I have an area in my yard that is planted with many small flowering plants. At one side of this planted area is a mature, ornamental plum tree. I've noticed that whatever I plant under the tree doesn't thrive.
Most plants die.
If this tree has not received deep, infrequent watering, a large part of its root system is close to the surface. The surface root are using most of the water and nutrients needed by the annuals and perennials. You may overcome some of the problem by giving the plants under the tree extra fertilizer and by installing 25 feet of soaker hose under the tree. Operate the soaker hose weekly or biweekly, depending upon how hot it has been, long enough to get the soil wet to a depth of 15 to 18 inches. This may take 12 hours or more.