Great Article on Invasive Shot Hole Borer

The Invasive Shot Hole Borer (formerly called the Polyphagous Shot Hole Borer) is still a threat to native and introduced trees here in California, and although in some cases it seems to be mellowing out just a little bit, it still has the capability of getting worse -especially during the curent severe drought. Here is an article by Rosi Dagit that summarizes that current state of affairs with this pest and also some opportunities to be involved in citizen science efforts to keep track of it.

Tree limb infested with Invasive Shot Hole Borer

Discoloration on bark indicating Shot Hole Borer attack

The Invasive Shot Hole Borer

Magnolia Misery

Question:
Two years ago, we removed most of the turf from our front yard only to watch our adolescent magnolia drop 70% of its foliage within a few months. We inferred it was for lack of water that previously irrigated the lawn. We installed a grey water system and watched it occasionally produce new flower buds, esp. after fertilization, but hardly any new leaves. It got scorched in the bad heat wave this past summer, and now looks terrible. Most branches are dry and break easily. Is it beyond saving? I probably need someone with the right knowledge to look at it, but this is the closest I can get (I think).

Thanks,
Tommy

Hi Tommy, 

The root system for Magnolias is fairly shallow. In nature its own leaves provide a fantastic mulch the helps prevent the soil underneath the canopy from drying out too quickly and causing stress on the tree. Before its demise your lawn was serving more or less the same  function. 

If you want to save the tree I would allow its own leaves to build up underneath the canopy, but before doing that give it a little boost by applying a 4"-6" layer of small to medium conifer bark to the soil underneath the historic canopy. This gives the tree a head start to having a luxurious mulch of its own leaves underneath it.  Also, you want at least 2/3 of the area underneath the canopy free and clear of anything but the tree's own leaves (and you initial bark mulch application).  You should also water your tree at least twice a week for 25 minutes until it revives.  And finally,  use absolutely no fertilizer -fertilizer of any kind (including grey water) softens the trees leaves so that the tree evaporates out more water than it should, causing stress on the tree.  It also acts like jet fuel to pests and diseases. 

Looking at the big picture, Southern magnolias like yours are natives to the southeastern part of the United States where they receive copious amounts of rain during the summer months. They are in no way drought tolerant plants. The have done well here up until now due to the availability of unfettered and low priced irrigation water. That situation has changed and the option of removing the tree and replacing it should seriously be considered. 

Below are a picture of a healthy Magnolia that has a 6"-12" layer of its own leaves providing the perfect mulch for the tree, and a picture of a suffering street-magnolia that has less than 1/4 of the area underneath it permeable to water.   -Frank

Magnolia at the Arboretum looking good; notice the thick layer of its own leaves underneath the canopy of the tree. 

Street tree Magnolias, notice most are at 20-30% of their health canopy size. The second Magnolia from the left has only 1/4 of the are underneath what would be its normal canopy permeable to water.