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:
- The large number of annuals that responded to monsoonal rains, including species that we had previously thought responded only to winter / spring rains.
- The large number of perennial species that we had previously thought responded only to winter / spring rains, that responded better to monsoonal rains!
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.
See Information about the order in which the species are presented, and the links from the Scientific Name and Common Name.
The three columns after the common name mark how each species respond to monsoonal rainfall, and to winter / spring rainfall. The columns are:
- M only: species responds only to monsoonal rain
- M best: species responds best to monsoonal rain, but also responds to winter / spring rainfall
- M/ Spring: species responds to both monsoonal rain and to winter / spring rainfall
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
# Famil Scientific name (*)Common Name M
Ann Per 1 Aizoa Trianthema portulacastrum horse-purslane X X 2 Amara Amaranthus fimbriatus fringed amaranth X X 3 Aster Pectis papposa var. papposa chinch-weed X X 4 Eupho Chamaesyce abramsiana Abrams' prostrate spurge X X 5 Eupho Chamaesyce albomarginata rattlesnake weed X X X 6 Eupho Chamaesyce arizonica Arizona spurge X X X 7 Eupho Chamaesyce micromera Sonoran spurge X X 8 Eupho Chamaesyce polycarpa small-seeded spurge X X X 9 Eupho Chamaesyce serpyllifolia ssp. serpyllifolia thyme-leafed spurge X X 10 Eupho Chamaesyce setiloba starfish (Yuma) spurge X X 11 Eupho Ditaxis neomexicana New Mexico ditaxis X X X 12 Eupho Stillingia spinulosa annual stillingia X X X 13 Fabac Senna covesii desert senna X X 14 Geran Erodium texanum Texas filaree X X 15 Malva Sphaeralcea angustifolia copper globemallow X X 16 Marty Proboscidea althaeifolia desert unicorn-plant X X X 17 Mollu Mollugo cerviana *carpet-weed X X 18 Nycta Allionia incarnata var. incarnata trailing four o'clock X X X 19 Nycta Allionia incarnata var. villosa large-flowered trailing four o'clock X X X 20 Nycta Boerhavia coccinea scarlet spiderling X X 21 Nycta Boerhavia coulteri var. palmeri Coulter's spiderling X X 22 Nycta Boerhavia triquetra var. intermedia fivewing spiderling X X 23 Nycta Boerhavia wrightii Wright's spiderling X X 24 Portu Portulaca oleracea *common purslane X X 25 Solan Datura discolor desert thornapple X X 26 Solan Physalis crassifolia thick-leaved ground cherry X X X 27 Solan Solanum elaeagnifolium *horse-nettle X X 28 Zygop Kallstroemia californica California caltrop X X 29 Zygop Kallstroemia parviflora few-flowered caltrop X X 30 Zygop Larrea tridentata creosote bush X X 31 Zygop Tribulus terrestris *puncture-vine X X 32 Poace Aristida adscensionis six-weeks three-awn X X 33 Poace Bouteloua aristidoides var. aristidoides needle grama X X 34 Poace Bouteloua barbata var. barbata six-weeks grama X X 35 Poace Dasyochloa pulchella fluff grass X X X 36 Poace Eragrostis curvula *weeping lovegrass X X 37 Poace Muhlenbergia microsperma littleseed 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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Updated 2 January 2016.