Plum Prunus domestica L.; Prunus salicina Lindl. Rosaceae Species description There are two primary species of cultivated plums, the European plum (Prunus domestica) and Japanese plum (Prunus salicina). Plum trees are small to medium-sized deciduous trees with rounded crowns. Plum leaves are oval to elliptic in shape depending on the species, with serrated margins. The flower buds form with leaf buds in early spring. Large, white flowers appear in April. European plums have 1-2 flowers per bud, and Japanese plums generally have multiple flowers per bud. Plum fruit, a drupe fruit with a single seed, matures from July to November, depending on the cultivar. The fruit skin is covered in a smooth, waxy layer, and the skin color may be green, yellow, orange, pink, red, purple, or blue. Interior flesh color varies from green to yellow to red. European plums are usually small, oval in shape and variable in color; these include varieties such as the prune-plums Stanley and Italian, Yellow egg, Reine Claude, and the Gage plums. Japanese plums are large, round to heart- shaped and have firm flesh; these are the most common fresh eating plums in the U.S. Natural and cultural history European plums are native to western Asia in the Caucasus Mountains. Japanese plums are thought to have originated in China on the Yangtze River; widespread cultivation for the last 2,500 years makes it difficult to determine its natural range. European plum pits have been found in archaeological remains in Switzerland. Plums are mentioned in Greek writings and were common in European gardens since at least the 1st century CE. Japanese plums were brought from China to Japan 200-400 years ago, and from there, they spread around the world. Japanese plums are widely cultivated in China, Japan, and Korea for sale as fresh eating plums. Candied fruits are also sold preserved, flavored with sugar, salt, and liquorice. In Japan, plums are used half ripe as a flavoring in a liqueur called sumomo shu. European plums were first brought to North America by Spanish Missionaries to the west coast, and later by English colonists to the east coast. Japanese plums arrived in the U.S. later and are now the most commonly cultivated plum, with California leading in production. Planting considerations and propagation techniques Plums can grow to a height of 15 to 25 feet with an equal canopy diameter and may live 15-30 years. Winter and spring temperatures determine bloom time; Japanese varieties usually require fewer chill hours (500-800) than European or American plum varieties (>1,000 hours). Plums are adapted to a range of climate conditions; European plums are more adapted to colder regions and Japanese plums are adapted to warmer climates. European plums are hardy to -30ºF, and Japanese plums are considered to be hardy to -20ºF, however blossoms are sensitive to late spring frosts. Choose a location with full to filtered sun exposure to encourage flowering and fruit development. A slope or raised area for planting the plum trees is best; do not plant plum trees in low areas vulnerable to colder temperatures. European plums are mostly self-fertile, whereas Japanese plums require cross-pollination for fruit set, and rely on insects for pollination. The two species will not cross-pollinate with each other. Plum trees can grow on a variety of soil types, from limestone to heavy clay, though selection of appropriate rootstocks will help them thrive in different conditions. Plum trees do best on moist, rich, well- drained loam soils, with a pH of 6-7. All Prunus species may be rooted from dormant hardwood, softwood, semi-hardwood, or root cuttings. Semi-hardwood and softwood cuttings taken in summer are the easiest to root. Traditionally, European plums were propagated by planting plum seeds or digging rooted suckers from underneath parent trees. Fruit from seedling trees is highly variable, and most plum trees are now grafted or budded for cultivar consistency. Seedling plum rootstock is used for European plums, and seedling peach rootstock for Japanese plums. To collect seeds, gather the ripe fruit and strain seeds out of pulp. Seeds require 2-3 months of cold stratification to germinate—plant seeds directly outside in autumn to overwinter in cold, moist soil. For spring sowing, stratify seeds in moist sand for 1-2 months in a greenhouse, then cold stratify (36-41ºF) for 2-3 months. Plant out early, when outside temperatures are still cool. Water needs Plums have moderate water requirements; as a general rule, irrigate with an inch of water each week during the growing season. Care Young trees are typically pruned to a single whip to encourage side branching. Three or four branches are selected for scaffold limbs to form an open vase or wine glass shape. Prune trees annually to keep them productive. Plums are more susceptible to disease issues in wet, humid regions than in arid regions. Plum diseases include rust, bacterial spot and plum pox virus. Harvesting and processing Plums are generally eaten fresh, but can be dried into prunes or processed into preserves, juice, or alcoholic beverages. The fruit quality, texture, juice content and flavor vary considerably among cultivars, and can be firm to soft, and sweet to highly acidic. References and resources Okie, W. R. 2008. Prunus domestica: European plum; Prunus salicina: Japanese plum. In The Encyclopedia of Fruit and Nuts, edited by J. Janick and R. E. Paull, CABI, Cambridge, MA. Plants for a Future (PFAF). Prunus domestica - L. http://www.pfaf.org/user/Plant.aspx?LatinName=Prunus+domestica Plants for a Future (PFAF). Prunus salicina - Lindl. http://www.pfaf.org/user/Plant.aspx?LatinName=Prunus+salicina Rieger, M. n.d. Plum – Prunus domestica, Prunus salicina. http://www.fruit-crops.com/plum-prunus-domestica- prunus-salicina/ 2 .