I cannot vouch for the lack of toxicity , but I can add is...Sycamore wood is on the less dense side of the chart. They are not the worst choice. Cottonwood is a very light wood and has a pretty fast decomposition rate, for example. And, you cannot necessarily judge decomp on density alone. Western Cedar is less dense than Sycamore, but has other attributes that make its decomp rate much longer. I wouldn't give a cottonwood log but a few years in a pond and would not add them to my pond. The sycamore should go twice as long IMO.
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