plum-plump! plum-plump! plump! plump!

There were many kinds of plums. When the red ones were all picked, the yellow ones were ripe. Then the blue ones. The largest of all were the very last. They were the frost plums, that would not ripen until after frost.

One morning the whole world was delicately silvered. Every blade of grass was silvery and the path had a thin sheen. It was hot like fire under Laura's bare feet, and they left dark footprints in it. The air was cold in her nose and her breath steamed. So did Spot's. When the sun came up, the whole prairie sparkled. Millions of tiny, tiny sparks of color blazed on the grasses.

That day the frost plums were ripe. They were large, purple plums and all over their purple was a silvery thin sheen like frost. - On the Banks of Plum Creek, Chapter 9, "Grasshopper Weather"

The frost plum is the American Damson or Frost Gage plum. The skin is deep purple, strewes with russet dots, and covered with a thin bloom, which Laura called frost. These plums ripen in October.
- posted by pioneergirl at 11:35 AM