Ask Mr. Smarty Plants

Morella cerifera (wax myrtle) is native to Virginia and should be well-adapted to living there. It likes a moist, rich soil. However, it appears to be native only to the eastern portion of Virginia, and Virginia is about the northern extreme of wax myrtle's frost tolerance. It is also referred to by our Native Plant Database as a "wispy" plant, which could explain some of the lower level of leafing out that you have experienced. If you live in the western part of Virginia and/or have had some unusually extreme freezes in the last couple years, that could be a partial explanation. However, we agree, normally this plant would do better than you are describing. 

So, let's turn our attention to the sumac in the waste ground behind you. There are 4 sumacs native to Virginia: Rhus aromatica (fragrant sumac), Rhus copallinum (winged sumac), Rhus glabra (smooth sumac and Rhus typhina (staghorn sumac). All the sumacs are pretty similar in how they grow and in that they are all native to, and grow profusely in, areas far further north than Virginia, even well up into Canada. Here are excerpts from Conditions Comments on sumac in our database:

"Because of its large, spreading habit, is not suited to small areas. They are fast growing, generally pest and disease-free, and drought-tolerant. Colonies are often single-sexed, formed from a single, suckering parent. Dry, rocky or gravelly soils. Colonies can be rejuvenated every few years by cutting them to the ground in mid-winter. Sumacs grow in dry waste areas, such as impossible slopes where even juniper struggle. They are fast growing, generally pest and disease-free, and drought-tolerant. Thin bark makes sumac especially sensitive to lawn mowers and string trimmers. Wounding, however, triggers development of replacement sprouts."

In summary, this plant is going to be tough for wax myrtle to compete with. And your idea of cutting down the shrubs behind your property is apt to backfire, in that colonies can be rejuvenated by cutting to the ground, and that damage from mechanical equipment results in more volunteers appearing. In short, this plant is a survivor. 

Unfortunately, the genus Rhus (sumac) sometimes features another form of competition, that of allelopathy. Allelopathy refers to the chemical inhibition of one species by another. The "inhibitory" chemical is released into the environment where it affects the development and growth of neighboring plants. Allelopathic chemicals can be present in any part of the plant. They can be found in leaves, flowers, roots, fruits, or stems. They can also be found in the surrounding soil. Target species are affected by these toxins in many different ways. The toxic chemicals may inhibit shoot/root growth, they may inhibit nutrient uptake, or they may attack a naturally occurring symbiotic relationship, thereby destroying the plant's usable source of a nutrient.

The sumac should certainly be cleaned up, by bulldozing if necessary, before anyone tries to  build on that lot. However, as we have already observed, this usually just inspires the plant to more vigorous colonization. You could try cutting a trench in the soil on your property line, and painting the cut-off roots with a broad spectrum herbicide, within 5 minutes after the cut, to at least slow down the spread of those roots. But, as you have already pointed out, you have colonization already on your side of the property line. 

We feel that your situation involves a not quite as well adapted species, wax myrtle, competing with a very well adapted species, sumac. This is a situation that will likely call for action on several levels, beginning with fighting the colony of sumac behind your property. If you want to try to have it killed chemically, please get a licensed and trained contractor to do that. Careless use of a herbicide could result in the loss of many more plants in the neighborhood, not to mention polluting the air. Next, you might consider another, more adaptable plant to replace the wax myrtles. When they are dug out, you will have another opportunity to try to destroy and dig out the roots of the sumac already on your property. You might even let that portion of your garden "rest" a year, while you continue to work to eliminate the remnants of the invasive plant.