How to prevent freeze damage in soft-seeded pomegranates? A detailed explanation of the causes and anti-freeze measures in the western Henan region
Causes of freeze damage to soft-seeded pomegranates in the western Henan region and anti-freeze measures
The pomegranate is one of people's favorite fruits, listed as a treasure among fruits for its rich nutrition and auspicious symbolism. In recent years, soft-seeded pomegranates have been regarded as the treasure of treasures, with market prices remaining high, and fruit growers' enthusiasm for planting has soared. Soft-seeded pomegranates have developed rapidly in Lingbao City; in Hangu Pass Town of Lingbao City alone, the planted area of soft-seeded pomegranates reaches 13,340 hm2. However, because soft-seeded pomegranates are an introduced variety, young trees are not adapted to local winter low temperatures, and each year the freeze damage rate of one- to three-year-old young trees is as high as 55%-83%. In some orchards, even the entire orchard suffers freeze damage, causing huge losses to fruit growers. To avoid causing more losses, the author conducted observational research on freeze damage to soft-seeded pomegranates and proposed a complete set of freeze damage prevention measures, hoping to provide reference for soft-seeded pomegranate production in Lingbao City.
1 Freeze damage and its manifestations
Pomegranate freeze damage refers to the harm to pomegranate trees caused by low temperatures below freezing point. Sometimes freeze damage occurs together with frost formation, so freeze damage is often also called frost damage. According to observations, frost damage is an important limiting factor for the development of soft-seeded pomegranates in the western Henan region. Hard-seeded pomegranate varieties in the western Henan region can often tolerate temperatures above -17 °C, but when the winter temperature drops below -8 °C, the branches and trunks of young soft-seeded pomegranate trees suffer freeze damage. The affected parts are mainly the phloem of the main trunk, the root crown, and the branch tips. After being damaged, the phloem and xylem turn brown, and the cortex becomes sunken. The phloem of the main trunk is most prone to freeze injury. After warming, the leaf buds at the branch ends can germinate normally, but because the water and nutrient circulation system is destroyed, the leaves soon wilt and die. When freeze damage is more severe, the branches show dehydration and shrinkage, the xylem turns brown, the leaf bud bases are frozen to death, and they cannot sprout after spring begins, so new buds can only sprout again from the root crown. More seriously, a sudden temperature drop with a long duration of low temperatures can cause the root crown of soft-seeded pomegranates to turn brown, the cortex and xylem layers to separate, the branch tips to dry up, and the entire plant to die. A late spring cold snap in spring can directly damage young buds and young leaves.
2 Causes of freeze damage
2.1 Variety characteristics
The place of origin where soft-seeded pomegranates grow has a mild climate, with a winter minimum temperature of 6 °C. The variety itself lacks cold-tolerance genes, which is the internal reason why soft-seeded pomegranates are prone to freezing. According to observations, soft-seeded pomegranates can suffer freeze damage at -8 °C in the western Henan region.
2.2 The structure of soft-seeded pomegranates themselves
Young soft-seeded pomegranate trees have relatively thin cortex, larger pith cavities, loose xylem layers, and poor cold tolerance.
2.3 Strong tree vigor and shallow dormancy
Soft-seeded pomegranate branches grow vigorously, and secondary and tertiary branches often sprout on vegetative branches. In particular, autumn rainfall in the western Henan region is concentrated, and autumn shoots grow in large amounts and stop growing late, resulting in high water content in the tree body and poor freezing tolerance.
2.4 Special climate changes
The western Henan region is located in the hilly area of western Henan Province, bordering the Yellow River to the north, and is the northern edge of the suitable pomegranate planting area. In winter, temperatures often drop sharply with strong winds, and low temperatures last for a long time. In early spring, temperatures fluctuate up and down, and there is often a late spring cold snap. Special climate changes are the main cause of freeze damage.
3 Technical measures to prevent freeze damage
3.1 Strengthen cultivation management and enhance the tree body's freezing resistance
3.1.1 Orchard site selection. Soft-seeded pomegranate orchards should preferably be selected in the middle and upper parts of wind-sheltered, sun-facing hillsides in mountain recesses. Flat land and the lower parts of valleys, especially trough-shaped valley bottoms and basins where cold air easily accumulates, are all danger zones for freeze damage. Soft-seeded pomegranates are drought-tolerant and light-loving, so fields with loose soil, good aeration, convenient drainage and irrigation, and a near-neutral pH value should be selected as orchard sites.
3.1.2 Seedling selection. According to observations, selecting grafted seedlings using local common pomegranate or sour pomegranate seedlings as rootstock can reduce the freeze damage rate by 24%-36%. Seedlings grown from seed are more freeze-resistant than root-sucker seedlings, cuttings have the worst freeze resistance, and larger seedlings have better cold tolerance than smaller seedlings.
3.1.3 Selection of planting season. Soft-seeded pomegranate seedlings are best planted from March to before Qingming Festival. For autumn planting, measures such as soil burial or wrapping must be taken.
3.1.4 Reasonable water control. In rainy years, pay attention to drainage to prevent waterlogging damage that affects root development. Control watering in the late growth stage to prevent excessive autumn shoot growth and make branches develop plump and full. Water thoroughly before freezing, because water has a high specific heat and absorbs or releases a large amount of heat when warming or cooling, which can stabilize ground temperature and prevent extreme low temperatures from freezing and injuring the tree body.
3.1.5 Scientific fertilization. Follow the principle of "promote early, control late." In the late growth stage, apply less nitrogen fertilizer and more phosphorus and potassium fertilizer, and spray potassium dihydrogen phosphate foliar fertilizer in combination with pesticide application. When autumn rain is plentiful and autumn shoots grow vigorously, spray 1,000 mg/kg paclobutrazol once. In addition, pay attention to increasing the application of organic fertilizer and applying base fertilizer early. Generally, after pomegranate harvest, apply base fertilizer in autumn in a timely manner. The base fertilizer should be applied deeply, and irrigation should be carried out promptly after application to promote root growth, form a root growth center, promote the return of water from the upper part of the tree to the roots, and increase the cell concentration of the tree body.
3.1.6 Disease control. Dry rot mainly damages the bark of the trunk, seriously blocking the upward transport and downward return of nutrients in the tree body. Early defoliation disease causes leaves to fall early, resulting in poor nutrient accumulation, which leads to tree decline and lack of resistance. Therefore, timely control of dry rot and early defoliation disease and protection of branches and leaves are important measures to prevent freeze damage.
3.1.7 Early winter pruning. Winter pruning can be carried out from late November to early December to prevent branches from drying out and help wounds heal.
3.2 Strengthen tree body protection and improve the micro-environment
3.2.1 Whitewashing the trunk. Before winter begins, whitewash the trunk in combination with orchard cleaning, which can both prevent diseases and insect pests on the trunk and prevent bark sunscald in early spring.
3.2.2 Smoking to raise temperature. After trees sprout in early spring, use crushed straw, discarded mushroom bags, or wood chips in the orchard to raise the temperature and prevent a late spring cold snap. Set the density at 45-60 piles/hm2, and thick smoke is best.
3.2.3 Burying soil or mounding soil at the root crown. For young, soft-branched seedlings, they can be pressed down and covered with soil. The soil covering thickness should reach 30 cm to prevent young stems from freezing in the frozen soil layer, and the soil should be removed promptly after spring temperatures stabilize. For older trees with thicker stems, before winter closure, soil can be mounded at the root crown in combination with intertillage, with a thickness of 20-40 cm, and removed before spring sprouting.
3.2.4 Wrapping protection. For trees with smaller crowns, a protection method of wrapping a small amount of straw inside and covering with plastic film outside can be used. The plastic film should not be wrapped too tightly, and ventilation should be ensured. For trees with larger crowns, cover the whole plant with corn straw in winter and tie it with thin rope outside to prevent it from being scattered by wind, while also paying attention to winter fire prevention. In addition, shelterbelts can be established to reduce wind damage.