We looked in The Backyard Orchardist by Stella Otto and How to Grow Food by Richard Gianfrancesco. Both contained information about growing plums, but only about growing trees from saplings or caring for trees that had already matured.
Fortunately, Amy Grant wrote an article for GardenKnowHow.com about growing plums from seed. The seed must be kept at temperatures around thirty to forty degrees Fahrenheit for ten to twelve weeks before it will germinate. (Sources vary on whether or not the seed needs to be removed from its protective casing, if the pit simply needs to be cracked, or if the whole pit can be planted as-is.) There are a few ways to accomplish this. Our patron could wrap the pit in a damp paper towel and put it in a plastic bag inside the refrigerator. After it sprouted, they would plant it two inches deep in an even mix of potting soil and vermiculite, keeping it cool and moist. Once there was no chance of frost, they could transplant it outside into the garden.
It’s also possible to simply plant the pit directly outside during the colder months. Grant suggests planting it three inches deep and marking the spot so that it can be found again.
Grant cautions that a plum tree grown from seed may or may not bear fruit, and the fruit may or may not taste the same as the original plum, as plum trees are generally propagated through grafting and not through seed. However, she assures that it is still a rewarding and worthwhile project.
Don’t Throw It, Grow It! by Deborah Peterson and Millicent Selsam, available at the Newton Falls Public Library, has more information on how to save kitchen scraps and grow them into plants, for anyone who is interested.