Bill Pramuk’s Blog

February 6, 2016

Trees and People

Bill Pramuk

Mysterious Decline of Sweet Olives

One of my favorite broadleaf evergreen shrubs is the sweet olive, Osmanthus fragrans. Cold-hardy enough to survive our typical winter low temperatures here in Napa Valley, this sturdy upright plant shows understated elegance with its simple, medium green leaves.  It tolerates  heavy clay soil, heavy pruning or no pruning at all, produces no messy fruit and surprises the gardener at least once per year with the delicious scent of its small and inconspicuous white flowers.

So far, it sounds just about perfect, doesn’t it?  But experience has shown that my dear, sweet olive has some weaknesses. Here’s a brief case history to illustrate the point.

In a well-tended garden, a hedegrow of sweet olives grew well for a few years, up to a height of about six feet.  Then the plants at one end of the row started looking pale and sparse of foliage.  Thinking the problem might be lack of fertility,  the gardener applied a little extra fertilizer.  The plants did not respond for a whole growing season.  A few months later, the plant at the end of the row appeared to be dead and those near it were looking weaker than ever.

Being asked to investigate, I grabbed my trusty ‘pick-a-hoe’, a small one-handed cultivating tool, and excavated the root collar.  Within a minute, carefully pulling the soil away from the base of the trunk, the problem and contributing factors became clear.

Just below the soil line, the bark was soft and the tissue beneath it was brownish yellow.  Scraping deeper I found a flat layer of bright white fungal tissue and detected a characteristic aroma: the strong, mushroomy smell of oak root fungus.

What was the “contributing factor”?  The cultivator uncovered drip irrigation tubing with emitters still located at their original position at the base of the trunk, an invitation for root disease.

Oak root fungus, any of numerous species of Armillaria, exists in soils all over the world, except in arctic regions and deserts.  Its biology and management is a huge subject, but here’s a very brief primer:

Armillaria waits indefinitely in the roots of infected or dead trees.  Normally inactive in summer’s dry soil it may become virulent with summer irrigation.  Susceptible plant species, even  trees that are normally resistant - like California native oaks - are increasingly subject to infection when they are old, weak or declining from other causes.

There is no direct chemical / medicinal cure. We can only manage the culture around the infected plants to favor vigorous plant growth and disfavor summertime virulence of the disease.  In some cases, soil excavation and surgery to remove infected tissue can be a part of attempting to save an infected tree.

For the sweet olives mentioned above, the  best option is to simply remove them along with dead and infected roots as much as possible and start over.

(Bill Pramuk is a registered consulting arborist.  Visit his website, www.billpramuk.com, email questions to info@billpramuk.com, call him at 707-226-2884 or visit Bill Pramuk, Consulting Arborist on Facebook)

January 23, 2012

Trees and People
Bill Pramuk
 
Root Collars and Rodents

Here are two incidents that are instructive and timely topics for January tree care.  Both have to do with small, warm-blooded  pests.

In the most recent case, a Tangelo Citrus tree was showing  abnormal, general yellowing of its leaves. In the other incident, which happened long ago, a crabapple tree just  up-and-died.  In both cases I was asked to figure out why. And in both cases one of the first routine steps in performing a basic visual tree examination - checking the root collar - led directly to the heart of the problem:  Rodents.

The root collar, where the roots branch out from the base of the trunk or main stem, is a critical juncture in trees and other woody plants.  If an individual root or branch is damaged, the plant might manage to survive, but when the root collar is severely damaged or diseased, the plant is at greater risk of losing its life.

In the case of the Tangelo, which was growing on a slope, a quick visual check revealed that the base of the trunk showed no outward taper where it entered loose garden soil.  Under normal conditions, the base of most trees should shows a flare, a noticeable outward spread where the buttress roots emerge from the trunk. This one had become buried, possibly by erosion on the slope.

The next step, excavating to locate and examine the root collar at the natural grade, showed that under the loose topsoil, just a few inches down, the bark had been eaten off of the entire circumference of the trunk.  The excavation also revealed a gopher run passing within a few inches of the trunk.

With this kind of evidence, there is no need to look further for explanations!

In the other incident, the crabapple was clearly dead.  This little tree was prominently located in a shrub border by a front entrance to a winery visitors center.  A large shrub had grown up and entirely enveloped the trunk of the crabapple tree.

Awkwardly, I managed to work my way through the branches to have a look at the base of the trunk.  The problem was instantly visible:  It had been gnawed away and looked like a classic cartoon picture of a tree trunk chewed away by a beaver.  I suspect that the culprits were meadow voles - chunky, short-tailed rodents resembling large mice.  They love feeding in brushy, secluded spots.

Unlike moles, which can cause a lot of disruption as they seek and feed on worms and grubs, gophers and voles feed directly on plant tissues. Rats and ground squirrels can do this as well.

Appropriate action depends on  proper identification.  So, before buying traps or putting out dangerous poison baits, do some reading on the subject and then choose your weapons wisely.  Many  warm-blooded pests and their control measures are described in a helpful booklet, Wildlife Pest Control Around Gardens and Homes (U.C. Cooperative Extension, Publication 21385).

In some cases, habitat alterations can help with control, but in all cases, careful observation is critical.

(Bill Pramuk is a registered consulting arborist. Visit his website, www.billpramuk.com, email questions to info@billpramuk.com, call him at 707-226-2884 or visit  his business page: Bill Pramuk, Consulting Arborist on Facebook.)