L.A. County Arboretum & Botanic Garden Plant Info: 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.