Creating A Shade Garden Where Soil Is Clay

Q: I am trying to create a shade garden under several large white oaks where the soil is dense clay. How can I amend it? I have seen something called an air spade that injects compressed air to break up the dirt and replace with amended soil. 

A: The air spade is a useful tool but there are several things to consider when using it under a tree. Digging radial trenches and putting amended soil into them is one approach. It may help the tree grow better roots, but it doesn’t amend the overall soil surface for the perennial plants you want to install. You can’t use the tool to blow all of the soil off the tree’s roots because many of the fine feeder roots will be damaged in the process. The tree won’t be able to absorb moisture in summer. Since white oaks don’t easily tolerate drought, you could end up killing the trees if you do the air spading now. You could remove and replace the soil in quadrants around the tree but I still think this would be better done in fall. You need to get advice from an ISA certified arborist who has lots of experience using an air spade.