Plants can fail for so many reasons. And since it depends on a lot of factors:

  • What the plants are.
  • Where you're located.
  • How old the plants are.
  • What's wrong with the soil, and how you're going to amend it.

With most ordinary perennial borders, with ordinary soil, it's sufficient to top dress once or twice a year with rich compost. If the soil isn't ordinary--heavy clay, pure sand, etc.--it should have been amended before the plants were put in, and then annual top dressing with compost is generally sufficient.

every plant outside of the tree is struggling worse than I have ever seen.

Are they competing with the tree?

Are they competing with heavy weeds, or with lawn grass next to the bed. You need to know whether you should water or rely on rainfall and what kind of soil you have, for example: clay or sand.