Hack n squirt in my area (SC) for my target species (Chinese tallow) is most effective in the late summer/early fall when the trees are getting ready to go dormant. This is when the nutrients are being transported down the tree into the roots for winter storage. This downward movement is what gets the herbicide down into the roots.
I ask my forester just last week that same question but for basal spray and he said anytime there wasn't snow on the root apron as that is where it absorbs most.I just sprayed locust last weekend with remedy and diesel