Ask Mr. Smarty Plants

You answered your own question when you said: "The soil is hard packed red clay, totally in full sun." If you follow this plant link to our webpage on Tecoma stans (Yellow bells), you will find these growing conditions:

"Growing Conditions

Water Use: Low
Light Requirement: Sun , Part Shade
Soil Moisture: Dry
CaCO3 Tolerance: High
Drought Tolerance: High
Cold Tolerant: yes
Heat Tolerant: yes
Soil Description: Well drained, rocky, limestone, sand, and loam soils
Conditions Comments: North American native varieties of this species can survive winters within their natural range but may die to the ground during especially harsh winters even there. Varieties sold in nurseries may be from tropical stock and not do so well in US cold. Yellow bells is drought tolerant and Southwestern varieties are adapted to monsoon rains with dry spells between. They may flower better if such conditions are emulated in planned landscapes, so allow ground to dry out between waterings."

You notice that clay is not listed in the soils this plant will tolerate and that it needs good drainage, plus a little shade. Just a guess, but we are envisioning a slight slope down from south to north. Natural gravity is draining at least an excess of water off that first tree's roots, and the last tree is sunk! There is the possibility that the southernmost trees are not even in clay. At some point in the past, fill dirt may have been put in to level or "enrich" your lawn. Fill dirt is always a chancy proposition - it may be called "topsoil" but it can be anything, including clay.

If you had amended the soil before you planted, or done so each time one died, we probably wouldn't be having this conversation. If you are asking only for plants that do well in clay, we will search for some of those for you. The problem with clay is that it is very tiny particles which quickly absorb moisture and then exclude oxygen. The tiny new hairlike rootlets of a tree need to be able to easily slide into the soil and access oxygen and nutrients. Clay has nutrients but it doesn't care to share. It would be very difficult now to go back and add compost, sand or even degenerated limestone to that clay soil where trees are already growing. Something you could do to possibly help the trees that are still alive is to spread mulch on the root area to protect those roots from the blazing sun. As that mulch deteriorates, it will gradually help to loosen up the soil beneath it. See our How-To Article on Under Cover with Mulch.

Your best bet is never to plant another replacement without digging out a bigger hole than you need for the tree, mix in one of the amendments we mentioned (our favorite is compost), then put the new tree in the hole and fill around it with the fresh fluffy dirt mix you have made. You might even need to be prepared to stake the new tree, because that better, looser soil will not necessarily support the new tree until the roots get a grip on their new situation. Literally. To water, stick a hose down in the loose dirt around the new roots and let it drip very slowly until moisture appears on the suface. While it is very hot and dry, do this two times a week. As the tree gets bigger and stronger and/or there is cooler weather and some rain, you can go down to once a week, and even twice a month. If you plant more Tecoma stans (Yellow bells), remember these are desert plants, native to Texas, although this USDA Plant Profile Map does not show it being reported as growing natively in Atacosa County, but does in nearby Bexar and Wilson Counties.

Before we try to find a list of trees that will grow in your garden and certainly before you put any more replacements into the ground, DO NOT plant anything now! In hot dry areas, we do not recommend that any woody plant (trees and shrubs) be planted in hot weather; November to January is best.

Now we are going to our Central Texas Recommended Species list and, on the right-hand side list of characteristics, select on "tree" for habit, and "sun" for Light Requirements, and clicked Narrow Your Search. This gave us 24 possibilities and, followiing each plant link to our webpage, we chose 6 that the Growing Conditions show as capable of growing in clay soil. Don't be fooled, though, you still need to amend that soil for a better chance of success. All the plants we chose to list are lower, more nearly shrubs, as is Tecoma stans (Yellow bells).

Small Central Texas trees that will grow in clay soil:

Cercis canadensis var. texensis (Texas redbud)

Chilopsis linearis (Desert willow)

Diospyros texana (Texas persimmon)

Ilex vomitoria (Yaupon)

Prunus mexicana (Mexican plum)

Sophora secundiflora (Texas mountain laurel)