don't go beyond the farthermost (or furthermost) tree.
Examples of furthermost
furthermost
When it was absolutely necessary we did it discreetly, on the furthermost borders.
From the denverpost.com
Men who fought big bluecoats are shrinking and creeping to the furthermost corners of their cells to get away from this baby.
From the sfgate.com
Atmospheric Greek ruins can be found at Segesta and Selinunte, while the rocky Egadi Islands sit in clear waters just off the island's furthermost tip.
From the telegraph.co.uk
When traveling west on Irvine, is it legal to be in the furthermost right lane and go straight through the intersection, or is that lane a right-turn lane?
From the ocregister.com
More examples
Farthermost: (comparatives of `far') most remote in space or time or order; "had traveled to the farthest frontier"; "don't go beyond the farthermost (or furthermost) tree"; "explored the furthest reaches of space"; "the utmost tip of the peninsula"