What are the practical measures to increase the fruit set rate of walnut?

  Because of the site conditions where the walnut is located and its own biological characteristics, as well as low tree nutrient levels, improperly arranged pollinizer cultivars, or lack of pollinizer trees, extensive management, poor flower bud differentiation, and severe pests and diseases, the fruit set rate can be reduced.

  1. Select cultivars with strong adaptability

  There are many excellent walnut cultivars, but their adaptability to environmental temperature differs to some extent. Therefore, at the beginning of establishing a walnut orchard, cultivar planning should be carried out, and cultivars with good adaptability to the local climatic conditions should be selected. Excellent cultivars with poor adaptability should be planted in areas with milder climatic conditions, and large trees of cultivars that have already been put into production but have poor adaptability and quality should be top-grafted and improved, eliminating inferior cultivars and developing good ones.

  2. Rationally arrange pollinizer trees and supplement with artificial pollination

  Dichogamy determines the importance of arranging pollinizer trees in walnut cultivation. Newly established walnut orchards are basically all provided with pollinizer trees of the protogynous and protandrous types paired with each other, which can ensure normal pollination. However, in generally extensively managed walnut orchards, or orchards improved by top grafting, pollinizer cultivars are often neglected, so specific pollinizer trees should be top-grafted in the orchard to improve pollination conditions.

  In order to increase the fruit set rate of walnut and raise yield, artificial pollination must be carried out. According to research, artificial pollination during the full bloom stage of female flowers can increase the fruit set rate by 17.3% to 19.1%, and carrying out artificial pollination twice can increase the fruit set rate by 26%.

  3. Spraying water and supplementing fertilizer on the tree

  Spraying water on the tree during the flowering period can increase air humidity and reduce the rate at which pollen and stigmas lose water and become inactive due to dryness, thereby increasing the fruit set rate. Spraying 0.1% to 0.2% boron fertilizer on the tree during the flowering and fruit-setting period can promote pollen tube elongation. Spraying 0.3% to 0.5% urea or monopotassium phosphate can also promote fruit set and reduce the occurrence of flower and fruit drop.

  Research has proven that technical measures such as applying growth regulators and rare earth solutions can all increase the fruit set rate to a certain extent. According to reports, for 40-year-old large fruiting walnut trees on mountainous land, spraying 5 to 7 mg/L IAA (indoleacetic acid) twice can increase the fruit set rate by 22.7% compared with control trees; for 8-year-old late-fruiting grafted walnut trees, spraying 1000 to 2000 mg/L paclobutrazol can increase the average yield per tree by 10% to 64.9% compared with control trees, and the effective period can last more than two years; spraying 300 to 800 mg/L NL-1 rare earth at the early stage of female flowers can increase yield by 48.6% to 66.5% compared with the control.

  4. Strengthen comprehensive management of the tree

  On the basis of rational fertilization and deep plowing of the soil, strengthen pest and disease control, protect the leaves, and enhance tree vigor. In addition, after the stigmas wither, spray 0.3% to 0.5% urea or monopotassium phosphate once every about 15 days, for 2 to 3 consecutive sprays, which can promote rapid fruit expansion and effectively improve fruit quality.

  5. Other techniques for increasing the fruit set rate

  In mid-to-late May, girdling the base of auxiliary branches or the trunk of vigorous trees, with a width not exceeding 0.6 cm, can moderate tree vigor, increase the fruit set rate, and promote the sprouting of new shoots below the girdling cut. These are consistent with the saying in the Shangluo mountainous area of Shaanxi, "cut once with a sickle, and it bears abundantly; cut once with an axe, and it breaks the thighs."

  The reasons for severe fruit drop and low fruit set rate in walnut are wind damage and excessive vegetative growth. Their harm is not direct mechanical damage, but because walnut has a large leaf area, severe transpiration water loss, and excessive nutrient consumption, causing a large amount of flower and fruit drop. Therefore, removing walnut leaves during the flowering period, leaving only the top 3 leaflets on each compound leaf, can increase the fruit set rate and raise yield.