I'm no expert, but I remember being told it should be done when the trees are taking nutrients back down to the roots in fall. If anything, my guess would be too early at this point. This popped up in a google search.
From the dark side, an article:
When Hack-and-Squirt is Effective “This method is effective almost any season except early spring,” said Jim. “When you have that heavy sap flow and sap rise, it can wash the herbicide right out of the cut. Otherwise, late winter, summer and fall are fine. In the South, the sap starts flowing in March or into April. It will start later in the North.”
I'm certainly no expert, but I'd say in MN now is the ideal time if girdling the tree as well. It's mid summer, the trees have been busy growing up until now as the rains fade, the heat increases, and the new growth hardens, you're choking replenishment from the roots and any nutrient transfer from the crown back down. Add some toxins to the mix and you've got sure death.
Not the cheapest option, but it's a great basal spray and the 2 gallons $45 makes goes a long way. I have a blue 2 gallon gas can I mix it in and then pour it into cheap spray bottles for application. The blue is supposed to be for kerosene, but we don't have any need for it so it's a good option to differentiate from gas or eventually straight diesel.
I take an old emty scent mist spray bottle. the kind that your finger pumps from the top of the spray bottle. I put the TVC in undilluted.
I then hack a cople spots and just put a mist spray in the cuts.
On huge trees i may do four cuts, and a little heavier mist.
I will probably be tired and done spraying before I spray an ounce or two.
It last a long time like this and just put the little lid back on and put it in your pocket until the next tree. This is the kind of bottle I use. It works for me.