I taught most of my dogs to dig on command. Just by digging with your own feet, they come investigate and so far it has always been contagious for them. Then you cheer them on and put it on cue; "Good job, dig, dig!" stop before they stop by giving them the command "Enough" that they learned as a young pup. I found it one of th easiest commands to teach because they enjoy it so much. And it wears down their nails in a fun way for them.
I start on the beach or any area with loose soil. I never went so far as to want to plant a tree though, mostly for burying their poop or a small dead animal.
Deja, GSD, sable WL, female, born 11 / 2013
Cam, Rough Collie, sable, male, born 8/2015