Planting under established trees
Hi, We just bought a new home. It has a lot of lawn under trees. I would like to plant native under story trees, shrubs and ground cover underneath. The trees include maples, black walnuts, oaks and others. I was planning on building up some soil over the grass and planting into that, but I read that adding soil above roots can be bad. So, I was wondering what is the recommended method, so that the roots of the existing trees are least disturbed? Thank you
Your plan is a good one as growing grass under heavy shade conditions is very difficult.
You are correct that you should not pile soil or mulch too deeply over tree roots, which can smother them and kill them slowly over a number of years.
You can safely mulch or add compost about 2-4 inches deep, but keep it from leaning against trunks and stems of plants. It is a good idea to leave some space simply mulched around the trees themselves, and perhaps make some island beds connecting to them. The trees are thirsty, and some have dense root systems that will preclude planting except in small pockets that you can dig and add soil to.
The choice of native plants is great, just be sure to suit the plant's requirements for sun, water, etc to the site conditions, and plan for the mature size.
Here is our page that should help you further: https://www.extension.umd.edu/hgic/topics/landscaping