A fruit tree grown in temperate locales, the plum is a member of the Rosaceae family and is related to the rose, apple, pear, apricot, and almond. A member of the genus Prunus, the plum contains more than 100 species, 30 of them native to North America. A plum is 86–88 percent water, 0.4–0.8 percent protein, 0.1 percent fat, 1.3–2.4 percent fiber, and 6.7–9.9 percent sugar, of which 1–4.2 percent is sucrose, 0.9–3.4 percent fructose, and 1.7–5.2 percent glucose. The content of sucrose, the sugar in sugarcane and sugar beet, increases as a plum ripens. One hundred grams of plum contain 120 to 190 milligrams of potassium, 0 to 3 milligrams of sodium, 6 to 8 milligrams of calcium, 4 to 7 milligrams of magnesium, 0.1 to 0.4 milligram of iron, 0.1 milligram of zinc, 4 to 11 milligrams of vitamin C, 0.02 to 0.05 milligram of thiamine, 0.04 to 0.05 milligram of riboflavin, and 0.2 to 0.9 milligram of niacin.
Humans have cultivated the plum tree since prehistory. One authority believes that among fruit trees only the apple is a more ancient cultigen. Europeans have grown the plum tree since the time of Christ and perhaps earlier. In 2003, the world produced tens of millions of tons of plums. Producing a near majority of the world's crop that year, China harvested millions of tons of plums. Romania ranked second, the United States third, Serbia and Montenegro fourth, and Germany fifth. China totaled the majority of the world's plum acreage in 2003, but tallied a low yield, nearly a sixfold reduction compared to Chile's yield, the world's highest yield per acre.
Plums are of three types: European, Japanese, and American. Most European plums are in the species Prunus domestica and originated in Eastern Europe or western Asia. The majority of the world's cultivars are of this species. A hexaploid, the European plum arose from the hybridization of the diploid Prunus arasifera and the tetraploid Prunus spinosa. The progeny were a triploid, but the chromosomes doubled, apparently in a mutation, to produce a hexaploid. The skin of the European plum may be red, blue, or intermediate colors.
Scientists group the European plum into four categories: Prunes, Raine Claude, Yellow Egg, and Lombard. The category Prune may cause confusion because it applies not only to a class of plum but also to the dried fruit of any plum. Prunes are oval, firm, thick, and freestone. The skin may be blue or purple. With a high sugar content, prunes are usually dried, a circumstance that gives this category its name. Prune cultivars include Agen, also known as French. Other cultivars are Stanley, Sugar, Imperial Epineuse, Italian, German, Giant, and Tragedy. The category Raine Claude, also known as Green Gage, is round with green-yellow or red skin. Raine Claude boasts sweet, juicy flesh. Plums of this category supply the fresh market and are canned. Cultivars include Raine Claude, a confusing name that applies to both category and cultivar. Other varieties are Bavay, Jefferson, Washington, Imperial Gage, and Hand. Yellow Egg is canned. Fruit is large, long, and oval, with yellow skin and flesh. Varieties include the confusing appellation Yellow Egg, Red Magnum, Bonum, and Golden Drop. A large plum, Lombard is oval with red or pink skin. Cultivars include Lombard, Pond, and Bradshaw.
Another European plum, Prunus insititia, yields small fruit. Prunus insititia cultivars include Damson, Bullecea, and Mirabelles. Damson has purple skin whereas Mirabelles has yellow skin. Seldom eaten fresh, Damson is processed into preserves and jam.
The Japanese plum, Prunus salicina, may have arisen in China, from where it spread to Japan between 1600 and 1800 CE. The islands gave this species its name and the impetus to spread worldwide. The tree has rough bark, which distinguishes the Japanese plum from its European counterparts. Because the Japanese plum tree flowers, early in spring it is susceptible to late frost.
The American plum comprises several species. Prunus americana has yellow or orange flesh. Cultivars include Desota, Hawkeye, Wyant, Weaver, and Terry. Made into jam and marmalade, Prunus hortulena is resistant to brown rot. Cultivars include Wayland and Golden Beauty. Also resistant to brown rot, Prunus munsoniana tolerates frost. Farmers plant Wild Goose, the chief variety of this species, in the lower Mississippi River valley. Prunus bessayi is not cultivated for its fruit but as rootstock for other varieties. Native to northeastern California, Prunus subordata is made into preserves and jelly.
When a plum tree is in bloom, it will not tolerate cold, wet soil, or wind. The Japanese plum needs less cold weather than the European plums to initiate dormancy in winter. One authority recommends an orientation toward the north, especially for the Japanese plum, to delay flowering in spring and thereby to reduce the risk of frost injury. Where possible the farmer should plant plum trees near large expanses of water to moderate the temperature. It is better to plant plum trees on a gentle slope rather than on flat land. The farmer should plant plum trees in spring to give them time to grow roots and branches before the onset of winter. Where winter is mild, the farmer may plant plum trees in autumn and winter. Dwarfs may be planted at a density of 320 to 480 trees per acre. Full-size trees are planted less densely. The farmers should choose a well-draining, deep loam, though European plums prefer clay and the Japanese plum does well in sand.
The farmer may graft a plum scion onto a plum, peach, Japanese apricot, or almond rootstock. In India, wild apricot is the rootstock of choice. During its first two years, a plum tree competes poorly against weeds. Irrigation should supplement rainfall. During its first year, a plum tree should be irrigated every two to three weeks. Because honeybees pollinate plum trees, the grower should rent beehives, placing them at a density of one hive per acre. When a plum tree is young, it needs little pruning. The European plum species remain attractive even when they are not pruned. By pruning, the farmer aims to let light penetrate the canopy. Where the climate is dry, a plum tree should be pruned heavily to encourage it to produce a small number of large fruit. Because the Japanese plum tree yields fruit more heavily than European plum trees, its limbs may break under the weight of its bounty. Accordingly, the farmer should prune the Japanese plum tree more heavily than European plum trees. In addition to pruning a tree, the farmer should thin fruit to encourage it to produce fewer but larger plums with uniform color. The farmer may thin by hand, machine, or chemicals. The third method is cheapest and yields the best fruit size and quality. The farmer should thin flowers when they are in full bloom and up to four weeks after the petals have fallen from them. A plum tree may be grown in sod because it competes against grass better than peach or cherry trees. The plum tree is sometimes intercropped with vegetables or other fruit. Where irrigation is unavailable, the farmer should not intercrop because the other plants compete against plum trees for water. Where irrigation is available, the farmer may interplant plum trees with rye grass, clover, peas, alfalfa, or other legumes. Soil near the tree may be cultivated to eliminate weeds, but one must be careful not to damage feeder roots near the surface. Since the 1950s, farmers have used herbicides to kill weeds. Effective are atrazine, simazine, diuron, oxyfluorfan, and glyphosate. A tree younger than one year should not be exposed to herbicides for fear of injuring it. One may eliminate weeds with a mulch of grass clippings, sawdust, pine needles, or black plastic. Mulch is desirable because it minimizes erosion and water runoff.
In 1928 scientists, focusing on the importance of potassium in plum nutrition, first documented its deficiency in soil in which plum was grown. Research on other elements followed. The amount of a nutrient in plum leaves may indicate the adequacy or inadequacy of that nutrient in the soil. A plum leaf should have 2.3–2.5 percent nitrogen and 3.2–3.4 percent potassium. The coupling of these elements is important because applications of nitrogen and potassium to the soil are more effective than either alone. A plum leaf has the highest concentration of nitrogen and phosphorus early in the season. The concentration declines as the season wanes. A plum leaf should have 0.14–0.25 percent phosphorus, 1.5–3 percent calcium, 0.3–0.8 percent magnesium, 0.02 percent sodium, and 0.3 percent chlorine. A plum leaf should have 6 to 16 parts per million (ppm) of copper, 40 to 160 ppm of manganese, 100 to 250 ppm of iron, 20 to 50 ppm of zinc, and 25 to 60 ppm of boron.
A plum tree removes from the soil 21.6 to 55 pounds of nitrogen per acre, 4 to 11.5 pounds of phosphorus per acre, 20.5 to 54.4 pounds of potassium per acre, 3.4 to 10.2 pounds of calcium per acre, and 1.8 to 5.9 pounds of magnesium per acre. One recommendation calls for 0.2 pound of superphosphate and 0.1 pound of muriate of potash per plum tree multiplied by its age in years. Another recommendation calls for the application of 197 to 295 pounds of a fertilizer with nitrogen, phosphorus, and potassium per acre. Various recommendations set the ratio of nitrogen to phosphorus to potassium at 45:45:45, 120:60:60, 120:30:15, and 200:240:300. Alternatively, the farmer may apply 60 to 105 pounds of nitrogen, 70 to 105 pounds of phosphorus, and 140 to 190 pounds of potassium per acre. Another variant calls for the application of 130 pounds of manure, 1.1 pounds of nitrogen, 0.55 pound of phosphorus, and 1.3 pounds of potassium per tree. A large application of potassium is important for trees that bear fruit heavily. The farmer should apply all phosphorus, potassium, and manure in December and January. Half the nitrogen should be applied before a tree flowers and the other half after fruit set. The farmer who applies aqueous nitrogen by drip irrigation may halve the amount with no loss in yield. Drip irrigation is also an effective method of apply aqueous potassium, though a plum tree is less efficient than apple or citrus trees in absorbing aqueous fertilizers. Water is commonly applied by drip irrigation. A plum tree needs as many as 36 inches of water per year.
A plum that will be shipped far should be picked when firm. The Santa Rosa variety should be picked about 94 days after flowering. In California, French needs 158 days after flowering to mature. Plums often ripen unevenly, requiring two or three pickings. Plums that will be canned may be harvested all at once. Most plums are picked by hand because machines damage them. Once harvested, a plum should not be exposed to sunlight. Machines are used, however, to harvest plums that will be dried.
Plums stored at 31°F or 32°F and 85–90 percent humidity may remain fresh 12 days. As a rule, plums cannot be stored longer than four weeks, though the plums of a few cultivars may be stored three months. Some plums deteriorate if kept too cold, in which case the temperature should be raised to 45°F after 7 to 10 days. Plums may shrivel during storage, losing 1.4–2.3 percent of their weight. Storage in an atmosphere of 1 percent oxygen and less than 0.2 percent carbon dioxide may increase the storage of a plum of the Victorian variety as much as four weeks. Santa Rosa and Sangold varieties may be stored two weeks at 33°F, 4 percent oxygen, and 5–7 percent carbon dioxide.
Late-ripening plums are made into jam because of their high sugar content and dry matter. Prune juice, a source of minerals and a laxative, is extracted from plums and is a popular breakfast beverage in the United States. Some prune juice is marketed as frozen concentrate. When the harvest is too large, the surplus goes to make prune juice. Plums are canned in several countries including the United Kingdom. Fewer plums are canned than peaches and pears. Italian plums are often canned. Plums that will be canned are picked before fully ripe. California and the Pacific Northwest produce 75 percent of the world's prunes. The French and Imperial varieties are the plums of choice for drying. Prunes were once sun dried, but now machines do the work. Prunes are dried to 4 percent moisture. Some prunes are pulverized into powder, which flavors wheat and rye bread. Plums may be made into wine. Plum wine is popular in Germany and the Pacific coast. The Santa Rosa variety yields the best wine. Plums may also be made into vermouth and brandy. Plum brandy is popular in Romania, Hungary, and the former Yugoslavia. The Alsace region of France produces plum brandy.
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