Sunburn on Plants a Major Desert Problem

Q. I have three 3-tier privets in the front of my house. One of them, showing in the two attached pictures, seems to have trouble at the top tier, which grew yellowish and small leafs, while the bottom two tiers grow normally. The top part of the trunk close to the sickly tier shows darker color.  Nursery people told me that the troubled shrub got too much sun and needs more water. But I have watered the shrubs (all three of them next to each other) with the same amount and same frequency. To tell me just the top tier of one shrub needs more water does not make much sense to me, right?

My question is: why only the top tier turn sickly yellow while others are growing normally? and what can I do to protect it from dying?  Thanks.

A. Thank you for the pictures. I agree with you. I do not believe it is water….directly, or can be improved by giving it more water. The problems is located on the branches or foliage showing the damage. We can eliminate an irrigation problem for exactly the reason you said and the foliage on the same side but below the damaged area is in much better condition. If the entire plant on that side had shown that kind of damage then I might be inclined to include a plugged emitter or not enough water to be a possible source of the problem.

               Whenever we see damage to a plant and it is localized like yours is then the usual problem is located on the branches or stems supporting the problem area OR on the foliage itself. Now, what I know because of my experience in the desert and you don’t know is that there are very few insect or disease problems on Ligustrum or privet. Most of the problems are sun related because it is not a true desert plant that can thrive in our environment easily. It does require a bit of pampering.

               My guess is that the stems supported the leaves or foliage has been damaged…mechanically. Mechanically just means that some outside force was at work to create the damage as opposed to a disease. Insects, such as borers, can also create mechanical damage by chewing or gnawing but I don’t think this is from insects since this plant does not have a history of that kind of problem here. That is my head knowledge telling me by deduction, not anything I can see.

               Here is a nice website that talks about privet problems but we currently don’t have very many of the problems listed as they do in California. 

http://homeguides.sfgate.com/privet-hedge-problems-43811.html

            The author does say this, 

Twig Kill

Repeated shearing keeps privets neat and compact, but also forces branching until the surface of the shrub is a thick mass of branches and twigs. Sudden cold snaps in winter, dry windy weather, or drought might kill small or weak branches. If the twigs at the end of a few branches die back, the branches themselves may be lost and the resulting open spot will have to fill in with growth from other branches. Twig kill might necessitate careful hand trimming rather than shearing to ensure that new branching expands into the void.

We do have one though that they don’t list…sunburn.

            The side damaged is toward the sun. I can’t see all of the plant but it does appear the sides away from the sun are healthier. If some over-aggressive shearing was done and opened up the top tier too much, it could open the inside branches to sunburn from our intense sunlight. That happens to a number of plants here whereas it milder climates it does not. I have seen that happen here to Podocarpus when an over aggressive gardener got carried away and pruned too much out. I will post that on my blog next week. One way to tell is to bend some of the branches on the top tier to find out if they are supple or stiff from sun damage. They might even snap if they have been damaged. 

Sunburn on Podocarpus after pruning and subsequent dieback.

            What to do? The damaged area could grow back but it will be slowly. You would live with this damage until you see some new growth “sprouts” coming from the inside branches. Many plants respond this way to damage, but not all. In the case of privet, the growth is slow, not rapid. Once this growth appears you can start to prune back the damaged stems to allow for this new growth to occur. This might take a couple of seasons.

            Another thing I would recommend if you don’t have it is to use several inches of wood mulch at the base of these plants. They definitely do NOT like rock mulch if that is what you have. Another less likely possibility is damage from mites to the foliage but I think this is less likely. I would say the higher probability is sunburn damage to the stems.

From Reader -

Thank for prompt and informative reply.  Your suggestion about aggressive pruning and western sun burn might be part of the reason which I will try to correct in the future.  I did add 3-4" organic mulch at the base of the plant and I'll do more in early spring when I begin fertilizing plants around my house.  In  the spring , I added iron chalets with regular shrub fertilizer, and mix some manure in the mulch. It seemed to work pretty well. 

Thank you again,