Background Information for Bloom Reports from the Anza-Borrego Desert
Table of Contents
Annual Germination, Growth and Blooms
General Requirements for Annual Germination
Bad Bloom Years Are All The Same; Good Bloom Years Are All Different
Peak Bloom: What Does That Mean?How Long Will An Annual Bloom Last
General FactorsIntroduction
See Current and Previous Bloom Reports from the Anza-Borrego Desert for a link to the latest summary of the bloom, as well as links to previoius bloom reports. This page gives background on this and previous bloom reports.
The bloom report page reports the current bloom status in Anza-Borrego Desert State Park and nearby areas below 3000 feet elevation. We began using this expanded area in 2010-2011. Prior to then, the pages were restricted to the Borrego Desert portion of the Park. That page also gives general information about the bloom for all species in this area, with emphasis on the annuals that are responsible for the widespread showy blooms that appear in some years on the desert floor.
This page also gives some general information on what is needed to germinate those annuals, and what is needed to sustain the annual bloom.
The information here is by no means a definitive list to what is blooming at all locations in the Anza-Borrego Desert; it only records the species we've seen in bloom on trips that occur roughly every fourth day, occasionally augmented by observations from other people. Because the locations change, the numbers of species in bloom, and the number of plants in bloom, cannot usually be directly compared from trip to trip. However, the information here will give the reader an idea of what the bloom is doing in the Anza-Borrego Desert.
Note that there is often quite a difference in the annual bloom between the moister canyons west of Borrego Springs and the drier areas around the Badlands. Similarly, even within those canyons on the west, there can be large differences between the north-facing and south-facing slopes, and between canyons with permanent water, like Borrego Palm Canyon, and drier canyons. In the drier areas to the east, there can be large differences between the edges of washes and the middle of washes, and between shady canyons and open areas. Location matters!.
Annual Germination, Growth and Blooms
General Requirements for Annual Germination
There are two main rain windows for annual germination in the Borrego Desert: monsoonal / summer rainfall, received from thunderstorms in July, August and September, and fall / winter rainfall received from October through January.
Monsoonal rainfall germinates 31 different species of summer annuals that bloom in September and October. Occasionally, fall / winter rainfall occurring in October will germinate some of these summer annuals. In addition, some perennial species respond best to monsoonal rainfall, and produce their best shows in September.
See Monsoonal Species for a list of the species that respond significantly to monsoonal rainfall.
The rest of this section discusses only the "normal" annuals seen by most visitors to the Borrego Desert in the late winter and early spring.
The timing of fall / winter rainfall is extremely important for the annual bloom. Rainfall received in the summer and early fall will not germinate the annuals that bloom in February and March. Rainfall received after January will either not germinate those annuals, or will germinate them too late for them to produce a robust bloom in most years. Thus rain must fall in October, November, December and/or January in order to germinate the annuals that produce the showy mass displays. The potential showiness of the bloom declines when the germinating rainfall gets later in January, since the annuals don't have enough time to grow very large before the increasing heat of March ends their bloom.
The amount in a single storm is also important. Native annuals require at least about an inch of rainfall, received over no longer than a period of something like several days, in order to germinate. Our native annuals have learned the hard way that less rainfall doesn't guarantee enough moisture in the soil for them to produce seeds. Many annuals won't even germinate with two inches of rainfall in the Badlands and similar soils.
Unfortunately, non-native annuals can germinate on less rainfall, and can sometimes get a head start over our native annuals if we get a first rainfall much less than an inch.
See Predicting Desert Wildflower Blooms - The science behind the spectacle from the Arizona-Sonora Desert Museum for information relating to Arizona desert blooms. Note, however, that there is a big difference between the Sonoran Desert germination in Tucson and in Borrego Springs.
Germinating rainfall typically occurs in Tucson in October and November, whereas it typically occurs in Borrego Springs in December and January. See Monthly average rainfall in Tucson and Borrego Springs from September to February (in the plot, month 13 is January and month 14 is February). Tucson receives an average of 1.5 inches of rain in October and November, whereas Borrego Springs receives an average of only 0.6 inches then, not enough to germinate annuals then in most years. Borrego Springs receives an average of 2.0 inches of rain in December and January, which is usually enough to germinate annuals there during that period. (As detailed above, since rainfall declines markedly to the east of Borrego Springs, the probability of germination in those areas in a given year declines as well.)
Bad Bloom Years Are All The Same; Good Bloom Years Are All Different
Janice Emily Bowers, with her vast experience in desert blooms, said it best in her book Flowers and Shrubs of the Mojave Desert, 1998, p. 4:
... maybe one in five [springs] will bring a good wildflower display. All bad springs are more or less alike in that wildflowers are scarce or not to be seen, but all good years are different in that no two have the same abundance of flowers or the same combinations of species. This is because different kinds of annual wildflowers have different requirements for germination and growth.This is just as true for the Borrego and Sonoran Deserts.
Peak Bloom: What Does That Mean?
The term Peak Bloom means different things to different people:
- To the casual visitor to the Borrego Desert, it means the time period when there are carpets of showy flowers on the desert floor that will immediately be obvious in a large-scale photograph. As a result, this is also what the Park Staff means by Peak Bloom as well, and what is reported by them in the postcard notifications to those who sign up for that.
Only a few species produce those showy carpets of flowers, and the timing of their bloom does not necessarily correspond to each other, let alone the ~200 other species that bloom in February and March.
Unfortunately, the carpets of flowers have become much diminished since about the year 2000 due to the invasion of Sahara mustard, Brassica tournefortii. This non-native weed has taken over most of the sandy areas that produce the big displays, crowding out our native species due to the unnatural advantage that non-native species have. Essentially the entire Borrego Springs city area and environs inside the boundaries of Anza-Borrego Desert State Park, along with the west half of the Borrego Badlands, are now close to 100% mustard. If you stand at nearly any place in the city of Borrego Springs early in the season after germination, the green you see around you and on hillsides is nearly all mustard.
The area of Coyote Creek Wash at the northern end of the payment in Di Giorgio Road was always an excellent wildflower display area, and often recommended as a place to go, until sometime around 2005, when mustard took it over. By 2010, there were only small patches of blooms left, with the mustard lurking nearby, which may be completely gone in the near future. (The linked pix was optimized to show the small spectacle-pod, Dithyrea californica, patch.)
Similarly, there used to be an extensive stunning field of hairy desert-sunflower, Geraea canescens, along the eastern portion of Henderson Canyon Road before the Sahara mustard, Brassica tournefortii, took over this area. See also picture taken on 2 March 2009 which shows how patchy this bloom has now become, amidst the extensive non-stunning field of mustard.
Park volunteers weed some areas of this species (Borrego Palm Canyon, Plum Canyon, and a number of patches along Henderson Canyon Road in Borrego Springs in order to recreate the sunflower displays there) in order to preserve some small areas for these native species. This effort has been tirelessly spearheaded by Park Botanist Larry Hendrickson.
For pictures of before and after the mustard invasion, see The Desert Museum's Sahara mustard (Brassica tournefortii) page.
Note that other species can produce fields of flowers, but not in the sense of carpets of flowers. For example, brittlebush, Encelia farinosa, can produce very showy displays on hillsides long after the carpets of annuals are gone.
- To a wildflower enthusiast, or a botanist, it might mean the time period when they can see the maximum number of species in bloom on a given one or two day trip. This in general occurs later than the time when showy carpets of flowers are present.
In fact, Tom has been in the desert at such a time of peak bloom, with perhaps 50 species in full bloom within a mile of his location, and had visitors ask him where the flowers are. Clearly, they were looking for carpets of flowers, and weren't interested in walking around to find all the species in bloom at that location.
Most of the time, we use the latter definition of Peak Bloom, but we also try to mention when the carpets of flowers are present.
If you are looking for a particular species in bloom, the time of Peak Bloom doesn't matter to you; you want to know only when that species is in bloom. Plant species bloom at different times; it is not possible to see every species in bloom even over the time period of a month.
For example, if you want to see the beautiful blooms of beavertail cactus, Opuntia basilaris, you'll need to come just after the showy annual carpets are finished. If you want to see the beautiful flowers of desert-willow, Chilopsis linearis ssp. arcuata, then you'll need to come here in summer, when few species are blooming except for it.
See observed dates of peak bloom in 2008-2009 and in 2009-2010 for various locations.
These annual species produce the showy carpets of flowers:
- hairy sand verbena, Abronia villosa
- brown-eyed primrose, Camissonia claviformis
- spectacle-pod, Dithyrea californica
- hairy desert-sunflower, aka desert gold, Geraea canescens
- Arizona lupine, Lupinus arizonicus
- desert dandelion, Malacothrix glabrata
- dune primrose, Oenothera deltoides
Other annuals can produce carpets of flowers, but are either more limited in their distribution, such as Bigelow's monkeyflower, Mimulus bigelovii, or purple mat, Nama demissum; or don't produce such showy displays, such as Fremont pincushion, Chaenactis fremontii (since fields of white don't show up well against the whitish background of the desert soil).
General Factors
Past Rainfall, Future Rainfall, and Heat are the main factors determining how long an annual bloom will last on the desert floor at about 1000 feet elevation:
- Past rainfall affects how big the plant is, and how many resources it has to keep blooming, especially the root system of the plant and how much moisture is available in the ground. Given enough resources, annuals can withstand conditions that would otherwise abruptly end the bloom.
- Future rainfall can allow a bloom to keep going even if resources are meager. Even annuals near death can produce significant new growth if they receive rainfall.
- Heat is the main factor that terminates a bloom. Tom has seen a very robust February bloom abruptly end in just two weeks in March when 90° heat begins and is sustained for a few weeks. On the other hand, the bloom can go into April in years with cooler March temperatures.
Number of Species and Plants in Bloom On Each Trip
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Comments and feedback: Tom Chester
Updated 17 February 2022
