aka Japanese Salted Pickled Plums! In Japan, umeboshi are always made in mid to late June, because that’s when the ume plums are ready. Ume plums are picked when they are hard and very sour. The kind I use are from the Kishuu region, which is in Wakayama prefecture. Kishuu ume are widely regarded to make the best umeboshi.
I understand that ume plums are now available in the United States. When you buy them, make sure you choose ones that are firm, plump and unblemished. Even small blemishes or cuts on the plums could lead to mold, which is the biggest reason umeboshi can fail.
Umeboshi improves with age for a few years. I usually start eating them 3 years after making them, though you can eat them the same year. At around 5 years I think they are at their best. After about 10 years or so they start to disintegrate and become mushy if kept wet, and rather shriveled like an old lady if kept dry - but they are still edible!
Click here for the recipe: http://justhungry.com/homemade-umeboshi-japanese-pickled-plums