The plums on my neighbor's tree are ripe. The tree hangs over into our yard, plus the house is a rental and they just moved out so I felt more free than most years to help myself and make jam. Anyway, they are the typical non-commercial small red-orange plums ubiquitous in Northern Ca. and make really tasty jam. They were bigger than most years (maybe 1.5 inch diameter), maybe because of the late rain. The jam is good on toast but my favorite is to use the jam in small jam tarts or a big free-form jam tart.
5 pounds plums
1 cup water
3.5 pounds sugar
Put plums and water in a large, heavy pot. Cover and cook over fairly low heat - to avoid burning - until plums are soft and falling apart. Push the plums through a medium sieve to remove skins and seeds, back into the pot. Add sugar, bring to a boil and cook until it thickens, or to about 220 F. Skim off the foam several times and pack in jars. This made about 9 cups.