Ask Mr. Smarty Plants

Actually, "evergreen" is not a specific plant, but a reference to whether or not a plant loses all its foliage in cold weather. That is opposed to "deciduous" which means the plant does lose its foliage. However, you are correct that some sort of evergreen plant would be most effective in making a sound barrier, because there would always be leaves to help shut out the noise.

In terms of what evergreen tree or shrub would be most effective, we would recommend shrubs for quicker effect. We also recommend that you plant natives to the Austin area. Because they are natives and already accustomed to this environment, they will need less water, fertilizer and maintenance than a non-native. How closely they can be planted to existing trees is a question of the amount of sun the shrubs need, whether the planting of shrubs will disturb the roots of a valuable tree, and whether that tree would even tolerate a plant under it. Some trees, for instance, oaks and pecans, have some allelopathic qualities, meaning they exude chemicals, usually when they are somewhat mature, to protect their space from being invaded by other plants. In addition to the shade cast by a mature tree, the chemical defense from the trees makes it difficult to grow much of anything beneath them, including grass. Generally speaking it is best not to plant shrubs inside the dripline of the trees; that is, the edge of the tree canopy. The roots of the tree will probably still extend beyond that, but it should be easier to get some shrubs into the ground beyond the dripline. The foliage of the tree is still going to buffer some of the street noise, even though there is an open space around the trunks. If you have the space, you might consider planting 2 rows of the selected shrub, planting them 6 feet apart, but staggering them so the space will fill in more quickly. If you only have room for one row, plant the shrubs 3 feet apart for quicker coverage.

Before we make our list, there are some considerations we want to explain. All these shrubs require a good deal of sun, so they are not candidates for planting under your existing trees.  Also, several are listed as "dioecious," which means that only the females have berries and there must be a male of the same species, blooming at the same time, within 30 to 40 feet, for the berries to develop. If you had a hedge of 12 yaupons, for example, a couple of males in that hedge would assure berries on the females. You can follow the links to each plant webpage and learn about their light and moisture requirements, speed of growth, etc. They are all commercially available, and if you have difficulty locating them, go to our Native Plant Suppliers section, type in your town and state in the "Enter Search Location" and you will get a list of native plant nurseries, seed suppliers and landscape consulants in the general area.

Finally, you asked when is the best time to buy and plant these shrubs. Now is good. It is better to plant woody plants in the Winter or very early Spring in this part of the country, while the plants are still semi-dormant. Newly planted shrubs are much more likely to be damaged by summer heat in Texas than by winter cold. It's easier on the gardener, too. 

Ilex vomitoria (yaupon) - can grow 12 to 25 ft. tall, female has bright red berries, dioecious.

Leucophyllum frutescens (Texas barometer bush) - can grow 8 ft. tall and 4 to 6 ft. in width. Gray-green foliage a backdrop for bright pink flowers which can bloom intermittently year-round.

Mahonia trifoliolata (agarita) - 6 to 8 ft. tall, red, edible berries, spiky leaves.

Morella cerifera (wax myrtle) - 6 to 12 ft. tall, fragrant folliage, blue berries on female plants in winter, attracts birds, dioecious. 

Ilex vomitoria

Ilex vomitoria

Leucophyllum frutescens

Leucophyllum frutescens

Mahonia trifoliolata

Mahonia trifoliolata

Morella cerifera

Morella cerifera