About the Desert Garden, Part 1

Saguaro cactus (Carnegiea gigantea) against a backdrop of the White Tanks mountains

Although I'm still hoping to get a vase up on the blog this week, I am taking some time out to work on a post that I've been planning for a long time - several posts, in fact.  I've wanted to take a long look at where the garden is located, to give some idea of the larger context and of the various factors affecting its growth.  This is the first of what will probably be three posts, each coming closer to the garden itself.  And when I am finished, I intend to edit and combine the three to make a more intelligent "About" page for the blog!

It's been over a year since I started the small, sunny garden.  In the meantime I've learned a lot about the location and some of its idiosyncrasies.  To begin, the garden resides just on (or beyond, according to many locals!) the northwest edge of the Phoenix, Arizona, metropolitan area.  This places it in the northern section of the Sonoran Desert.  So I'll begin with a look at this wider area.

The Sonora is the hottest of the North American deserts and at the same time contains the largest diversity of plant species.  Native Sonoran plants range from the Saguaro cactus (Carnegiea gigantea, which occurs in the wild only in this desert) to riverbottom trees such as Arizona ash (Fraxinus velutina), to a host of shrubs and subshrubs and herbaceous wildflowers.

Carnegiea gigantea, the gentle giant of the Sonoran Desert

Larrea tridentata, the shrub which lends its distintive, crisp scent to moist days

Sphaeralcea ambigua, whose warm cup-flowers enliven the roadsides after rain

It is a large desert, encompassing sections of southeastern California, much of southern Arizona, and 

northwestern sections of Mexico, where it extends at least to the Gulf of Baja (Baja California itself is now considered by many to be a separate ecosystem).  Throughout this desert there are multiple subdeserts and a wide range of elevations, with the lowest point being California's Salton Sea, which lies below sea level.

There are now some major cities in the region, with Phoenix being the largest -- and a rapidly-expanding one.  Others include Tuscon, AZ, and Palm Springs, CA, as well as Hermosillo and Saltillo in Mexico.

The region is home to a number of native American tribes; but the Hohokam, who originally erected irrigation canals in the Phoenix area, left them during the 14th century, as massive flooding and associated alterations of the Salt River destroyed the system or rendered it unworkable.  The canals then lay more or less unused until the 19th century.  As settlers and prospectors entered the area in the 1840's and '50's, the ancient canals were promptly repaired and expanded, providing arable land for crops of hay, cotton, citrus, etc.  These supplies went to Fort McDowell to the northeast and to Vulture Mine to the west.  Vulture Mine was the richest producing gold and silver mine in Arizona; shut down with other gold mines during the Second World War, it failed in a later attempt at a comeback and remains closed to the present day.  In the meantime Phoenix became a transport and agriculture hub and eventually a major metropolis on its own account, growing out into the surrounding desert at a great rate.  Job and retirement opportunities have fueled much of the growth, and the sunshine is offered as respite from cold, grey winters elsewhere in the country.

Cattle-ranching has also been a major factor

The weather itself is part of what defines the region.  During autumn and spring and part of winter, it makes the garden here feel like an outpost of Paradise.  In the dead of summer one is apt to compare it to the other place.  Summers are proverbially hot, with many days reaching over 100 F/38 C and extended periods of temperatures over 110 F/43 C.  (A record 122 F/50 C was reached in Phoenix on June 26, 1990.)  Low humidity makes it more bearable for the humans, but not for plants!  Winters, on the other hand, are mild, with a smattering of freezing temperatures, greater as one moves beyond the city to the north.  This limits the use of tropical plants, but still allows citrus to be grown through much of the area.

Rain is, of course, scarce -- about 7 in/18 cm annual rainfall on average -- and occurs over two separate periods during the year.  The largest amounts come in the summer monsoon season, the first relief from the searing heat of June.  Though temperatures in July and August remain high, the rain and associated humidity offer a respite to plants (and mosquitoes!).  The second period is during winter.  But the rainfall is in any case too widely spaced to sustain grass cover, let alone heavier vegetation.  The soil is basically dry; and without the water to flush mineral build-up away, it is often alkaline and to some degree salty.

Soils can range from sandy and rocky to clay.  In some locations, such as hillsides, it can include caliche, which is a layer of calcium carbonate hardpan.  Thin layers can be broken up; thick layers (sometimes over three feet deep) present a serious challenge to plant-growing.  Clay replaces lighter soils as one moves away from mountains.

There are many mountainous upthrusts across the area.  These are the Vulture Mountains, which take their name from the Vulture Mine

But the most important factor in many ways is simply the sun.  Almost ever-present (Phoenix boasts an average of 330 days of sunshine per year), it is also very intense due to the latitude and low humidity.  This is no longer the "full sun" of standard gardening nomenclature, but a thing to itself, requiring a sharp lookout as many plants require some protection from its full force.  This, in turn, makes shade a unique opportunity for the gardener, allowing the use of plants beneath plants, using species that would never tolerate such conditions in more northerly locations.  Even many succulents, including aloes, require some shade here for best growth.  And in some cases, for survival.

All of which creates the unmistakable growth patterns of the Sonoran Desert.  Scrubby desert shrubs spaced sparsely across rocky or gravelly hillsides and plains.  Thickets of little trees in low areas, leafless in the drought of summer, but flourishing in the rains that flood across the land periodically.  Tall stands of cactus looking out at the sun.  Low-growing agaves, whose shallow roots collect the water where it is -- at the surface -- and whose heavy leaves store the water against the many dry days ahead.  Sweeps of wildflowers that suddenly bloom, then dry and blow away on the ever-present wind.

Kallstroemia grandiflora was abundant during the exceptionally wet summer of 2014

I love this place.  I love the confounding brilliance of the daylight, with its flattened hues and clean-cut shadows.  The way the earth changes from gold to grey to gold again with the passing of a single cloud.  I love the sense that one can lean against the sun -- it's so close here -- to get warm.  And the cleanness of desert soil, washed daily by sunlight and kept scoured by wind and flood.

It is a beautiful land.  But certainly it is an intimidating one for the gardener.  And information must be collected haphazardly as one finds books and knowledgeable gardeners because there is simply not the storehouse of old wisdom nor centuries' worth of gardening tomes.  I am collecting my own little storehouse -- one year's worth now -- here on the blog.  It's already been wonderful to be able to go back and check my own records and theories; and perhaps it will be helpful or at least interesting to other gardeners, in the region and out of it!

Looking up at a Saguaro cactus (Carnegiea gigantea)