Shibui I thought the same but there was one tree near me I took about 5 cuttings and two rooted back in september and are now growing ferociously. I used that red gel hormone from bunnings. It took me about three years to get everything right to make it work. Perlite, sand and potting soil was my mixture in a quite randome ratio.
I did not use any greenhouse etc as the winters here are warmer. They were out in the open getting about 4 hours of winter sun a day.
My first attempts years ago I used greenhouses based off info from people living in freezing northern american states, I figured I simply dont need that here. Rarely sub zero, and no snow. No real need to be so picky on protecting. I find the thickness of the cutting to be quite important too, the thinner ones did not root.
Though that was a standard palmatum. I tried sango kaku cuttings, and graftings with zero success.
Also I did actually try osakazuki seeds from ebay 2 years ago. The only one which sprouted did have a very similar leaf shape, but it died when I went on holiday. Neighbour thought it was a good idea to keep it drenched in a bowl of water
do you think it is this cultivar?
