Oaks, Watered to Death

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.

Please post a comment - I want to know what you think.

Visit my web site to learn about my new book on drip irrigation and other gardening books.

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Indicating Past Fires

I’m chopping firewood and remembering all the old native Pacific madron trees (Arbutus menziesii) that used to grace pockets of the forest around my house. The trees rarely occur in pure stands. Some had trunks three feet or more in diameter at breast height (DBH). (I don't know who's breast.) Over the past 22 years all but a handful have died a natural death and were resurrected as firewood.

At only 80-100 years, these are so short-lived trees compared to the mixed coastal oak (Quercus agrifolia), Douglas-fir (Pseudotsuga menziesii), tanoak (Lithocarpus densiflorus), the California coast redwood (Sequoia semperviren) – with a DBH of 8-25 feet - and California bay trees (Umbellularia californica) in “my” forest. This is because the madrone, while drought resistant, is a colonizer of fire-scorched earth. They are among some of the first trees to sprout after a fire. It is said the seeds need fire to germinate. (However, most germination in nurseries is done by freezing the seed before planting in flats or tree tubes.) These trees at first grow more rapidly than the Douglas fir trees associated with a more mature forest. They help to stabilize the soil. Eventually the Douglas fir trees grow above the madrones and shade them to death.

To maintain madrone in a mixed Douglas fir, oak, and redwood forest, the tree needs fire. Not a firestorm in the tops of mature trees, but frequent fires about every five to 50 years. Pacific madrone depends, on these periodic fires to eliminate or greatly reduce the beginning of a Douglas fir overstory.

However, the forest here hasn’t burned in over 100 years. (A 1/2-acre fire last week a mere two miles from my house was quickly extinguished. What a close call and a relief!) The amount of “kindling” (dead trees and shrubs as well as the invasion of foreigners along parts of the forest’s edge like Scotch broom (Cytisus scoparius), which act like little balls of standing gasoline because they burn so easily and hot) on the ground is tremendous. When the next fire comes the burning grasses or Scotch broom will leap up the trunks of the trees to form a crown, or canopy, firestorm. Nothing, except the noble redwoods, will be left. Including my house.

This photo is of one of the few Pacific madrone trees left on the property as it snakes toward sunlight from beneath the maturing Douglas fir trees.

Please post a comment - I want to know what you think.

Visit my web site to learn about my new book on drip irrigation and other gardening books.

NOTE: The comments section at the bottom of the post has disappeared. Click on the "___ Comments" button or the title under the "Blog Archives". Thanks, Robert

This Tree Needs Caterpillars

Caterpillars are pests aren't they? Not always.

It turns out the native coast live oak (Quercas agrifolia) that grows around my house co-evolved with a tiny caterpillar, appropriately named the oak-leaf caterpillar. Without the caterpillar, the oak tree might die during years of stress.

A nursery in Los Altos, CA has a gorgeous coastal live oak about 75 feet in diameter. During the drought of the early ‘70s a lot of trees were very stressed. A neighbor near the nursery sprayed their oak, “to get rid of the messy droppings and fallen leaves”. The nursery carefully watched their tree lose many leaves and be riddled with leaves partially eaten by the oak-leaf caterpillar.

Guess which tree died?

The neighbor’s. Because it had 100% of it’s green leaves to transpire moisture. Even an organic spray, such as Bacillus thuringiensis var. kurstaki, would have lead the tree to an early death. At the nursery, the tree survived as much of it’s leaf area was consumed by the caterpillar, offering less area for transpiration.

Judging by the moths at my screened windows and the partially-eaten leaves, the first round of oak-leaf caterpillars is here. There are two to three groups of moths each season depending on how dry the year is.

Please post a comment - I want to know what you think.

Visit my web site to learn about my gardening books.

NOTE: The comments section at the bottom of the post has disappeared. Click on the "___ Comments" button or the title under the "Blog Archives". Thanks, Robert