Autumn Rosa
Plum (Japanese) in the US

Starting Japanese Plums from seed, it's a bit of a marathon, not a sprint, taking about two years until you get your first proper crop. Sow them indoors, about an inch deep, in trays or small pots with some good potting mix. When to sow depends a fair bit on where you are. For folks in zones 3 and 4, you're looking at April or May. Zone 5 can start a bit earlier in March, going through May. Further south, zones 6, 7, 8, 9, and 10, you've got options in the fall too, October, November, even December for the warmest spots, alongside some spring sowing. Just check your local last frost date for spring planting.

Once your seedlings are big enough to handle, carefully prick 'em out and pot 'em on into bigger pots. They'll need a good, sunny spot on a windowsill or in a cold frame to grow on sturdy. Before planting out in their permanent spot, you'll need to harden them off for a week or two, gradually getting them used to the outside. Plant them once all danger of frost is past, giving them plenty of space, you know, a good 15 to 20 feet (about 4.5 to 6 meters) if you can, depending on the rootstock.