tree peonie losing to herbaceous graft

Hi Folks,
Newbie here.
My established TP is being overwhelmed by its herbaceous stock. After 5 or 6 yrs. of apparent dormancy, the HP appeared last season and quickly surrounded my TP.
Last Fall I ruthlessly spaded out any HP parts I found but the HP has returned with a vengence this year.
I considered using a weed killer but that would certainly effect the TP.
Any suggestions? What am I up against? What brought this on?

Thanks for your help.
Vivo

 

If it's not already try adding soil so the graft union is buried.

 

Thanks, Ron B. That makes sense. I'll check and follow up here. Any suggestions on how to get rid of the rascal HP?