I rained 4+ inches yesterday. This got me thinking about water and native coastal oak trees.
I live in the arid Mediterranean zone of California where the oak trees get no rain from May through September. To native oaks, summer water is a poison which can quickly kill. These trees spent a millennium, or more, evolving to thrive without summer rain. In the process, oaks were able to tolerate a number of hitchhiking fungal hosts. These predatory mycelium—examples include crown, armillaria, and heart rots—need two things to thrive: moisture and warm soil. As evolution arranged it, the soil is plenty moist during the wet California winters—but too cold to promote fungal growth. During the dry summer, the soil is warm enough to stimulate fungal growth, but the upper layer of the soil has dried out enough to retard most fungi. These seasonal alternations insure that there's very little time when both necessary ingredients are in abundance together. Thus whatever limited amount of fungus survives is not enough to threaten the 300 year life expectancy of the oak.
Adding irrigation in the summer insures the rapid growth of the sometimes fatal fungi. There is little more unnatural than watering beneath a oak tree in the summer. Such trees can die from rot in 25 to 50 years—a rather quick demise for something that could live another 100 to 300 years. Most houses are owned by two or three families in the quarter century it takes the tree to slowly die. Thus, the actual culprit who installed the irrigation system isn't around when the tree meets its brutal end.
One of the debates about oak trees revolves around how far away summer irrigation should be kept from an oak's trunk. The conservative answer is a zone free from summer irrigation for radius of six to eight feet from the trunk. Yet the more prudent gardener would agree that an unirrigated area equal to the width of the canopy—called the dripline—is the minimum distance. As described in my book Roots Demystified, the greater portion of any tree's roots, especially the majority of the tiny absorbing root hairs, are well beyond the dripline. In a heavy clay soil; roots, as a general rule, roots growth half again as wide as the dripline. A sandy soil, because of its lower resistance to root penetration, allows tree roots to grow as much as three times wider than the canopy. If nature selected oaks for no summer water, than the natural landscaper should avoid all summer water within the entire root system—up to three times the canopy's width, or more. Alas, if there are two or more oak trees on the typical suburban yard, this means the entire yard should be irrigation free—period. This brings up the delicate art of denial. Most homeowners rationalize—if they even give it any consideration—that some lawn or a few irrigated shrubs under part of the oak's foliage is alright. But this does shorten the oak's natural life—period
Fortunately, nature has developed plenty of species of plants which thrive beneath California oaks without summer moisture. The trick is to plant these special plants in the fall, in October or early November, so the winter rains can establish a healthy root system before the parched winds of summer arrive.
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