It was April 2021, our dear friend Jim of Nashville Tree Conservation Corps called saying two dozens of TreeDiaper^® treemats they installed earlier that year were flat after a lot of rain they just received. All the other 1000ish are doing just fine.

We immediately thought it must be a defect issue and we went on to check the lot number and test the batch of products. Jim was very helpful in terms of sending pictures and checking the contents etc. Everything seems just fine! Then what went wrong? A replacement of those flat took care of the problems for the time being. But it kept puzzling us...

In September, I was in Nashville for the NRPA conference. I visited the site where they spotted the problems. Although most of the problems have been taken care of, there are still a few rather flat ones. I took one back to my hotel room to test it. In the hot water of my hotel room shower, TreeDiaper^® treemat charged up to 80% capacity in 40 minutes. Now what!?!

It was raining the next morning, I drove to the site and try to figure out the problem. I noticed the rain water would not penetrate the mulch layer, instead it runs off to the side. After poking around, I found that the mulch is wet on the top 1/4 inches during the rain. Underneath it, it was very dry. More importantly, it was fused together into one big chunk. See the pictures.

The fungi mycelium network is connecting the mulch pieces and formed a hydrophobic and anaerobic layer. Then why most TreeDiaper^® treemats were fine and why only two dozens out of 1000ish TreeDiaper^® are not? It is the thickness, texture, particle sizes of the mulch, and the positions of the mulch that makes difference. The finer the wool/bark pieces, the more likely a dense mycelium network would form, which is then blocking water and air from getting to the roots. When the people installed them, they supposed to spread the water absorbing pellets around evenly. If they accidentally kept them around on corner and then piled a thick layer of this kind of mulch on top of it, then it would not do the initial charge and would remain flat even you have a lot of rain.

To prevent the mulch tuning hydrophobic and anaerobic, we strongly suggest people use arborist chips, which have much coarser structures than double shredded mulch. If you have questions about wood chips, read Professor Linda Chalker-Scott's articles about it: 

Using Arborist Wood Chips as a Landscape Mulch (Home Garden Series)
Wonderful wood chips - The Garden Professors