Hi coing
To make umeboshi, you have to have shiso ready to color red. if you din't grow Shiso this year, you make plume wine. you use vodka or shochu to make plum wine.
It is about time to hurvest Ume in Japan.
Since I don't know vareity of Plume tree, size of fruit, it is hard to guess that you have early bloomer, mid bloomer or late bloomer. also depend on weather of your area. It is about time to hurvest Ume in Japan.
keep record of ume growth for future reffrence.
This year, wait untill few fruits chenging color to yellowsh, then you harvest rest of ume on the tree.
Nanko, kaga, shiro kaga, koshu koume are popular varety of umeboshi production.
yama

Hi yama, thanks for your reply. Actually I do have some shiso growing, I have grown for years more as an ornamental than for using its flavor. But this is the first year my tree had fruit. I think the tree is more an ornamental variety, but I will give it a try. The fruits are now about 2 or 2.5 cm in size, and there are a lot of them. I will wait as you say, and try it as an experiment.

Thanks again, your information is very helpful!

Coing

hi coing
Shiso has many use too.
flower
you can use as gernish, tempra, seed can be use as sort of pickle/ splinkle on salad, pasta, in rice ball gernish.

leave
can use as kimchee, toping of salada( have to chop off fine), ifred shiso can be use as food coloring/red
but stain your finger and nail ^^

Enjoy gardening...............yama