Ask Mr. Smarty Plants

We are not sure "thrive" is the right word; we would go more for "survive." You listed several different growing conditions that we would definitely tag as causes for the decline of your Morella cerifera (wax myrtle). We'll try to deal with them one at a time, but we have a feeling that all of them are contributing factors. 

The first, and most obvious to us, was the necessity to use a jackhammer to makes holes for your new plants. Our Native Plant Database page on this plant stipulates:

"Soil Description: Slightly acidic, moist, deep sands, loams, clays." 

Even if you refilled that hole with soil amendments like compost or topsoil, you still have to ask what happens when those expanding roots (usually farther out than the width of the plant) hit that rock. Plants don't come equipped with jackhammers, and being unable to extend further out for nutrients and moisture from the soil probably zapped some of them. 

Next is the question of watering. Granted, we had good Spring rains, but then it dried up, and when Central Texas dries up, it really dries up.  The early season moisture encouraged plants, always optimistic, to put on more leaves for more vigorous growth. Then, when the water stopped, the plant could have gone into semi-dormancy, not trying to get the now-restricted supply of water to upper and outer branches and leaves. Again, from our Database:

"Conditions Comments: Requires constant moisture to get established, but both drought- and flood-tolerant once established. Water Use: High"

So, some of your plants could be suffering from transplant shock, the symptoms of which can show up as much as 3 to 5 years after the initial planting. Again, you said you planted them "last year." If this was in the summer, or nearly any time except late Fall or Winter, that could certainly contribute to transplant shock.  We're afraid a few months in the ground does not constitute "established." One watering since the drought came back, with blistering heat, probably would be called murder if you were in Plant Court.

You did not say if the plants uniformly had sun or shade, perhaps from large native trees. If there are trees around, you can be sure they are competing with the new shrubs for every drop of moisture or nutrients in that shared soil. We would regard shade and competition as a possible cause for the bushes that are tall and skimpy; they are reaching for sun. Back to the Database:

"Light Requirement: Sun , Part Shade"

We consider "sun" to be 6 hours or more of sun daily, "part shade" 2 to 6 hours of sun. 

On to the dangers of planting that many of the same species together. This is probably not a full-blown monoculture, but it presents the possibility of one plant having a disease (possibly even brought with it from the nursery) and spreading to all the others over time. Furthermore, if you plant a diversity of plants, and one species dies, you are not left with the prospect of totally replanting.

As for the diagnosis of lethal wilt, we are not plant pathologists and couldn't judge on that account. However, we did find this article from the University of Florida Extension Myrica cerifera, which details several forms of fungus that can attack wax myrtles, with pictures. We usually think of fungus or wilt as being associated with too much moisture, rather than too little, and most wilts have a specific insect carrier. However, again, this is out of our line of expertise, so we are not sure if the seaweed concentrate correctly addressed that problem.

And, finally, a question that we always ask is "Is this plant in the right place?" While wax myrtle does grow natively in Texas, and we often recommend it for hedges, here is our database description of its preferred habitat:

"Native from New Jersey west to eastern Oklahoma and east Texas, south through Mexico to Central America as well as through much of the Caribbean, this popular evergreen ornamental is used for screens, hedges, landscaping, wetland gardens, habitat restoration, and as a source of honey."

It is found natively mostly in East Texas, where the deep, sandy acidic soil it likes are available. 

Our conclusion is that you planted a really good plant, native to Texas, in a place most of the plants don't seem to like. The moral of this is that it is necessary to thoroughly investigate a plant before you purchase and plant it. We hope with more watering, most of your plants will survive, and offer our sympathy if they don't.

From our Native Plant Image Gallery:

Morella cerifera

Morella cerifera

Morella cerifera

Morella cerifera