Saturday, November 3, 2018

Putrid Plummeting Pomegranates Puzzle Patron

I just had an email inquiry that included the following:

"I have a very sick pomegranate tree that is dropping yellow leaves all over the place and producing few fruits, all of which are rotten and eventually dropping. Also, there are black spots on many of the leaves.

I've attached a photo of the tree to show you what is going on and would be eternally grateful if you could - via this photo, if possible  - provide a diagnosis and suggest a cure.  I believe this tree is 90 years old, and I'd love to preserve it if I can."

Pomegranate tree in question.  
As you can see the tree looks fairly healthy, although the fence seem to have shaded the tree's lower half, causing any leaves that might form their to appear chlorotic. I puzzled over this photo trying to find something wrong, then I saw something and blew up the photo to find out what it was.
Obvious animal damage to fruit. 

It was animal damage, most probably rats. As you can see in the upper photo the Pomegranate bush grows along a fence that most probably is providing access to the fruits for the rats.

The solution? Mine would be not to grow fruit, however this is not what I think the person making the inquiry would want. If you have a rat problem like this the key to control is to restrict the rats access to the fruit tree or bush. In this case I would trim the thing so that the no part of the tree is closer than 1 foot from any part of the fence. Also make sure the tree is not 1 foot or closer to any utility lines, if it is, trim it, unless the utility lines are power lines, then call your utility company to trim it for you as trimming trees near power lines is extremely hazardous.

As far as the black spots go, I cannot see any in the photograph, so I will have to wait until the patron brings in some leaves.

Thursday, November 1, 2018

Something Bad is Happening to Our Magnolias, But There May Be Some Hope

In the last several years I've noticed that many Southern magnolias (Magnolia grandifloras) in the greater Los Angeles area are taking a turn for the worse. They're losing branches, their canopies are thinning and they are, in general, looking terrible. For example below is a picture of a southern magnolia cultivar 'Samuel Sommers' in 2012 (on the left) and just recently (on the right). Notice in the recent photo that only 50-60% of the trees canopy is left. The side-by-side photo below is one of the milder examples of how Southern magnolias have declined over the past several years.
Magnolia "Samuel Sommers" left: in 2012, right November 2018
Below also are some examples of Magnolia grandiflora street trees growing in an Alhambra neighborhood (there is nothing special about Alhambra except that they seem to have a large amount of M. grandiflora planted as street trees).

Magnolia grandiflora planted as street trees in Alhambra California
Magnolia grandiflora planted as street trees in Alhambra California. Notice the dead branches appearing randomly in the canopy of the tree, a process called 'flagging'. 
These Magnolia grandiflora trees once had much larger canopies, with some of the trees canopies shrinking to 1/2 their previous size.

What's causing these once beautiful, magnificent trees to decline? There doesn't seem to be any one cause, but there are many culprits. The number one culprit is that this tree is like a fish out of water here. Native to the deep south and south eastern portions of the United States, Magnolia grandiflora thrives in cool, but not snow covered winters and hot, humid, and rain punctuated summers that its native range provides. It does do well here it it has plenty of humidity and water, however these conditions are becoming harder to maintain. But climatic differences aren't the only culprit.

Magnolia grandiflora suffers from a significant pest and two very damaging diseases. The pest is the polyphagous shot hole borer (PSHB), for which Magnolia grandiflora is a host.  The diseases are Xylella and Botryosphaeria 

But most of the damage to Magnolia grandiflora and many other street trees can be attributed to what is essentially a very bad interpretation of a famous landscape designer's radical new idea. Frederick Law Olmsted, who previously had designed Central Park on Manhattan Island, New York, was commissioned to design one of the first planned suburbs in the United States, Riverside Illinois. His plan specified walkways and trees lining the avenues of the community. These became so popular that the idea of tree lined urban and small town streets became an archetype of America and a specification of many an urban plan. Unfortunately this vast majority of these Law Olmsted imitators did not specify the same amount of space between street and sidewalk as Law Olmsted -10 feet between the two, ample room for the the roots of most trees.

The planting of trees in spaces much smaller than the almost 300 square feet than Law Olmsted gave the Riverside trees lead to an entire nation with millions of trees stressed out. The idea of tree lined streets took off, but the idea that those trees required adequate space to set out their roots did not.  And although smaller trees like Privets and Crepe Myrtles can be planted in the small spaces between urban and suburban streets and sidewalks, larger trees planted in such small spaces are basically doomed.

So though there may not be hope for the thousands of Magnolia grandiflora planted as street trees here in the Southland, there may be some hope for the tree if it is planted with plenty of room (and regular watering) to let it grow. In fact just behind the two waning Magnolia grandifloras located just west of the Crescent garden here is a compact variety of the tree that seems to be thriving, Magnolia grandiflora 'Victoria'. Although there is a small amount of shoot dieback on the tree it is relatively healthy. The reason? I'm not completely sure, however the tree is surrounded by other trees and is next to a lawn, this would mean plenty of water and humidity. But the one thing I think is making a big difference is that the tree is free and clear of lawn or any other kind of plant underneath its canopy and the only thing there is a thick layer of its own leaves. I can't tell you the number of times I've observed evergreen trees doing better because a thick mulch of their own leaves was permitted to remain underneath their canopies.
Magnolia grandiflora 'Victoria'
Notice that the canopy of this tree is healthy and thick compared to the Magnolias pictured above. 

Closeup of Magnolia grandiflora 'Victoria'  

Substantial mulch layer under the canopy of Magnolia grandiflora 'Victoria' composed of its own leaves.  

The mulch layer of leaves is at least 3-4 inches deep. 









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