Hi Tim,
That grew fast! Two months and it's a jungle lol
I've left Japan now until December, so I worry about mine too, especially the back garden — it was already fairly overgrown when I left. At least my neighbour sends me photos on LINE from time to time.
On winter — growth pretty much stops once it gets cold, so what you find at Christmas should be I think what's standing there in autumn.
That black sheet in my photo is something you'll see in gardens all over Japan — it's called bōsō sheet (防草シート), literally "weed-prevention sheet". I had to Google it (well, "Claude" it, as I did not know its name):
It's a woven fabric, so rain drains through, but nothing grows up through it. Cheap rolls at any home centre (Nafco, Komeri, Cainz…), pegged down with plastic pins. A lot of people put gravel on top, which looks nicer and protects the sheet from the sun. Part of my garden is covered with it, and that bit has stayed completely clear while everything around it went wild.
I think it depends what you want from the garden. If you're planning trees and proper planting, some maintenance is unavoidable. I want to have a nice semi-Japanese garden but need my gardener back, lol. But if you want the lowest-maintenance option, covering most of it with this sheet, or gravel, or a combination is a good way to go I think.
I'm actually in Korea at the moment — partly a holiday, partly seeing how I feel about maybe a second base here one day, so I could spend time in both countries and alternate (especially once I retire, though that's still a long way off sadly). I recorded a lot of raw footage on this Japan trip too, so there's more than enough interesting content coming between now and December.
Enjoy the Christmas trip — winter in these old houses is its own experience, I made a note to make a video about it at some point. Mine is not too bad actually in winter once I turn the AC on but that is one of the reasons I wanted to buy in Kyushu due it its very mild winters!