Question
I recently bought a home in Houston tx that has several cottonwood trees that are about 15-20 yrs old. Many of the roots are above ground but I have noticed one of the the above ground roots appears to have rot and is holding water after rain or watering.  Can I fill this part of the root system with soil?  Overall the trees appear to be healthy. We had a serious drought last year and I do not know if the previous owners watered

Answer
Since the roots are above ground it sounds like the root was hit and cut with a lawn mower and a decay fungi has gotten into the root. Not really a big problem but that root will die but the other roots can take up the slack. I would cover the exposed roots with sand. Do not put more than 2 inches over the roots, more would cause the weight of the sand to damage the roots. Sand is best since it is porous and allows the water to reach the roots. I would also fertilize the tree with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and watered in good. Apply the fertilizer just before a rain storm and you will not need to water. This will increase the health of the roots and the tree.Put the sand down before you fertilize.