pleaching mature hornbeam
how late is too late to pleach a carpinus caroliniana
If you mean to braid the branches, you can do this any time the branches are still flexible enough to move into the desired position without breaking or cracking them. This is typically done with young branches or young plants. Of course, when branches have fewer leaves it is easier to see the branches and the form. Here is what I mean by pleaching--
"Pleached: To braid or interlace. Pleached trees are planted in rows (palissades) or a quincunx (five trees in a square with one in the middle), and the branches are woven together, pruned, tied, and trained to form an archway or raised rectangular hedge with smooth contours. Pleached refers to the more architectural form of clipped plants, while topiary is the more decorative form."