What are the key techniques for open-field cultivation of sweet cherries?
In recent years, the planting range and production scale of sweet cherries have continued to expand, forming the Bohai Bay production area, the old course of the Yellow River production area, the low-altitude production area of the Loess Plateau, the highland production area of the southwest, and the **desert-edge production area, spanning more than 20 provinces in China, among which Shandong, Dalian and Qinhuangdao in the Bohai Bay region are advantageous production areas.
Sweet cherries grown in the open field have a poor ability to withstand disastrous weather, so cultivation techniques are of great significance for improving returns.
I. Site selection
Sweet cherries are prone to damage from low temperatures, drought and waterlogging, strong winds, hail and other disasters. When establishing an orchard, choose a plot with high terrain, a low groundwater table and no tendency to accumulate water; windbreak belts can be established as needed. At the same time, choose a sandy loam or loam with deep soil, good aeration and a neutral to slightly acidic reaction (a pH of 6.5-7.5 is suitable), and it must not follow peaches in the same ground. Before planting, the soil must be deeply plowed to increase soil permeability and organic matter content and reduce soil bulk density. Loose soil with high organic matter content has a strong buffering capacity against rainwater and is relatively suitable for sweet cherry cultivation.
II. Selection of rootstocks and varieties
At present, the main sweet cherry rootstocks include Daqingye, the Gisela series, Mahaleb, Colt and others; they each have their own characteristics and differ in drought resistance, waterlogging resistance, disease resistance and other respects. Therefore, the corresponding rootstock should be selected according to actual conditions; for example, Gisela 5 has a strong dwarfing effect, while Mahaleb is resistant to crown gall.
According to the ripening period, sweet cherries can be divided into early-maturing varieties (fruit development period 30-40 days), mid-maturing varieties (fruit development period 40-50 days) and late-maturing varieties (fruit development period more than 50 days). Early-maturing varieties include Hongdeng, Huangmi, Italia Early Red and others, while Van and Napoleon are late-maturing varieties. Excellent varieties of the corresponding ripening period should be selected according to local climatic conditions. In recent years, China has successively introduced self-fertile varieties such as Stella, Lapins, Yanhong and Xinxing from abroad; they have good yield and quality and are worth promoting in production.
III. Training and pruning
Traditional sweet cherry tree forms usually adopt the open-center shape. In recent years, newly built orchards have generally adopted trunk-based cultivation, and the spindle shape is a practical tree form for early fruiting and high yield in dense planting. Planting density is determined according to site conditions, variety, training method and management level, with a spacing of 2-5 m x 3-6 m between trees and rows. For varieties with a compact tree form and easy high yield, such as Summit, Early Van, Sparkle and Van, a spacing of 2-2.5 m x 4 m can be used; for vigorous varieties, such as Hongdeng, Meizao and Samito, a spacing of 3 m x 4 m can be used.
After sweet cherry planting, most varieties begin to bear fruit after 3 years. For growing-season pruning, mainly bud notching, pinching, shoot twisting and thinning are used; in May-June, pinch and twist the current year's shoots, and in July-August, thin out overly dense branches. Winter pruning mainly involves appropriately adjusting the tree form and renewing fruiting branch groups.
IV. Pollination
From flower bud break, sweet cherries go through flowering, leaf expansion, shoot growth, fruit development, flower bud differentiation, leaf fall and dormancy; the fruit development period (from flowering to fruit ripening) is 30-60 days, and open-field sweet cherries begin to ripen in late May.
Most sweet cherry varieties are self-unfruitful or have a low fruit set rate, so suitable pollinizer trees must be arranged in production, and the planting ratio of the main variety to the pollinizer variety should preferably be 3:1 or 2:1. For the pollinizer variety, it is best to choose one with good fruit economic traits, a large amount of pollen and essentially the same flowering period as the main variety. It has been found in production that Van, Lapins, Stella and Rainier are relatively ideal pollinizer varieties.
V. Frost protection during the flowering period
Sweet cherry is a fruit tree that likes warmth and is not cold-tolerant, and open-field cultivation requires an annual average temperature of 7°C-14°C. Above 15°C, there are many flowers but few fruit set. Low temperatures and cloudy rain during the flowering period can cause poor pollination and fertilization, reducing yield. The critical temperature for frost damage at the flower bud stage of sweet cherry is 1°C-2°C, and below -3°C for more than 4 hours all flower buds will be damaged by frost. Orchard smoking is an effective measure to prevent spring frost, and the key to this method is to make the smoke cover the area above the orchard.
VI. Fertilizer and water management
After sweet cherries enter the high-yield period, the level of fertilizer and water management not only affects the current year's yield of sweet cherries but also affects production in the following year. According to tree age, tree vigor, soil fertility and the fertilizer requirements of the variety, the type, amount and timing of fertilizer should be well controlled. Fertilization should pay attention to several key periods. First, apply base fertilizer in autumn, in September-November, mainly organic fertilizer, at 2000-3000 kg per mu, supplemented with an appropriate amount of nitrogen, phosphorus and potassium fertilizer or micronutrient fertilizer. Second, apply topdressing before flowering, mainly nitrogen fertilizer, with 1.5 kg of urea per bearing tree to promote bud break and flowering. Generally, no topdressing should be carried out during the hard-core development period, so as not to affect fruit quality. Third, apply topdressing after fruit harvest. About 10 days after harvest, flower buds differentiate in large numbers, so a quick-acting compound fertilizer should be applied, mainly phosphorus and potassium fertilizer, at 0.4-0.6 kg per tree, to increase nutrient accumulation and promote flower bud differentiation.
Sweet cherry watering can be carried out at the following key periods. Pre-flowering water: carried out from bud break to the flowering period, to meet the water demand for leaf expansion and flowering and help flower bud formation. Fruit-promoting water: carried out from after petal fall to before fruit ripening. At this time the tree grows vigorously and is most sensitive to water supply, and watering can be done 2-3 times. Post-harvest water: carried out after fruit picking. At this time flower bud differentiation is concentrated, and the amount of watering should be just enough to wet the soil surface. Freezing water: carried out after autumn base fertilizer application, and it should be watered sufficiently and thoroughly. From the fruit turning white stage to the beginning of coloring stage, flood irrigation is strictly prohibited to prevent fruit cracking.