I harvested several hundred seeds from my Japanese maples last year. I stratified them and a few hundred germinated. They are about 4 months old and I am just wondering how I know when they are outgrowing the container they are in. Some of them I moved to 4.5" pots a couple of months ago but they dont seem to be doing as well as the ones still in the starter trays. Any advise would be great.
i have started jm seeds here although just a bakers dozen nothing like you have done - i waited until the roots started coming out the bottom before moving to larger pots. i used no fertilizer so mine are now in 1 gallon containers and are 4 years old. i have planted two out and expect they will grow faster than the others next year.
You could keep them in there for another year. I like to transplant when the roots reach the end of the pot not just one root but many roots. Do not over pot the plant (too big of a pot) the roots can start to rot from too much moisture. I have seedlings in a 3" pot and they are about 3 foot tall with no branching. I don't want branching for grafting reasons.