Walnut Cultivation: Climate Requirements, Soil Needs, and Variety Selection

Walnut cultivation can be carried out in suitable ecologies, similar to many fruit varieties, given Turkey’s significance in production, consumption, and trade and its rich fruit culture. Turkey ranks third in global walnut production after the United States and China. The yield per tree in the country varies between 33-37 kg, and orchards established with grafted walnut seedlings can achieve 50-60% higher yields compared to non-grafted ones. While wild walnut trees start bearing fruit at 7-10 years, grafted walnuts begin to bear fruit at the age of 4. From 6 kg of walnuts obtained from grafted trees, 3 kg of kernel can be extracted, whereas the same amount of walnuts from wild trees yields only 1.5-2 kg of kernel. In orchards established with recommended grafted seedlings, a 5-year-old walnut tree can produce 3-4 kg, an 8-year-old tree can yield 8-10 kg, and a 12-year-old tree can produce 15-20 kg of walnuts. To optimize walnut cultivation, understanding the climate requirements, soil needs, and appropriate variety selection is crucial.

WALNUT CULTIVATION

Orcharding in Turkey, a significant country in terms of production, consumption, and trade of orchard products, has a rich fruit culture, allowing the cultivation of walnuts in suitable ecologies, much like many other fruit varieties. Our country ranks third in global walnut production after the U.S. and China. The yield per tree in Turkey ranges from 33-37 kg, and when orchards are established with grafted walnut saplings, a 50-60% increase in yield can be achieved compared to non-grafted ones. Wild walnuts start bearing fruit at 7-10 years old, while grafted walnuts start at 4 years old. Grafted trees yield at least 3 kg of kernel from 6 kg of walnuts, whereas wild trees yield only 1.5-2 kg of kernel from the same amount. When orchards are established with recommended grafted saplings, a 5-year-old walnut tree can produce 3-4 kg, an 8-year-old 15-20 kg, a 12-year-old 30-40 kg, a 14-year-old 50-75 kg, and a 20-year-old 80-120 kg of fruit. Planting 10 saplings per acre is expected to yield 1-1.5 tons during the productive period.

Pollination Biology: Cultivation Characteristics

Climate Requirement One of the essential qualities of walnut trees is their adaptability to various climates. The chilling requirement of walnut trees ranges from 800-1800 hours and is observed from sea level up to 1700 m in altitude. Extreme summer heat can cause burns on green bark, leaf scorching, and fruit shriveling. Late spring frosts are crucial limiting factors for walnut cultivation. The climate requirements of varieties should be well determined, considering especially the risk of late spring frosts. Late spring frosts cause more damage to buds during swelling. Late fall frosts can also limit walnut cultivation. In varieties with very late leafing, the short vegetation period leads to unripe shoots being damaged by early fall frosts. Young walnut trees developing until late fall can suffer damage at -10°C, and non-woody shoots are damaged at temperatures between -9°C and -12°C during early fall frosts. Buds can withstand up to -3°C during the closed bud stage and -1°C during full bloom. Cold damage is more common during flowering. Generally, walnut trees exhibit resistance up to -25°C and +38°C.

Soil Requirement Walnut trees are not very selective about soil, but they thrive in well-drained, non-retaining, and moisture-retaining deep soils with base water levels not exceeding 2.5-3.0 meters. Walnut roots are taprooted, and soils that are moist, deep, and soft, particularly at the lower parts where they extend, are more suitable for cultivation. Excessive moisture and stagnant water impede the absorption of necessary oxygen for the roots, resulting in slow root development and halted tree growth. Leaves become light green with open vein spaces, and veins turn brown. It is observed that fertile soils or those improved through cultural measures like fertilization and irrigation lead to accelerated growth and increased fruit yield in walnut trees. Walnut trees are more adaptable to sunny summers and mild winters in sheltered valleys, but they can also grow in areas with harsh winters, taking into account the chilling requirement, waking, and winter rest periods when establishing an orchard.

Variety Selection: The most important characteristics to consider in selecting productive and quality walnut varieties include pollination biology, leafing times, vegetation periods, especially resistance to cold and drought, uniform development of the tree canopy, superior yield, and quality of fruits, thin-shelled fruits with easy kernel separation, fruit weight of at least 10 grams, and kernel yield of over 50%. Varieties with most of these features should be grafted onto wild walnut seedlings.

BİLECİK

Exhibits an upright, spreading, and vigorous crown development. Suitable for cultivation in all regions except coastal areas. Especially recommended for areas with late frosts. – Thin-shelled, easily separable from the shell, suitable for consumption as a dry walnut. – Kernel weight is 13 g, kernel yield is 53%, oil content is 68%, and protein content is 18%. Harvested in late September. – Pollinates with Şebin, Yalova-3, and Yavuz-1 (KR-2) varieties. – More resistant to internal kernel pests compared to Şebin walnut variety. – Tolerant to late spring frosts due to its late leafing compared to Yalova varieties.

ŞEBİN

Has a dense, spreading crown development. – Can be grown in all walnut-growing regions except coastal areas. Recommended, especially for areas with late frosts. – Oval fruit structure with thin, easily separable shell. High yield, harvested in late September. – Plump kernel, kernel weight of 12 g, kernel yield of 63%, oil content of 67%, and protein content of 17%. Blooms with male and female flowers simultaneously (homogeneous) and is self-fertile. – Pollinates with Bilecik and Yavuz-1 (KR-2) varieties. – Sensitive to walnut internal kernel pests, drought, and sunburn.

GÜLTEKİN-1 (KR-1)

Exhibits an upright, vigorous crown development. – Suitable for cultivation in all walnut-growing regions, especially recommended for areas with late frosts. – High-quality and attractive fruits, harvested in mid-September. – Kernel weight is 17 g, kernel yield is 40%, oil content is 72%, and protein content is 17%. – Pollinates with Yalova-3 and Yavuz-1 (KR-2) varieties. – Tolerant to walnut internal kernel pests, but sensitive to drought. – Tolerant to late spring frosts due to its late leafing compared to Yalova varieties.

YAVUZ-1 (KR-2)

Exhibits an upright, spreading, and vigorous crown development. – Suitable for cultivation in all walnut-growing regions, especially recommended for areas with late frosts. – High-quality fruits with thin shells, harvested in mid-September. – Kernel weight is 16 g, kernel yield is 48%, oil content is 70%, and protein content is 18%. – Pollinates with Bilecik and Gültekin-1 (KR-1) varieties. – Tolerant to internal kernel pests, drought, and sunburn. – Tolerant to late spring frosts due to its late leafing compared to Yalova varieties.

YALOVA-3:

Exhibits an upright, spreading, and vigorous crown development. – Suitable for cultivation in all walnut-growing regions, especially recommended for areas with late frosts. – High-quality fruits, harvested in mid-September. – Kernel weight is 15 g, kernel yield is 45%, oil content is 71%, and protein content is 18%. – Pollinates with Şebin, Bilecik, and Gültekin-1 (KR-1) varieties. – Tolerant to internal kernel pests and drought. – Tolerant to late spring frosts due to its late leafing compared to Yalova varieties.

Pollination

Walnut trees are dioecious, meaning male and female flowers are on separate trees. For optimal fruit production, it’s essential to plant compatible pollinators within a certain distance. For example, planting a certain number of pollinator trees for every main walnut tree can enhance yield. Some varieties, like Yalova-3 and Şebin, have both male and female flowers on the same tree (homogeneous), allowing them to self-pollinate, but planting other compatible varieties nearby can still improve yield.

Planting Density and Arrangement

The recommended planting density is 10-12 trees per acre. – For square planting: 8×8 m – For triangular planting: 10×10 m – For alley cropping: 12×6 m It’s essential to avoid shallow planting, as it may cause the tree to tip over during strong winds. In orchards with late spring frost risk, planting on the northern slope is recommended to delay flowering and reduce the risk of frost damage.

Fertilization and Irrigation

Fertilization should be applied based on soil and leaf analysis results, considering the nutrient requirements of the tree. – It’s crucial to provide sufficient irrigation, especially during dry periods, to support tree growth and fruit development. Irrigation is particularly essential during the first years after planting. – Drip irrigation is a suitable method for walnut orchards, providing efficient water use and avoiding wetting the foliage, which can lead to diseases. – Mulching around the base of the tree helps retain soil moisture and suppress weed growth.

Pruning

Pruning is essential for shaping the tree, promoting air circulation, and managing the size for easier harvest. It’s recommended to start training the tree in the early years and continue with maintenance pruning as needed. Removing dead, diseased, or crowded branches helps maintain tree health.

Harvest and Post-Harvest

Walnut harvest usually takes place in September-October. Harvesting is done when the husk starts to crack, and the kernels have reached their maximum size. – Proper drying and storage are crucial to maintaining walnut quality. Drying should be done in well-ventilated areas with controlled humidity to prevent mold formation. – Post-harvest processing includes hulling, cleaning, sorting, and packaging. – Walnut storage should be in a cool, dry place to prevent mold and maintain kernel quality. Properly processed and stored walnuts have a longer shelf life and better market value.

Pest and Disease Management

Regular monitoring of orchards is essential to detect and manage pests and diseases promptly. – Common walnut pests include aphids, mites, codling moths, and walnut husk flies. – Diseases like walnut blight, anthracnose, and crown gall can affect walnut trees. – Integrated pest management (IPM) practices, including cultural, biological, and chemical control methods, should be employed to maintain a healthy orchard.

Walnut cultivation in Turkey holds significant economic potential, and with proper orchard management practices, farmers can optimize yield and quality. Choosing suitable varieties, considering climate and soil requirements, and implementing effective pest and disease management are key factors in successful walnut cultivation.