Answer: Planting around a mature tree is always somewhat of an experiment. Cottonwoods tend to take all available moisture out of the soil, most actively where the feeder roots are. Close to the trunk, as with any tree, the canopy will be dense and prevent rain from reaching the soil. Astilbes require a rich and moist soil to grow well -- hostas are more tolerant and more resiliant and might have a better chance. You might try using a water holding polymer in the amended soil where you plant and add lots of organic matter such as compost to the soil. Also, do not heap soil up against the trunk in the planting process as this can cause rot. Food luck with your project!
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