(Note: This article is adapted from the presentation I gave to the Plants of the Desert class on the main UA campus in July 1992 and repeated at the Arizona Native Plant Society meeting in Sierra Vista in August 1992.)
The next time you go to Tucson, notice the plants that you see growing in the desert along the way. Tucson is at about 2,400 feet in altitude, almost a half mile lower than Sierra Vista, and more than a quarter mile lower than most other towns in Cochise County.
There is a saguaro cactus with split arms growing in front of a house in the Country Club Estates area in Sierra Vista. The only saguaros that grow in Sierra Vista are in yards where they receive care and maintenance. At about 4,600 feet in altitude there is not enough warmth in the winter in Sierra Vista for free-growing saguaros to recover from the tissue damage they suffer when it gets below freezing.
Have you seen the boxes for Tombstone pizza in the frozen foods sections of the grocery stores? Nationally, the saguaro cactus with its arms, is the best-known symbol of the desert. Tombstone is about 18 miles northeast of Sierra Vista and just about as high. There are only 2 saguaros in Tombstone and neither one has arms.
Just down the street from the saguaro there is a palo verde tree in a wash. You will find no palo verdes native to Sierra Vista either. Where it is wet enough, as where water runs off from the street into the wash, a seed of a landscape palo verde has become established. Both the blue and little-leaf palo verdes are used in landscaping in Sierra Vista. The little-leaf palo verde is more tolerant of cold than the blue. In fact, only the little-leaf palo verde grows free in the higher reaches of the Tucson area, in the foothills and the east.
Agaves grow in Sierra Vista. Look for them along Highway 90 as one approaches Sierra Vista from the north and Highway 92 as one goes south towards Ramsey Canyon and turns east towards Bisbee. The Palmer agave grows free in both Tucson and Sierra Vista, but it grows better in Tucson. The Huachuca agave is used in landscaping in Tucson, but it is native only to the Huachuca Mountains near Sierra Vista. The Palmer agave also grows along Interstate 10 in Texas Canyon on the way to Willcox.
Two flowers that bloom white in the summer are the Arizona prickly poppy and the New Mexico fleabane or white daisy. Both these flowers grow free in Tucson and Sierra Vista.
Most vacant lots in Tucson have bare spots all year. Most such lots in Sierra Vista become filled with grass. Rainfall in Sierra Vista is about fourteen inches per year compared with Tucson's eleven. The difference all falls during the summer monsoons, mostly in July and August.
The golden rabbitbrush gets its name from its bright yellow flowers which have just finished blooming. They will bloom again in September. It grows free all through Cochise County and in Sierra Vista yards both as weeds and landscape plants. If one is lucky, one may see small golden rabbitbrush in well-sheltered and well watered areas as weeds in Tucson. Rabbitbrush is a marker of the high desert.
The Arizona mesquite grows both in Tucson and Sierra Vista as well as throughout Cochise County. In Tucson leaves appear on the mesquites in February or March. In the high desert the appearance of the leaves of the mesquites in mid- to late April is almost always a sign that winter weather is certainly over.
Texts that define the desert in the classroom say that the Sonoran Desert ends just as you go east of Tucson on I-1O, and the Chihuahuan Desert begins about 200 miles further east, just west of Deming, New Mexico. It is colder for longer periods of time in the winter in Lordsburg, Deming, Las Cruces, New Mexico, and El Paso, Texas, than Sierra Vista. There are no saguaros in these areas, even as landscape plants.
Just after you go east from Tucson towards Cochise County, you climb into the high desert. Except when you drop into the San Pedro River valley at Benson, you do not go below 4,000 feet in altitude until you get to Las Cruces and the Rio Grande Valley. This is a characteristic of the Chihuahuan Desert. Other characteristics of the Sonoran Desert do not change so quickly. Further east in the Chihuahuan Desert there is a period of rain in late April and early May which you rarely get in Cochise County. In the western part of the county, Sierra Vista and Benson get more breaks from the cold in winter than Willcox and Douglas, further east. On the average, Willcox is colder than Sierra Vista in the winter, even though it is about 500 feet lower in altitude.
Differences between Tucson and areas in Cochise County like Sierra Vista provide challenges to plants from one area before they can become established in the other. In using native and other plants these differences must be taken into account. The differences in individual areas are the main reason why the native plant life in Cochise County is so diverse.
Scientific names of plants mentioned in this article:
Saguaro cactus: Carnegia gigantea
Blue palo verde: Cercidium floridium
Foothills palo verde: C. microphyllum
Palmer agave: Agave palmeri
Huachuca agave: A. parryi var. huachucensis or A. huachucensis
Arizona prickly poppy: Argemone platyceras
New Mexico fleabane (wild daisy): Erigeron divergens
Golden rabbitbrush: Chrysothamnus nauseosus
Arizona mesquite: Prosopis velutina