Q.What are the best plants to grow under hardwood trees?
Tree roots, dry ground are additional considerations.

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Something to consider will be that planting around tree roots is very difficult. There is another way to go about this, though. Using a layer of mulch around the tree, so long as it isn't crowding the tree base, can get around needing to compete with tree roots. They will grow through the mulch or soil instead. In this way, you have many more options for plants, since they won't be needing access to the actual soil underneath. This article will give you more information: https://www.gardeningknowhow.com/ornamental/flowers/fgen/gardening-around-tree-roots.htm