Ask Mr. Smarty Plants

We are just as puzzled as you are. From personal experience, we have always considered Morella cerifera (wax myrtle) an excellent native choice for Texas, with few pests or other problems. About all we can do now is to look at various possible causes, and leave it to you to examine your shrubs and see if you can determine the true problem. 

To begin with, you referred to the bushes as thinning out, and then looking straggly. Our website on the wax myrtle (link above) refers to is as a "wispy, multi-trunked evergreen shrub." Perhaps you were expecting it to thicken up and become more dense, when that is not in the plant's nature to do so. However, that should not cause the dieback. There will always be some attrition in leaves, even in evergreen plants. Old leaves will die and drop off to be replaced by fresh new leaves. That is just normal.

Again, according to our Native Plant Database information on the wax myrtle: "Wax myrtle is a fast growing plant that tolerates poor drainage and drought. Essentially a shrub, it serves as an excellent screen plant. It is also good for wetland gardens. Height and legginess can be maintained with a line trimmer or the shrub can be allowed to develop into an airy shrub."  However, it is a high water user, at least until it's well-established, so you might not be watering it enough in our severe drought conditions.

Here are our ideas on what might have caused the problem and how (or whether) to treat them:

1.  Examine the undersides of the leaves for possible insect infestation. Scale, aphids and whiteflies all produce honeydew when they feed and this, in turn, attracts a sooty black mold. The mold itself can cause problems and is unattractive. We don't encourage the use of pesticides in a situation like this, but rather try a hard spray of water on the undersides of the leaves; once these bugs are washed off, they have difficulty getting back on. Or you can spray with a weak solution of Safer Soap.

2.  Examine the pruning wounds for signs of possible canker disease. This University of Illinois Extension website Cankers and Dieback Diseases of Woody Plants has a lot of information on identifying and treating cankers, including  pictures. 

3.  We suspect you're being too nice to your plants and over-feeding them. The advantage of using plants native to an area is that they have evolved to be able to take care of themselves. Wax myrtles, in particular, are sensitive to overuse of nitrogen fertilizer, and some of the symptoms you have described bear that out. We have heard of liquid seaweed as an organic fertilizer, but have no personal experience with it. It's just our opinion that it needs to stop. 

Check out these possibilities and see if you can detect and correct the problem on your own. We don't believe the spacing of the shrubs would be a problem. The wax myrtle likes sun to part shade for its light requirements, and prefers a slightly acidic soil, while the soil in Austin is pretty alkaline. If you decide it is the pH of the soil that is causing the dieback, although we think it unlikely, then you may want to replace the shrubs with a plant which thrives in alkaline soil. 

Morella cerifera

Morella cerifera

Morella cerifera

Morella cerifera