The Oak Tree Versus the Insidious Clay

How the black clay soil in Central Texas responds to drought/flood cycles

I've read many posts about how well live oak trees thrive in Central Texas. They say, "You don't even need to water them, even in drought! They will do just fine." Well that advice is good up to a point. But my poor live oak tree suffered grievously from drought and the quirks of clay.

[Image: cracked black clay soil, just a little dry, not even in drought]
Clay soil has a life of its own. It expands and contracts depending on available moisture. These effects become very obvious during drought; the bare soil cracks, sometimes deep into the ground if the drought lasts for awhile. That is the contraction phase at work. Then when the rains finally come, the soil expands again, and the cracks, no matter how deep, disappear.

This constant movement of the ground is a major issue for any flat concrete structure on the ground, notably slab home foundations. In the clay sections of Central Texas, home foundations, as well as sidewalks and driveways suffer from this continual expansion and contraction. It's hard to find a driveway in my neighborhood that doesn't have large cracks in it. (On a lighter note, in my driveway it gives my bluebonnets a great place to grow.)

What you don't hear about is what this does to plants. If these changes can crack concrete, what could they do to roots? The effect isn't nearly as obvious; it's under the ground, and most plants don't die back from it.

However, the 2011 drought in Texas was extreme. I trusted my live oak to take care of itself, and it seemed to do fine during the actual absence of rain. This extreme drought was broken by an extreme flooding rain, and suddenly the tree wasn't doing well at all. In the end, fully half the main branches of the tree died. Similar patterns appeared with some shrubs in the yard, but the most affected was the poor live oak.

I couldn't figure it out. Why would rain almost kill the tree when it did so well during the drought? I talked to arborists - no one had any idea. Then I finally connected the properties of sudden clay expansion to what it could do to tree roots. That had to be it. With the flooding rain, the ground expanded so fast it actually sheared off a large section of the roots of my live oak, and the root system could no longer support the branches.

Ever since, I've been careful to keep the tree watered at least some during drought. It's never happened again.