This is kind of an odd question, sorry!

I have a fence that was installed by the home's previous owners about 20 years ago. I'm slowly replacing the fence panel-by-panel, and as I do I've discovered that several of the posts were only barely set in the ground! Most of them were dug less than 1' down, then they poured cement around it that just created a type of pancake.

I have 2 posts that are completely leaning, almost fallen down! I'm nowhere near ready to replace those panels, though, so I'm trying to support them for maybe a year before I'll be ready to make a permanent fix.

Right now I'm using ratchet straps that are attached to a neighboring tree. That's working fine, but I kinda need my straps back! So I'd like to replace them with rope.

The question is, how do I tie the rope in such a way that there is NO slack in the rope... so that, when I release the ratchet strap, it won't lean forward?

Bracing from the front isn't really an option because of plants near the fence. I considered nailing a 2x4 to the post and the tree, but I'd rather consider that a last resort since I don't want to accidentally do permanent damage to the tree... and I already have the rope :-/