I think I can remember reading that the window of opportunity to paint the stump is only a few minutes after cutting it if you want the max effect. Apparently they seal over pretty quick
Yes. It is very important to do the cut stump treatment within 30 minutes of cutting the tree or sapling down and this applies to most species. The stump and root system, doesn't 'know' the rest of the tree is gone and it just keeps pushing sap/nutrients up to the cutoff, which then seals over the surface and prevents the herbicide solution from entering the part of the vascular system that goes down to the roots.
I usually do the slash (hack) and squirt of 50/50 remedy ultra/diesel in the fall right before tree foliage begins to change color. (Remedy Ultra's active ingredient is triclopyr) It is during this period, that most of the activity in the vascular system is down rootward, which is what causes the leaves to change color then fall off. A few slashes with a hatchet, about 20-30" above ground, with the cut angled down toward the ground... not a straight horizontal slash. Only takes a few squirts with a spray bottle. I try not to completely circle the girth of the trunk. If you do, there is no movement of sap to the root system and the root system will receive no herbicide.
A tree this size, I would put about 4, maybe 5 cuts that size around the trunk, leaving some peripheral area untouched.
The diesel will probably kill grass around the tree but the triclopyr itself does not.
I have killed hundreds of sweetgum, chinese tallow, ironwood, pine, and cedar with the above method.
Everything I've heard or read says that new cutdown and cut stump treatment is effective regardless of the time of year on most tree species except in the dead of winter when there is very little vascular movement in either direction.
I do try tho, to do the slash(hack)/squirt thing before a seed bearing tree begins producing seed.
Lots of youtube vids on how to do both cut stump and hack/squirt treatments.
