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 Factors


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:

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:

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:

Number of Species and Plants in Bloom On Each Trip

See latest plots of bloom numbers.

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Copyright © 2008-2022 by Tom Chester, Kate Harper, and Mike Crouse.
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Updated 17 February 2022