Is it too late to work?

Back and squirt is what I do to a tree in the morning after eating and drinking way to much the night before.

Worked for me earlier

Taco Bell says "Thanks for your business"

Probably depends on where you are. Most of my leaves in Monroe county are just starting to change colors. I imagine that the moisture(what little there is with no rain for months) is headed back to the roots at a good rate. If your leaves are already falling I’d suppose it might be a little late but I’m new to this control method myself. I did a stand of sweetgum at the end of September and they are turning noticeably brown on the tops.

Never too late. Some claim that during spring green up it'll push out the cuts, but I've killed a bunch of sweet gum in early spring. I generally use the Harper mix, Garlon 3, water and Polaris AC. But I've also used Triumph (generic Tordon RTU) and recently tried some 20% Imazapyr in water on Chinese Tallow trees. Need to go back and verify that it's working.

Hack and squirt will work anytime of year, but generally spring tends to be the least effective timing as sap flow is upwards and less herbicide gets to the roots. I’d do it whenever your schedule allows.

I use to do this method before the ole lady go her hysterectomy done..

It’s an effective method and the timing is very important.

Will it kill an oak next that's growing couple inches from it?

Waffle House is a perfect example.

If you use a soil active herbicide and get sloppy, potentially yes. As far as root grafts between two different species it’s not likely.

I have had this happen several times. I think you can use straight triclopyr to prevent that.

It depends on chemistry used and as k bush said, how sloppy you are. Generally when I see injury to nearby vegetation, it is either 1) root exudates 2) too much applied to hack area and you get runoff in the next rain and roots take up a soil active herbicide like imazapyr or 3) you habe root grafting between same species. Root grafting really isn’t a concern across species.