributed 304 observations. Birgit Knorr contributed 154 observations, and "ebergman" contributed 143 observations. No other observer contributed more than 55 observations.

Calflora records

Calflora records contributed no additional species.

Floral Regions and Coverage Maps

The checklist contains a total of 245 taxa, with separate columns for the Sweeney Pass area flora and for the Volcanic Hills Flora. The floral boundaries are given in Fig. 3, which also shows all GPS points for species in this area.

Fig. 4 shows separately the GPS locations where species were recorded by Walt Fidler (6,332 species GPS points) and/or Tom Chester (857 species GPS points), and the 2,425 species GPS points from iNat.

An additional 1,721 GPS points were taken by Walt Fidler to map the Cylindropuntia ramosissima locations in this area. See Figs. 7 and 8 in Cylindropuntia ramosissima Geographic and Elevational Distribution, and Habitat for a discussion and maps of those points.

Fig. 3. The Floral Boundaries are shown for the two sections of this flora, along with all species GPS points recorded in this area by the authors and by iNat observers (map not yet updated with the iNat points).

Fig. 4. Left: GPS points recorded by Walt Fidler and/or Tom Chester. Right: GPS points recorded by iNat observers, mostly by Fred Melgert and Carla Hoegen. Click on the maps for larger versions.

Checklist for Sweeney Pass and Volcanic Hills Area

For more information on this checklist, see:

The family name is abbreviated to the first six characters in order to save space in the table rows.

An asterisk before the Common Name indicates a non-native taxon.

The column with label BW links to the photo gallery page for each species at Fred Melgert and Carla Hoegen's BorregoWildflowers site.

The last two columns give a minimum estimate of the number of plants from the field surveys, up to a maximum of 99 plants, for the Volcanic Hills area (column label VH) and the Sweeney Pass area (column label SP). The main intent of these estimates is to indicate the species for which we found very few plants. If the columns contain V, it means we have not seen the species but it is vouchered from that area. If the columns contain iN, it means we haven't seen the species; the species hasn't been vouchered; but there is an iNaturalist observation. If the columns contain FC, it is found only in the Fred Melgert / Carla Hoegen plant list for this area.

Version for printing, without other text on this page: html (7 pages) or pdf Clickbook booklet (2 double-sided pages). (See printing instructions for an explanation of these options)

#FamilScientific Name
Link goes to the Jepson eflora
(*)Common Name
Link goes to Calphotos
1SelagiSelaginella eremophiladesert spike-mossBW9999
2PteridCheilanthes covilleibeady lipfernBW16
3PteridCheilanthes parryiwoolly lipfernBW9999
4EphedrEphedra asperaMormon teaBW2560
5EphedrEphedra californicadesert teaBW99
6AcanthCarlowrightia arizonicaArizona carlowrightiaBW99
7AcanthJusticia californicachuparosaBW9999
8ApiaceApiastrum angustifoliumwild celeryBW9999
9ApocynAsclepias albicanswhite-stemmed milkweedBW6991
10ApocynAsclepias subulatarush milkweedBW1726
11ApocynFunastrum hirtellumrambling milkweedBW9953
12ApocynFunastrum utahenseUtah vine milkweedBW13
13ApodanPilostyles thurberiThurber's pilostylesBWV
14AsteraAdenophyllum porophylloidesSan Felipe dogweedBW9999
15AsteraAmbrosia dumosaburroweedBW9999
16AsteraAmbrosia salsola var. salsolacheesebushBW9999
17AsteraBahiopsis parishiiParish's goldeneyeBW913
18AsteraBebbia juncea var. asperasweetbushBW9999
19AsteraBrickellia desertorumdesert brickelliaBW4
20AsteraBrickellia frutescensshrubby brickelliaBW23
21AsteraCalycoseris parryiyellow tackstemBW9999
22AsteraCalycoseris wrightiiwhite tackstemBW9999
23AsteraChaenactis carphoclinia var. carphocliniapebble pincushionBW9999
24AsteraChaenactis fremontiiFremont pincushionBW99
25AsteraChaenactis stevioidesdesert pincushionBW9999
26AsteraEncelia farinosa var. farinosabrittlebushBW9999
27AsteraEricameria brachylepisboundary goldenbushBW31
28AsteraEriophyllum ambiguum var. paleaceumannual woolly sunflowerBW52
29AsteraEriophyllum lanosumwhite easter-bonnetsBW499
30AsteraEriophyllum wallacei var. wallaceiWallace's woolly daisyBW199
31AsteraGeraea canescenshairy desert-sunflowerBW1299
32AsteraGutierrezia sarothraematchweedBW9911
33AsteraHypochaeris glabra*smooth cat's earBW1
34AsteraLactuca serriola*prickly lettuceBW1
35AsteraLasthenia gracilisgoldfieldsBW65
36AsteraLogfia arizonicaArizona herba impiaBW9999
37AsteraLogfia depressadwarf filagoBW9999
38AsteraLogfia filaginoidesCalifornia filagoBW9999
39AsteraMalacothrix glabratadesert dandelionBW9999
40AsteraMalperia tenuisbrown turbansBW599
41AsteraMonoptilon bellioidesdesert starBW9999
42AsteraPalafoxia arida var. aridaSpanish needleBW5189
43AsteraPerityle emoryiEmory's rock-daisyBW9999
44AsteraPeucephyllum schottiipygmy-cedarBW99
45AsteraPleurocoronis plurisetaarrow-leafBW9999
46AsteraPorophyllum gracileodoraBW9961
47AsteraPrenanthella exiguabright-whiteBW6299
48AsteraPsathyrotes ramosissimaturtlebackBW299
49AsteraRafinesquia neomexicanadesert chicoryBW9999
50AsteraSenecio mohavensisMojave ragwortBW9999
51AsteraSonchus oleraceus*sow thistleBW1
52AsteraStephanomeria exigua ssp. exiguaslender wreathplantBW9999
53AsteraStephanomeria pauciflorawire-lettuceBW9999
54AsteraStylocline gnaphaloideseverlasting nest-strawBWFC
55AsteraStylocline intertextatangled nest-strawBWV
56AsteraStylocline micropoidesdesert nest strawBW1020
57AsteraTrichoptilium incisumyellow-headBW9999
58AsteraTrixis californica var. californicaCalifornia trixisBW9999
59AsteraUropappus lindleyisilver puffsBW1699
60BignonChilopsis linearis ssp. arcuatadesert-willowBW4
61BoragiAmsinckia intermediacommon fiddleneckBW2199
62BoragiAmsinckia tessellata var. tessellatabristly fiddleneckBW247
63BoragiCryptantha angustifolianarrow-leaved cryptanthaBW9999
64BoragiCryptantha barbigera var. barbigerabearded cryptanthaBW1099
65BoragiCryptantha maritimaGuadalupe cryptanthaBW9999
66BoragiCryptantha micrantha var. micranthared-root cryptanthaBW9999
67BoragiCryptantha nevadensis var. nevadensisNevada cryptanthaBW2899
68BoragiCryptantha pterocarya var. cyclopteraTucson wing-nut cryptanthaBW199
69BoragiCryptantha pterocarya var. pterocaryawing-nut cryptanthaBW9999
70BoragiCryptantha utahensisscented cryptanthaBW~34
71BoragiEmmenanthe penduliflora var. penduliflorawhispering bellsBW9999
72BoragiEucrypta chrysanthemifolia var. bipinnatifidaeucryptaBW69
73BoragiEucrypta micranthadesert eucryptaBW1699
74BoragiNama demissa var. demissapurple matBW99
75BoragiPectocarya heterocarpachuckwalla pectocaryaBW9999
76BoragiPectocarya peninsularisBaja pectocaryaBW99
77BoragiPectocarya platycarpabroad-fruited combseedBW3699
78BoragiPectocarya recurvatacurvenut combseedBW9999
79BoragiPhacelia cicutaria var. hispidacaterpillar phaceliaBW9999
80BoragiPhacelia crenulata var. ambiguaheliotrope phaceliaBW1
81BoragiPhacelia crenulata var. minutifloralittle-flowered heliotrope phaceliaBW9999
82BoragiPhacelia cryptanthalimestone phaceliaBW77
83BoragiPhacelia distanscommon phaceliaBW9999
84BoragiPhacelia minorwild canterbury bellsBW4
85BoragiPhacelia neglectaalkali phaceliaBWV
86BoragiPhacelia pedicellatapedicellate phaceliaBW9999
87BoragiPholisma arenariumsand plantBW68
88BoragiPlagiobothrys jonesiiJones' popcorn flowerBW499
89BoragiTiquilia palmeriPalmer's coldeniaBW991
90BoragiTiquilia plicataplicate coldeniaBW99
91BrassiBrassica tournefortii*Sahara mustardBW9999
92BrassiCaulanthus cooperiCooper's jewel-flowerBW1
93BrassiCaulanthus lasiophyllusCalifornia mustardBW4699
94BrassiDescurainia pinnatawestern tansy-mustardBW2399
95BrassiDithyrea californicaspectacle-podBW199
96BrassiDraba cuneifoliawedge-leaved drabaBWV32
97BrassiEruca vesicaria ssp. sativa*garden rocket1
98BrassiLepidium fremontiidesert pepper-grassBW1
99BrassiLepidium lasiocarpum ssp. lasiocarpumhairy-podded pepper-grassBW9999
100BrassiSisymbrium irio*London rocketBW9999
101BrassiSisymbrium orientale*Oriental mustardBWV
102BrassiStreptanthella longirostrislongbeak streptanthellaBWV
103BrassiThysanocarpus curvipes ssp. eradiatusfringe-podBW128
104CactacCylindropuntia bigeloviiteddy-bear chollaBW9999
105CactacCylindropuntia echinocarpasilver chollaBW9999
106CactacCylindropuntia ganderiGander's chollaBW996
107CactacCylindropuntia ramosissimapencil chollaBW9999
108CactacCylindropuntia wolfiiWolf's chollaBW9999
109CactacEchinocereus engelmanniiEngelmann's hedgehog cactusBW2899
110CactacFerocactus cylindraceusCalifornia barrel cactusBW9999
111CactacMammillaria dioicaCalifornia fish-hook cactusBW5597
112CactacMammillaria tetrancistradesert fish-hook cactusBW518
113CactacOpuntia basilaris var. basilarisbeavertail cactusBW9999
114CampanNemacladus glanduliferusglandular nemacladusBW9999
115CampanNemacladus orientaliseastern glandular nemacladusBW9999
116CampanNemacladus rubescensdesert nemacladusBW9999
117CampanNemacladus tenuis var. aliformisdesert threadplantBWV
118CampanNemacladus tenuis var. tenuisdesert threadplantBW9999
119CaryopAchyronychia cooperifrost matBW116
120ChenopAtriplex hymenelytradesert hollyBW3699
121ChenopAtriplex polycarpacattle saltbushBW9966
122ChenopChenopodium murale*nettle-leaved goosefootBW36
123ConvolCuscuta denticulatadesert dodderBW1iN
124ConvolCuscuta psorothamnensisindigo bush dodderBW9999
125CrassuCrassula connatapygmy-weedBWiN99
126CrassuDudleya arizonicaArizona chalk dudleyaBW1926
127CrassuDudleya saxosa ssp. aloidesdesert dudleyaBW5
128CucurbCucurbita palmatacoyote melonBW235
129EuphorChamaesyce micromeraSonoran spurgeBW50
130EuphorChamaesyce polycarpasmall-seeded spurgeBW9999
131EuphorChamaesyce setilobastarfish (Yuma) spurgeBW756
132EuphorCroton californicusCalifornia crotonBW691
133EuphorDitaxis lanceolatanarrowleaf ditaxisBW9999
134EuphorDitaxis neomexicanaNew Mexico ditaxisBWiN
135EuphorStillingia linearifolialinear-leaved stillingiaBW99
136FabaceAcmispon glaber var. brevialatusshort-winged deerweedBW5976
137FabaceAcmispon maritimus var. brevivexillusshort-bannered coastal lotusBW9999
138FabaceAcmispon rigidusdesert lotusBW481
139FabaceAcmispon strigosusstrigose lotusBW9999
140FabaceAstragalus nuttallianus var. cedrosensisCedros milk-vetchBW699
141FabaceAstragalus nuttallianus var. imperfectusturkey-peasBW99
142FabaceDalea mollissilky daleaBW22
143FabaceDalea mollissimadowny daleaBW43
144FabaceLupinus arizonicusArizona lupineBW9999
145FabaceLupinus bicolordwarf lupineBW2
146FabaceLupinus concinnusbajada lupineBW9949
147FabaceLupinus sparsiflorusCoulter's lupineBW2
148FabaceProsopis glandulosa var. torreyanahoney mesquiteBW22
149FabacePsorothamnus emoryiEmory's indigo-bushBW9999
150FabacePsorothamnus schottiiindigo bushBW9999
151FabacePsorothamnus spinosussmoke treeBW9957
152FabaceSenegalia greggiicatclawBW9999
153FouquiFouquieria splendens ssp. splendensocotilloBW9999
154GeraniErodium cicutarium*redstem filareeBW9999
155GeraniErodium texanumTexas filareeBW9999
156KramerKrameria bicolorwhite rhatanyBW9999
157KramerKrameria erectaPima rhatanyBW9999
158LamiacHyptis emoryidesert-lavenderBW9999
159LamiacSalvia columbariaechiaBW9999
160LamiacSalvia vaseyiVasey's sageBW9973
161LamiacScutellaria mexicanaMexican bladder sageBW99
162LoasacMentzelia affinisyellow blazing starBW1+
163LoasacMentzelia albicauliswhite-stemmed blazing starBWiNiN
164LoasacMentzelia hirsutissimahairy stickleafBW7399
165LoasacMentzelia involucratabracted blazing starBW9999
166LoasacMentzelia veatchianaVeatch's blazing starBW'1+?
167LoasacPetalonyx thurberi ssp. thurberiThurber's sandpaper-plantBWV
168MalvacAyenia compactaayeniaBW741
169MalvacEremalche exiliswhite mallowBW28
170MalvacEremalche rotundifoliadesert five-spotBW4699
171MalvacHibiscus denudatusrock hibiscusBW9999
172MalvacHorsfordia newberryiNewberry's velvet mallowBW991
173MalvacSphaeralcea ambigua var. ambiguaapricot mallowBW63
174MontiaCalyptridium monandrumsand cressBWFC23
175MontiaCistanthe ambiguadead man's fingersBW99
176NyctagAllionia incarnata var. incarnatasmall-flowered trailing four o'clockBW223
177NyctagBoerhavia triquetra var. intermediafivewing spiderlingBW2V
178NyctagBoerhavia wrightiiWright's spiderlingBW99
179NyctagMirabilis laevis var. retrorsaBigelow's desert four-o'clockBW2999
180NyctagMirabilis tenuilobaslender-lobed four o'clockBW9920
181OleaceMenodora scabra var. glabrescensbroom twinberryBW3
182OnagraCamissoniopsis pallida ssp. pallidapale suncupBW13
183OnagraChylismia cardiophylla ssp. cardiophyllaheartleaf suncupBW241
184OnagraChylismia claviformis ssp. peirsoniibrown-eyed primroseBW9999
185OnagraEremothera boothii ssp. condensataBooth's evening primroseBW9999
186OnagraEremothera chamaenerioideslong-fruit suncupBW3499
187OnagraEulobus californicusCalifornia suncupBW9999
188OrobanOrobanche cooperiCooper's broomrapeBWV
189PapaveEschscholzia minutiflora ssp. minutiflorasmall-flowered poppyBW9999
190PapaveEschscholzia parishiiParish's poppyBW9999
191PhrymaMimulus bigelovii var. bigeloviiBigelow's monkeyflowerBW9999
192PlantaAntirrhinum filipesdesert twining snapdragonBW1943
193PlantaMohavea confertifloraghost flowerBW9999
194PlantaPenstemon clevelandii var. clevelandiiCleveland's beardtongueBW11
195PlantaPlantago ovatadesert plantainBW9999
196PolemoAliciella latifolia ssp. latifoliabroad-leaf giliaBW9999
197PolemoEriastrum eremicum ssp. eremicumdesert woolly-starBW9999
198PolemoGilia stellatastar giliaBW9999
199PolemoLangloisia setosissima ssp. setosissimabristly langloisiaBW9999
200PolemoLeptosiphon aureus ssp. decorusgolden linanthusBW99
201PolemoLinanthus filiformisyellow giliaBW99
202PolemoLinanthus jonesiiJones' linanthusBW9999
203PolemoLoeseliastrum matthewsiidesert calicoBW9999
204PolemoLoeseliastrum schottiiSchott's calicoBW9999
205PolygoChorizanthe brevicornu var. brevicornubrittle spineflowerBW9999
206PolygoChorizanthe corrugatawrinkled spineflowerBW2
207PolygoChorizanthe rigidadevil's spineflowerBW9999
208PolygoEriogonum deflexum var. deflexumflat-topped buckwheatBW2299
209PolygoEriogonum fasciculatum var. polifoliumCalifornia buckwheatBW9999
210PolygoEriogonum inflatumdesert trumpetBW9999
211PolygoEriogonum reniformekidney-leaf buckwheatBW3599
212PolygoEriogonum thomasiiThomas' buckwheatBW9999
213PolygoEriogonum trichopeslittle desert trumpetBW9999
214PolygoEriogonum wrightii var. nodosumWright's buckwheatBW9999
215PolygoPterostegia drymarioidesfairy bowties, threadstemBW9999
216RanuncClematis paucifloravirgin's bowerBW1
217RanuncDelphinium parishii ssp. subglobosumintermediate larkspurBW21
218ResedaOligomeris linifolialineleaf whitepuffBWiN99
219RosacePrunus fremontiidesert apricotBW1781
220RubiacGalium stellatumstar-flowered bedstrawBW9999
221RutaceThamnosma montanaturpentine broomBW99
222SimmonSimmondsia chinensisjojobaBW9999
223SolanaDatura discolordesert thornappleBW216
224SolanaLycium andersoniiAnderson's boxthornBW36
225SolanaLycium fremontiiFremont's boxthornBW51
226SolanaLycium parishiiParish's boxthornBW1834
227SolanaNicotiana obtusifoliadesert tobaccoBW4399
228SolanaPhysalis crassifoliathick-leaved ground cherryBW3451
229UrticaParietaria hespera var. hesperapellitoryBW9999
230ViscacPhoradendron californicumdesert mistletoeBW3099
231ZygophFagonia laevisCalifornia fagoniaBW9999
232ZygophFagonia pachyacanthasticky fagoniaBW9999
233ZygophLarrea tridentatacreosote bushBW9999
234AgavacAgave deserti var. desertidesert agaveBW9999
235AgavacHesperocallis undulatadesert lilyBW9999
236PoaceaAristida adscensionissix-weeks three-awnBWiNV
237PoaceaAristida purpureapurple three-awnBW6
238PoaceaBromus madritensis ssp. rubens*red bromeBW240
239PoaceaBromus tectorum*downy bromeBWV
240PoaceaFestuca bromoides*brome fescueBW20
241PoaceaFestuca octoflorasix-weeks fescueBW5
242PoaceaHilaria rigidabig galletaBW9999
243PoaceaSchismus barbatus*Mediterranean schismusBW9999
244RuscacNolina bigeloviiBigelow's nolinaBW18
245ThemidDichelostemma capitatum ssp. capitatumblue dicksBW99

Voucher data provided by the participants of the Consortium of California Herbaria (ucjeps.berkeley.edu/consortium/)

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Copyright © 2012-2021 by Walt Fidler, Tom Chester, Carla Hoegen, Fred Melgert, Don Rideout, Nancy Accola, Gaylee Rogers, Steve Rogers, Joe Woods, Jim Roberts, Abbyann Sisk, and Kate Harper
Permission is freely granted to reproduce any or all of this page as long as credit is given to us at this source:
Comments and feedback: Tom Chester
Last update: 30 January 2021