Please help: I have almost 2 month old Zoysia sod that is mostly fine, but one area under an oak tree where sod does not grow roots, sod is still not attached to the ground after 7-8 weeks. Sod is thinning there. Is anything I could/should do to make it grow roots? or Zoysia simply cannot grow under the oak (I trimed branches, sunlight is filtered during whole days under the tree). Thank you!