Plant Species of the Borrego Desert: Monsoonal Species

Fig. 1. Left: Datura discolor, desert thornapple, a summer annual photographed by Kate Harper along S22 in Agua Caliente County Park on 10 December 2015. Right: A field of Pectis papposa, chinch-weed, a summer annual photographed by Tom Chester on the Mescal Bajada on 2 September 2014. Note the scale given by the car in the background on the linked larger picture.
Click on the pictures for larger versions.

Tom first began botanizing the Borrego Desert in earnest in 2005, which was a glorious year with good monsoonal rainfall and good winter rainfall. He observed a number of species in that year that he wasn't to see again for many years, such as Chamaesyce arizonica, Datura discolor and Ditaxis neomexicana. On repeat visits to the same areas where he had seen those species, he couldn't understand why those species were no longer present, even though those years had good rainfall which germinated many other annuals.

Finally in 2012, after a decent monsoonal rainfall, it became clear to Tom that these were all species germinated by warm summer rainfall from thunderstorms in July, August and September, and which would not germinate from winter rains. Tom first added the list of Summer Annuals Germinated by Monsoonal Rainfall to the Bloom Reports from the Anza-Borrego Desert in 2012.

In the fall of 2013, after some very good monsoonal rains, Kate and Tom began botanizing the Borrego Desert earlier than normal, beginning on 29 August 2013. Both were stunned by two things:

Table 1 gives the list of plants that respond significantly to monsoonal rainfall. The list derives mostly from observations in late 2013 / early 2014.

Most of these summer annuals won't be seen in a given year at any time unless there has been sufficient summer rainfall (Aristida adscensionis is one of the few summer annuals that is also a spring annual). Good monsoonal rainfall occurs in fewer than half of all years. Monsoonal rainfall is always spotty, with thunderstorms soaking an area of perhaps one square mile, and not touching surrounding areas.


The column with header Famil gives the first five letters of the Family name.

An asterisk before the common name indicates a non-native species.

The three columns after the common name mark how each species respond to monsoonal rainfall, and to winter / spring rainfall. The columns are:

The next two columns indicate whether a species is an annual or perennial, or can be both at various times.

Table 1. Species That Respond Significantly To Monsoonal Rainfall

#FamilScientific name
Link goes to the Jepson eflora
(*)Common Name
Link goes to Calphotos
M /
1AizoaTrianthema portulacastrumhorse-purslane X   X 
2AmaraAmaranthus fimbriatusfringed amaranth X   X 
3AsterPectis papposa var. papposachinch-weed X   X 
4EuphoChamaesyce abramsianaAbrams' prostrate spurge X   X 
5EuphoChamaesyce albomarginatarattlesnake weed  X  XX
6EuphoChamaesyce arizonicaArizona spurge X   XX
7EuphoChamaesyce micromeraSonoran spurge X   X 
8EuphoChamaesyce polycarpasmall-seeded spurge  X  XX
9EuphoChamaesyce serpyllifolia ssp. serpyllifoliathyme-leafed spurge  X  X 
10EuphoChamaesyce setilobastarfish (Yuma) spurge  X  X 
11EuphoDitaxis neomexicanaNew Mexico ditaxis X   XX
12EuphoStillingia spinulosaannual stillingia X   XX
13FabacSenna covesiidesert senna  X   X
14GeranErodium texanumTexas filaree   X X 
15LoasaEucnide rupestrisrock nettle X   X 
16MalvaSphaeralcea angustifoliacopper globemallow X    X
17MartyProboscidea althaeifoliadesert unicorn-plant X   XX
18MolluMollugo cerviana*carpet-weed X   X 
19NyctaAllionia incarnata var. incarnatatrailing four o'clock  X  XX
20NyctaAllionia incarnata var. villosalarge-flowered trailing four o'clock  X  XX
21NyctaBoerhavia coccineascarlet spiderling  X   X
22NyctaBoerhavia coulteri var. palmeriCoulter's spiderling  X  X 
23NyctaBoerhavia triquetra var. intermediafivewing spiderling X   X 
24NyctaBoerhavia wrightiiWright's spiderling X   X 
25PortuPortulaca oleracea*common purslane X   X 
26SolanDatura discolordesert thornapple X   X 
27SolanPhysalis crassifoliathick-leaved ground cherry  X  XX
28SolanSolanum elaeagnifolium*horse-nettle X    X
29ZygopKallstroemia californicaCalifornia caltrop X   X 
30ZygopKallstroemia parviflorafew-flowered caltrop X   X 
31ZygopLarrea tridentatacreosote bush   X  X
32ZygopTribulus terrestris*puncture-vine   X X 
33PoaceAristida adscensionissix-weeks three-awn   X X 
34PoaceBouteloua aristidoides var. aristidoidesneedle grama X   X 
35PoaceBouteloua barbata var. barbatasix-weeks grama X   X 
36PoaceDasyochloa pulchellafluff grass X   XX
37PoaceEragrostis curvula*weeping lovegrass X    X
38PoaceMuhlenbergia microspermalittleseed muhly  X  X 

We thank Larry Hendrickson and Mike Bigelow for noticing that Boerhavia coccinea was previously missing from the table.

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Copyright © 2012-2018 by Tom Chester and Kate Harper.
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Updated 19 January 2018.