Root Rot, Phytothphora spp.

Another round of rain is on the way. The soil is still a bit warm due to 75-85F days. It's time to watch out for root rot.

Many western and Mediterranean plants, especially those from certain desert and chaparral communities, are quite sensitive to overwatering or even to a single irrigation in an otherwise normally dry summer. A primary cause of death for such drought-resistant plants is crown- or root rots caused by Phytothphora spp. and other species of fungi in moist, warm soils.

In the wet winter the soils are to cold for the Phytothphora to survive. In the warmth of summer there isn't enough moisture neat the surface to encourage Phytothphora. Add water during the summer and you have a recipe for disaster.

Rosemary, as pictured above, is a good indicator plant for Phytothphora problems. The plant doesn't always die, but shoots are killed off by a partial girdling of the crown of the roots. These yellow-browns "strikes" are the first signs of Phytothphora in the soil or a soil too heavy with clay to allow enough drainage. That why I always recommend planting on a mound (as seen above), especially Mediterranean and California natives from dry parts of the state.

Let me know what you think.

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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.

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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

Garden Rot [not roots]

Every eccentric garden I’ve been in pays some homage to death. Here the face offers a slightly oblique reference to death. Rot. All gardens have compost, mulch, and rotting fibers and critters. Rot [as well as death] is just a natural part of every garden.

[This unique comical sculpture was carved by the wonderfully-eccentric Marcia Donahue in Berekely, CA.]

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Pop goes the weasel (tree)

As written in my book Roots Demystified, Change Your Gardening Habits to Help Roots Thrive, amendments are materials such as sand, peat moss, compost, and rice hulls have been added to the planting holes to supposedly improve drainage and keep the soil loose and friable. Fertilizers, such as blood meal, cottonseed meal, greensand, and wood ashes, are those ingredients added to provide mostly nutrients. Some amendments, such as compost, are thought to do both improve drainage and act as mild fertilizers.

One of the best studies of the effect of amendments for fertility was done at Oklahoma State University at Stillwater by Joseph Schulte and Carl Whitcomb. They planted 108 silver maple trees with 11 different soil treatments and a control (untreated planting hole). One conclusion was: "no benefit was derived from the use of soil amendments either with a good clay loam soil or a very poor silt loam subsoil." They found that the control plantings with no additional amendments generally outperformed the plantings with amendments for drainage and fertility.

The loose soil of the amendments in a traditional planting hole makes something like an underground swimming pool full of water when it rains hard, drowning important root hairs. Adding a lot of amendments only leaves the roots unprepared for the shock of what lies beyond the amended area. (And nobody can amend the area of the mature root system seeing as how much wider it is than the foliage.) Often the roots fail to make it out of the well-amended hole and merely circle around in the loose planting medium, rendering the trees likely to blow over during a storm. The trees most tolerant to wind are those with the widest root systems. Or, as the photo shows, not spreading the roots when planting can allow the tree to simply pop out of the ground when hit by a car or pulled out by hand.

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Doughnuts of Death, Mounds of Mulch

I just got back from the land of “toxic” mulch – the donuts (as spelled by Dunkin’ Donuts) of death in St. Louis, MO. Every landscape contractor (and maybe home gardeners mimicking the landscape companies) seems to be having a contest as to how high they can pile mulch around tree trunks. Some have mounded mulch 16 inches high or higher. These donuts of death are disasters in the making. Root rots (Phytophtora spp.) like a warm and moist/wet soil. Mulching tree trunks so high has all the ingredients needed for the rot of trunks to kill trees. I saw one tree that had mulch apparently piled over the graft of the now dead tree. (Unfortunately I didn’t have my camera with me at the time.) As the photo says, we must put a stop to this uninformed practice. Landscapers must abandon these donuts of death.

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