|M. R. James in his study|
It's pretty clear that the ghost of an unjustly executed man is trapped in the post. The situation is evidently based on an old Danish superstition about how to lay a ghost: you sort of herd it out to an empty field and imprison it with a strong stake. It seems that the summerhouse was built around the post, but note that while what's left of the house is decayed and falling apart, that post is still firmly rooted and strong. Was it preserved somehow by its job as a ghost-holder?
I thought it was interesting that the title of the story is "The Rose Garden," but there's not a single actual rose in the story. Instead it's a bleak and rather unattractive section of land that's too overgrown with laurels and other bushes. The rose garden is never created.