Arizona-Sonora Desert Museum Digital Library
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Are you planning to tour southern Arizona and want to know when the desert wildflowers or cacti are most likely to be in bloom? Do you want to know when the palo verde trees or ocotillos bloom? This phenology website can answer most of your questions.
Would you like to know when it rained and there isn't a weather station nearby? Would you like to know if the past year was warmer or cooler than usual and don't know how to navigate through the ocean of data on the NOAA website? The flowering patterns (phenology) of some plants can answer these questions for you.
Phenology translates as “the study of appearances”. More precisely, it is the observation and study of the cyclical events in the lives of plants and animals: the leafing out and flowering of plants, or the hibernation, migration, and reproduction of animals. The science includes the correlation of weather events with these biological phenomena.
The summer rainy season's storms are usually very localized. There can be a deluge in one place and no rain at all a half-mile away. I do a lot of field work, and I like to know what the recent rainfall has been at the sites I visit. There are usually no rain gauges nearby. I can't tell by looking at the ground; the surface is dry a day or two after a summer storm. And I've learned that most people are not reliable storm reporters. Many times I've been told “There was a whopper of a storm here last night; it rained really hard.” If there is a rain gauge nearby, it often shows that only a few hundredths of an inch fell. A “hard” rain that lasts just a few minutes is impressive if you're caught in it, but it may not amount to a significant total. Even an “all night” rain might be so light and intermittent that the total is quite small.
Numerous desert plants (and animals) are exquisitely sensitive to rainfall. This makes sense: if you live in a place where rain is sparse and unpredictable, you'd better be ready to take advantage of it when it comes. By knowing the response times and sensitivity of some species, you can tell when it rained, and about how much.
Ocotillo leafing out? It rained 2-5 days ago. Of all Sonoran Desert plants, ocotillo has the fastest response to rainfall that an observer can easily see. Bare stems will unfurl small, light green leaves only 2 days after a summer shower. In about a week they will be full-sized and maturing to a darker green. Ocotillo is also one of the most sensitive of desert plants; only two-tenths of an inch of rain with summer humidity will trigger leafing. (Our phenology database currently contains only flowering data. Ocotillos flower around Tucson in April without fail, regardless of rainfall.)
Ocotillos leaves turning yellow? The monsoon is over. Ocotillo foliage turns yellow and begins to fall as soon as the soil and air dry out after the summer rainy season ends. In Tucson's typical rocky or caliche soils, leaves begin yellowing about 10 days after the last summer rain. In valley soils leaves may stay green for a month or more after the last rain. So you need to know the soil type to use this gauge.
Pincushion cactus in flower? It rained 5-10 days ago. Southern Arizona's commonest pincushion cactus, Mammillaria grahamii, produces flower buds during the end of its growth season in late summer. They wait until the next warm-season rain to expand and bloom. That's usually some time in July of the year after they were formed. The first flowers open just five days after at least a quarter-inch of rain. A plant stays in bloom about a week, so by looking for the relative number of buds and spent flowers you can judge whether it is just beginning or ending its flush of bloom. This pincushion may bloom a second and third time after subsequent rains.
A number of other plants also bloom in direct response to summer rain, but this species is the quickest.
Creosotebush in flower? Good rain about three weeks ago. Nonsucculent plants become dormant when the soil is dry, often dropping their leaves and appearing to be dead. It takes them longer to “wake up” when the rain returns. Nonsucculents also have deeper roots so it takes a greater rainfall to soak down to their level. Creosotebush will produce new green leaves in about two weeks after at least a half-inch of rain, and flowers begin to open in three weeks. This commonest Sonoran Desert bush will flower any time of year, but in the cooler season it takes longer to respond than in the summer.
Devil's claw has first flowers? Gullywasher three weeks ago. Half the species of plants in the Sonoran Desert are annuals, and most of these will grow only during the cool season. But there are some that grow during the summer rainy season. The showiest of these are devil's claw (Proboscidea spp.) and summer poppy (Kallstroemia grandiflora). The annual devil's claw (Proboscidea althaeifolia) will not germinate unless about an inch of rain has fallen. The first flower opens about three weeks after that deluge. You might be fooled if there are two or three smaller storms within a week or so. Devil's claw seeds can measure cumulative rainfall if the soil remains moist, and will germinate when the one-inch total is reached. If the soil dries out, the seeds' internal rain gauge is reset to zero.
There is also a perennial devils claw, P. arenaria. It grows from a large tuberous root. Like its annual cousin, it will sprout only after a good soaking rain. It may lie dormant for several years waiting for a sufficient downpour to wake it up. It too flowers just a few weeks after a big rain.
Wildflowers blooming in abundance? Fall and winter were wet. Half of the Sonoran Desert's plant species are annuals, and most of them respond only to cool season rains. The seeds germinate and grow only if fall and winter rains are well above average. Carpets of color means the winter was soaking wet (for a desert). See http://www.desertmuseum.org/programs/flw_predicting.php for more details.
Barrel cactus beginning to bloom in September? Summer rains were late. The fishhook barrel cactus (Ferocactus wislizeni) usually begins flowering by the first of August (and continues through the end of September). But if the rains fail in July, flowering can be delayed by a month or more.
