Desert Wildflower Blooms
Spring Flowering Season
The spring flowering season in the Arizona Upland subdivision spans from mid February to mid June with a peak from mid March to late April depending on rainfall and temperatures during the growing season. In the warmest areas of the Lower Colorado River Valley subdivision it is normally a couple of weeks earlier, though it sometimes starts as early as November. The different life forms which dominate at different times vary in their showiness and reliability. The early-blooming winter annuals can create an incredible display, but do so only rarely. Later-blooming species bloom more dependably, but mostly not in great masses of color. The progression of spring bloom described below is for average years near Tucson. It may be three weeks earlier or later depending on weather, elevation, and latitude.
Winter Annuals
Winter annuals such as poppies (Eschscholtzia mexicana), lupines (Lupinus sparsiflorus and others), and owl clover (Castilleja exserta, syn. Orthocarpus purpurascens) create the vast carpets of color for which the Sonoran and Mohave deserts are so famous. This event may occur between late February and mid April, usually in mid March. Annuals are highly dependent on rainfall. Massive and widespread displays occur only about once a decade, when the winter rainy season is both earlier and wetter than normal. Good shows happen in localized areas every three or four years. A good bloom cannot be reliably predicted more than a week or two before it begins (see next page for details), and it lasts at peak beauty for no more than two weeks. Seeing one requires being able to travel on short notice, and perhaps great distances. Death Valley may be spectacular in a year when Organpipe Cactus National Monument is poor. The high Mohave Desert may peak two or three weeks later than the lower elevation and more southerly Sonoran Desert. A good bloom may occur in a remote area and remain undiscovered.
Herbaceous Perennials and Small Shrubs
Herbaceous perennials and small shrubs such as penstemon (especially Penstemon parryi), brittlebush (Encelia farinosa), and fairy duster (Calliandra eriophylla) also require rain to bloom but are less sensitive to its timing. They are somewhat more dependable than the annuals, making a good show in about half of the years and peaking some time in March. These species usually grow as individuals or in small patches and do not create masses of color.
Cacti
Cacti, because they store water, are fairly independent of rain. They bloom well nearly every year though wetter years produce more flowers. The greatest diversity of spring-blooming species can be seen in April. The cactus show continues as the abundant prickly pears bloom in early May, followed by saguaros from mid May to mid June.
Trees and Large Shrubs
Trees and large shrubs are fairly dependable bloomers, though flowers will be sparse in dry years. Creosote bush (Larrea tridentata) and whitethorn acacia (Acacia constricta) both bloom mainly in spring and sometimes again in summer. Blue palo verdes (Cercidium floridum) turn bright yellow in late April, followed two weeks later by the much more abundant but paler yellow foothill palo verdes (C. microphyllum). Desert ironwood trees (Olneya tesota) bloom heavily about every other year with masses of lavender flowers, usually in late May. The abundant ocotillo reliably produces spikes of red flowers through-out April. These species bloom about two weeks earlier in western Arizona.
Summary
If you want to see the famous carpets of color, keep abreast of local news from Palm Springs to Tucson and from Death Valley to northern Mexico. Begin checking in January lest you miss an early show. (The dunes can sometimes begin in November, but this catches nearly everyone off guard so it's pure luck to discover such an early show.) You'll find masses of annuals somewhere in this area about once every three or four years. If you want dependability and will settle for less quantity, success is almost guaranteed in the middle half of April.
For timely Spring Wildflower information:
Summer Flowering Season
The summer flowering season season begins a few weeks after the first summer rain and continues into late fall. Though there are many beautiful species to be seen, there are rarely massive displays of color in this season, because the summer rains are more sporadic and localized than the winter rains and the soil dries rapidly in the heat.
Summer Annuals
Summer annuals such as summer poppy (Kallstroemia grandiflora) and devil's claw (Proboscidea parviflora) germinate within a few days after the first soaking summer rain and begin to flower as soon as three weeks later. Chinchweed (Pectis papposa) is the most widely-adapted summer annual; it ranges from New Mexico into the central Mohave Desert where it is the only summer annual (summer rains are uncommon in the Mohave). It can form showy carpets of yellow when rains are abundant.
Herbaceous Perennials and Small Shrubs
Herbaceous perennials and small shrubs bloom opportunistically if they get enough rain. Trailing four-o'clock (Allionia incarnata) and desert marigold (Baileya multiradiata) are nonseasonal, flowering in response to rain in all but the coldest months. Fairy duster will also bloom again in wet summers, but not as profusely as in spring. Sacred datura (Datura wrightii) is mainly a summer perennial though it may begin flowering as early as April in warmer areas. There are several woody shrubs that bloom in late fall. Most are composites such as burroweed (Isocoma tenuisecta) and desert broom (Baccharis sarothroides). Desert senna (Senna covesii) and Coulter hibiscus (Hibiscus coulteri) flower in response to any warm rain and peak in summer when most such rain occurs. Desert zinnia (Zinnia pumila) is truly biseasonal, flowering well in both rainy seasons.
Cacti
Cacti include several summer-flowering species. The pincushion cactus Mammillaria grahamii microcarpa makes buds during its previous growing season, then goes dormant during the dry season. The buds burst into bloom five days after each of the first two or three summer rains. The fishhook barrel cactus (Ferocactus wislizeni) is much larger thn the pincushion and less dependent on rain; it flowers throughout August and September.
Trees and Large Shrubs
Trees and large shrubs are nearly all spring bloomers, but a few bloom again in summer if rains are generous. Whitethorn acacia (Acacia constricta), velvet mesquite (Prosopis velutina) flower heavily in spring and often again in summer. Desert willow (Chilopsis linearis) flowers from spring through fall if it has enough water.
Summary
Though the Sonoran Desert has two flowering peaks, there is almost always something in bloom. The only exceptions are after a hard winter freeze or during severe droughts.