As a tree grows and its branches and roots expand, the area around it can become a barren wasteland. The tree roots quickly soak up all available water and do a great job of blocking the sun, so few plants thrive there. However, it is possible and relatively easily to create a lovely, dense fill of small plants at the base by following advice in this article.
Tips include protecting the roots of your tree by using small trowels rather than shovels, planting small seedlings with small root balls, called “liners”, and giving them plenty of water. Use multiples of the same plants and choose plants for their attractive foliage rather than flowers. Fill in around the tree instead of planting a ring, and plan for drought.
Find these and other good tips at https://www.thespruce.com/tips-for-planting-under-trees-4119908