I have a very old and very large (75+' tall)  cottonwood tree that has a garden in front of it.  Over the last three years it has been sending up saplings along the roots.  It is a huge task keeping up with pulling or cutting them as there is literally thousands of them.  I noticed this year there are some areas in the garden where the root is at the soil surface.  Should I cover those with soil?  The backside of the tree has some turf, a small patch of raspberries and a garden of all native plants.  This year there are more saplings popping up but not anywhere near the quantity as the front side garden.

A couple years ago it was suggested this was drought stress so I worked at watering more consistently.  Another suggestions was competition for water due to the garden at its "feet."  What are your thoughts on the cause and what can I expect in future years?  Will it subside or continue?  What are the best actions I can do to preserve/protect the tree while maintaining the garden?  Thank you in advance for your guidance.