Ask Mr. Smarty Plants

Sophora secundiflora (Texas mountain-laurel) is native to the Austin area, so we can't blame the problems on it being out of its territory. You are absoutely correct that even native plants, well adapted to our uncertain climate, have suffered this past couple of years. However, everything we looked at researching this plant says that its mature height is 10 to 15 ft.,  but none of the material said how old it was expected to get. Generally speaking, the slower a plant grows, and this one grows very slowly, the longer it will live. So, it would seem your tree is mature, but not necessarily senile, ready to die. The only possible change we can think of that might cause some decline in a tree that well developed is if there has been a change in the drainage. This is a plant accustomed to growing in desert areas and limestone, which is why it is so slow-growing. It needs good watering, especially in Spring and Summer, but also good drainage. If it has not been receiving supplemental watering during the very dry last two years, and now there has been rain, it may have feet standing in water beneath the surface of the soil, especially if you have clay soil. 

We certainly would not recommend taking down a mature and apparently healthy tree just yet. Don't fertilize it.  Often, that is a knee-jerk reaction when a plant does poorly, to fertilize. A plant under stress should never be fertilized, and with this tree, with the blooming already past for this season, you sure don't want to send a message to inspire new growth. Furthermore, this is a plant that really should never be fertilized, it is so well-adapted to its environment. Make sure it is getting plenty of deep watering, but that the water is draining. If water stands on the plant 30 minutes before it disappears into the soil, you have a heavy clay soil. Try mulching the root area, reducing the amount of water put on the roots each time, but increase the frequency. We would definitely give this tree another year or so, before it was eliminated from your landscape.

From Our Native Plant Image Gallery:

Sophora secundiflora

Sophora secundiflora

Sophora secundiflora

Sophora secundiflora