How can i kill the grubs that are eating my plant's roots?

There is organic type products you could use such as Scanmask or Margosan-O. Margosan- O is an organic grubicide made from Neem oil. They are beneficial nematodes that will search out and kill the grubs... There is also milky spore disease (no your not adding a disease to your lawn), just the name of it and it works up to 10 yrs. Other natural ways is to collect the beetles that lay the eggs when you see them in your yard or encourage birds, they eat them too. Good luck!