Ask Mr. Smarty Plants

It's okay, nobody in Texas likes privet, either. From this article on Ligustrum lucidum from Floridata, we learned that it is an invasive weed, particularly in California, which you already know. There are several ligustrums, all native to the Far East, China, Korea and Japan, that fall into the "privet" category: invasive, useless, ugly and possibly damaging to other, more valuable, plants. 

Actually, you have two problems, the invasive weed plant and the falling leaves, and we are not at all sure they are actually related. So, let's start with the tree, Quercus lobata (valley oak), which this USDA Plant Profile shows is native to Santa Clara County and quite a bit of the rest of California, usually in valleys. Because there are so many things that can cause leaf drop, about the best we can do is give you some of the possibilities, which you can compare to your tree's symptoms and perhaps find a solution.

In the first place, we noted that this tree grows best where it can tap into groundwater, and is less tolerant of drought. If you have been going through a drought period, we would suggest some supplemental watering of the tree, as well as putting a good quality of shredded hardwood mulch over its root area to protect the roots from heat as well as holding moisture in the soil. Often, we attribute leaf loss to transplant shock, where a tree has been planted in weather too hot, or incorrectly dug up or otherwise abused on its way to a new hole. Transplant shock can appear up to 3 years after actually planting the tree, but 20 years?  Nope, don't think that's it. Have you or a landscaper spread "weed and feed" fertilizer on lawn beneath the tree or over its roots? The purpose of this fertilizer is to also kill broad-leaf weeds that appear in narrow leaf, or monocot, grasses in the lawn. Guess what? The oak tree is a broad-leaf plant. How about sprayed herbicides of any kind in the area? A puff of wind or poor aim, and the trees gets the treatment. Since we are not on the spot, and can't actually see the trees (and probably couldn't figure it out if we were), we suggest you contact the University of California Cooperative Extension Office for Santa Clara County. They are more likely to be in touch with plant pathologists who diagnose this sort of thing, and much closer to the situation.

Just one more thing about this that we are puzzled about. Most oaks trees exhibit some sort of allelopathy; that is, they emit substances toxic to competing plants trying to grow beneath them. But, we remember having privet come up under huge old native oaks in a previous garden, before we ever heard of allelopathy. Privet must be one of the rare plants immune to the oaks' defense systems, which no doubt is part of why it is so invasive. 

And while we're thinking about it, NO on the weed-wacker. They can do fatal damage to the bark of trees; at the very least, making openings for insects and disease to get to the inside of the tree.  

Okay, on to the "Texas Privet." The Texas Invasives.org-Invasives Database on Ligustrum lucidum has some pretty specific instructions for removing this plant.  The only real non-chemical technique they recommend is to dig the stuff up. If the privet is very closely entwined with the oak, this is going to be hard to do, again avoiding damage to the bark of the oak. The other take on getting herbicide to the privet is one we have often suggested. We have no reason to recommend for or against herbicide and prefer it be used more or less as a last resort, and with great care. Get a small bottle of a broad-spectrum herbicide, like Round-up, a jar and some small disposable sponge brushes. With long-handled lopping shears, cut off every branch of the privet as close to the ground as you can get it. You need to do one branch at a time: cut, paint cut edge with undiluted herbicide from the jar with sponge paintbrush. This needs to be done within 5 minutes of the cut before that cut edge can begin to heal itself. You are trying to get to the roots of the privet, and the privet will try to keep you from doing it. Between applications, put the lid on the jar, so it won't be in danger of toppling over, and be very careful that none is transferred to any other plant.  Then, keep a close eye-when another privet sticks its head up, wham, brush it again. Without leaves to nourish the roots, the plant will eventually starve, but with invasives like that, you can never relax. There are always more in the neighborhood and our friends the birds will be happy to plant some for you.  

Pictures of Quercus lobata (valley oak) from Google