Good Morning Beth, 

Thank you for contacting the U of M Extension Service. 

There may be several issues with your plant area. Have the trees surrounding the planting area grown up and provided much for shade for the plants so they are struggling a little? 

Also, perennial plants eventually need splitting to thrive. Please click on the following link from the U of M Extension Yard and Garden website: https://extension.umn.edu/planting-and-growing-guides/dividing-perennials

This information states: 

After a few years in the garden, these perennials may start to produce smaller blooms, develop a 'bald spot' at the center of their crown, or require staking to prevent their stems from falling over. All of these are signs that it is time to divide.

The above information has details about rennovating your plants. 

If the trees have grown up above your planting area and the plants are within the "drip line" of the tree (under the canopy where the rain falls) the plants may be competing with your trees for water and nutrients. 

Good Luck!