| The city of Phoenix, in south-central Arizona in the U.S. Southwest, is located near the northern edge of the Gila Semi-Desert subregion of the horseshoe-shaped Sonoran Desert. Extending northward from Mexico and covering over 120,000 square miles, the subtropical Sonoran is the hottest and driest of North America’s four deserts. It also is the only desert in the world that has two rainy seasons. Summer temperatures can reach or surpass 120Â° F, and most of the Sonoran receives less than eight inches of precipitation annually… The five seasons here typically begin with a warm and dry March and April, followed by a hot and dry May and June growing period. This ends with the heat of wet July and August, a hot and dry autumnal growing season of September through November, and then the three mild and sometimes wet winter months of December through February… Per the Sunset Western Gardening Guide, Phoenix is in Hardiness Zone 13 (Low or Subtropical Desert). One hundred miles to the southeast, Tucson is in Zone 12 (Intermediate Desert). The crucial climactic difference between these is harder frosts over a longer cold winter in Tucson. The two cities’ temperature and precipitation records usually don’t coincide.
Some teachers have observed that Phoenix is one of the most challenging places on earth to grow bonsai — outdoor dwarfed plants in shallow containers — especially because of the long, hot, dry summers. We have risen to and continually take the challenge.
The spirit of the art of bonsai extends beyond any specific climate or geography. And that is why it even found root in the Valley of the Sun and has flourished.
Whether it be a strong wind or a light breeze, the dry Summer air will quickly dehydrate and burn the thin edges of the leaves of many types of plants. The strong sunlight helps decrease the size of bonsai here for several seasons: the leaves don’t have to be their usual size in order to absorb their optimum light energy. Exposure to south-central Arizona’s sun causes faster growth in plants at temperatures up to about 105Â° F. And the mid-Summer intensity virtually stops new growth until the start of “cooling” in late August. When growth picks up then, a second set of leaves can be expected for plants whose Spring set was fried.
A coarse, well-draining soil mix is of absolute importance here. When the Summer air temperature, even in the shade, is 110Â° F, extra water sitting in the bottom of a bonsai container has a similar temperature as it slowly cooks the roots of your tree. A coarser mix than is usually recommended elsewhere allows for drainage of that extra water and provides a wee bit of insulation as well.
[See “The Effect of Heat on Root Growth” by Andy Walsh, Bonsai Journal, American Bonsai Society, Vol. 34, No. 2, Summer 2000, pp. 49-53, for background and empirical research on what we’ve long learned and known.]
With a quicker-draining soil mix we might have to water more often during the hottest, driest days of Summer, but at least we have living trees to take care of. The soil mix dries out more quickly here, but we still need to keep watering in balance. The leading cause of death of our trees — as it is for bonsai and houseplants almost everywhere — remains overwatering.
Over about 90Â° F air temperatures, fertilizer added to the soil works much faster, sometimes with disastrous results as extra levels of especially Nitrogen draw moisture out of the tree.