Well Steve I start backing my fertilizer down about end of Aug first of Sept. But I stay a lot warmer longer than u do I never really quit fertilizers just only do 1/4 of dose in winter. In summer I feed ever two weeks full doses in winter 1/4 dose every 6 weeks. But with this tree going dormant I would back off to half dose now then to 1/4 then 1/8 and then nothing by the time it goes dormant. Unless you plant on greenhouse or keeping from dormancy this winter. Hope my 2 cents helps.
Trace

You're going to leave it out in the frost?

Yes. I left PT outside until the temps dropped into the teens. I then took it in and stored it in a basement with a sunless window She busted out in growth next spring about 2 months before last spring frost. I had to move the 200 pound tree several time before I could leave her out permanently. She has grown very well. I have on PT planted in ground. It is important that tender growth has hardened in time for cold.

Steve, fertilizing depends on how much heat and light your trees will get during the winter, along with soil mix...

I know you keep yours this way, warm and under lights, actively growing. So yes continue to fertilize..

One like me, who subjects them to cold and the mercy of whatever sunlight I can naturally provide would stop by the end of summer...

That is a nice looking tree.

Mike