Planting and Pruning Plums
When selecting a site for plums, choose a location with loamy, well-drained soil in full sun. Avoid low-lying areas where frost settles. If possible, remove wild plums to prevent them from spreading disease to your new trees. When planting plums, keep the graft union an inch above the soil line. Space trees 20 to 25 feet for standard varieties, 15 to 20 for dwarfs.
During their formative years, Japanese plum trees should be trained to an open center. They grow vigorously and so require relatively heavy pruning to keep fruiting wood close to the trunk. These varieties will bear fruit on 1-year-old shoots as well as on long-lived spurs. Remove 1-year-old shoots to promote bearing only on the older spurs to prevent trees from overbearing. European varieties bear fruit only on spurs older than 1 year, so they bear less heavily and require less pruning. Don't prune out the 4- to 6-inch fruit-bearing spurs; remove young shoots instead. European plum trees should be trained to a conical shape with a central leader, much like apple trees. Unless the crop is unusually heavy, European plum trees require little fruit thinning. The closely related Damson plums require none at all. Japanese plums, on the other hand, tend to overbear and should be moderately thinned. All plum trees naturally thin themselves after the fruit has formed. During the spring drop, nearly every other plum may fall. Don't panic; it's natural. Follow up with a hand-thinning soon after to end up with fruits spaced about 3 to 4 inches apart. European plums should be spaced one fruit every 2 inches. This encourages larger, better tasting plums.
Wait until some fruits soften before harvesting. The plums should come off easily with a slight twist. European plums are best when they ripen on the tree. But they, as well as Japanese varieties, can be harvested just as they begin to soften, and then stored in a cool place to ripen fully. Late-maturing varieties may last for several weeks in a cool place if picked when firm but quite near the softening stage. Though plums on early-bearing trees may ripen at different rates (allowing two or three harvests), most of the fruit on plum trees will ripen at roughly the same time.
Photography by National Gardening Association.