Plums are placed within the Prunoideae subfamily of the Rosaceae, which contains all the stone fruits such as peach, cherry and apricot. Plums are the most taxonomically diverse of the stone fruits and are adapted to a broad range of climates and soils.
The plum tree is a small to medium-sized shrub or tree, but may also grow up to 10 m tall. Twigs are glabrous, becoming lustrous red-brown. The shiny bright green oblong-ovate leaves are carried on 1- to 2-cm-long petioles with several glands.
The white flowers are borne on short spurs, mostly in clusters of 2–3 and appearing before or together with the leaves. Honeybees are the major pollinators. The fruit, a drupe, is globose-ovoid, 3–7 cm in diameter, yellow, red or greenish in color, glabrous, and often pointed at the apex. Fruits are classified as freestone or clingstone type, which indicates whether the stone separates easily from the flesh.
Plums do not produce true from seed. Consequently, and to maintain ‘true to type’ cultivars, they must be propagated asexually by using grafting.
Budding or grafting can only be carried out if suitable rootstock material is available. Myrobalan (Prunus cerasifera) seedlings have been used as the principal rootstock for plums. Their roots are adapted to a wide range of soil and climatic conditions. They are resistant to drought and root and crown rot, but susceptible to root knot nematodes and bacterial canker. Contact Oxfarm Organic Ltd for more information about grafted plums trees.
In Kenya, plums are usually budded onto preferably nematode-resistant peach seedlings where they tend to produce earlier and set more consistently. Peach seedlings are compatible with most commercial plum cultivars, although a large scion overgrowth may sometimes develop as the tree grows older.
Establishment Plums are grown on a wide range of soils. However, a deep, well drained, medium-textured soil with a pH of 5.5 – 6.5 is generally best. Plum trees are the most tolerant of all stone fruits regarding heavy soils and water logging, and they tolerate drought better than do peach trees. plums are very productive at altitudes of about 1,700–2,600 m, where their chilling requirements (hours below 7 °C) of around 100–800 are met. Therefore, it is important to choose suitable cultivars adapted to a specific location.
Depending on future market outlet the grower must decide on such aspects as appearance, storage quality, marketability, palatability, pest/disease resistance, tree growth and productivity. Although it is known that some cultivars like Methly, Beauty and Santa Rosa are self-fertile, adequate cross-pollination is needed for other cultivars to ensure sufficient fruit setting for commercial purposes. Worldwide, many new improved selections are introduced each year and are gradually replacing older and less desirable cultivars.
In most cases a planting distance of 5 m x 6 m is recommended. A spacing of 4 m x 5 m is sufficient if trees are regularly pruned, which is done to maintain a balance between vegetative growth and fruiting. In case crosspollinating varieties are needed, these should be distributed at a rate of about every 3rd tree in every 3rd row.
The preferred tree shape is the ‘open center’ with good light distribution even for larger tree sizes. Most of the young plum trees obtained from the nursery will be in the shape of a straight whip without lateral branches. Thus, pruning at planting usually consists of cutting this whip back to 50–60 cm from the ground. This stimulates side branching below the point of cutting.
During the tree’s formative years is light: interior branches and water sprouts are controlled by pruning or bending, and growing laterals (scaffolds) are headed to induce branching. At maturity, vigorous upright shoots are removed since fruiting occurs mostly on spurs. To maintain fruit size, renewal of fruiting wood is necessary since spurs live for only about 5–8 years. To start a new crop cycle, trees should be defoliated. In most locations, the leaves do not detach easily, and
therefore chemicals like sodium chlorate (0.5–1%), copper or zinc sulphate and, recently, cyanamide (all at about 2% concentration) are used.
To avoid problems associated with heavy crops, it is necessary to thin the fruits. This should be done after natural drop and before seed hardens (about 6 weeks to 2 months after flowering), since thinning at a later stage will have little or no effect on fruit size. Remove all but the best plum per cluster or spur.
Depending on cultivar, the remaining fruitlets should be spaced 7–15 cm apart. Orchards are clean cultivated, preferably with a beneficial mulch cover around the trees. During the rainy season weeds are slashed, or a cover crop may be grown.
The need for nutrients varies from tree to tree depending on, among other factors, the cultivar, age, and soil condition. In this context, leaf analysis will help determine deficiencies and/or excesses of the nutrients needed for good growth and production.
Depending on cultivar, plums are ready to be picked 80–120 days after flowering. Fruits may be picked before they are completely ripe since they will finish ripening off the tree. Fruit maturity first begins at the top of trees and later at the bottom, which usually necessitates more than one picking stretched over a period of 7–10 days. Plums are highly perishable and must be picked and handled with care. If they are picked to be eaten fresh, leave the stem attached to the fruits and place them in lined buckets. For older cultivars, 4–10 tons/ha is a fair yield. For the newer ones, such as Harry Pickstone or Reubennel, 20–30 tons/ha may be expected.
Most plums are marketed as fresh fruit. To avoid an oversupply at certain months of the year they may be kept for up to several weeks—depending on cultivar—in controlled cold storage at 0.5º–1 ºC and 90% relative humidity.
The application of pesticides and/or fungicides is sometimes necessary to control seasonally appearing pests and diseases. Among these are aphids, beetles, bugs, fruit-piercing moth, nematodes, scale insects, caterpillars, and red spider mites.
Common diseases of plums are blossom wilt, brown rot, scab, die back, and rust. These are all fungi which thrive under wet conditions and thus they pose less of a threat if the growing season is not very wet. Outbreaks can be prevented and/or controlled by using appropriate inputs. If in doubt, and to avoid mistakes and unnecessary expenditure, farmers should contact a horticulture extension officer for assistance in diagnosing a pest or disease problem, and advice on suitable control measures.
Plum Cultivars Include;
- shiro (syn.: Ogden)
- Harry pickstone
- santa rosa
- Burbank (syn. Wright’s early)
- satsuma (syn. red Cardinal; Blood plum)
- Cherry plum