Nursery Site Selection, Seed Collection, and Seed Storage Methods

  1. Selection of the nursery site. Generally, when selecting a nursery site, one needs to consider plots with deep soil layers, high soil organic matter content, a low groundwater table, low salinity and alkalinity, and shelter from the wind and facing the sun; sandy loam or light clay soil is preferable. The nursery site must have irrigation conditions, able to irrigate in dry periods and drain in waterlogged ones. At the same time, note that the previous crop must not be walnut trees or a walnut nursery; other crops need to be planted for 5~8 years before the land can be used again for walnut seedling cultivation.

  In autumn, deep-plow the nursery site 20~25 cm; before deep plowing, apply base fertilizer mainly of organic fertilizer, with 4000~5000 kg of farmyard manure per mu, mixed with 50 kg of superphosphate. After deep plowing, there is no need to rake it level, so as to use the winter sunlight, rain and snow for "freezing and weathering" to promote soil maturation. Before spring sowing, shallow-plow once more, rake level, make beds, and prepare for sowing.

  2. Seed collection. First select the mother trees for seed collection; they should be vigorous mature trees with no pests or diseases, full kernels, uniform fruit size, and stable yields year after year. Seeds from old and declining trees are not suitable as rootstock seed.

  Walnuts used as seed should be harvested appropriately late so that the kernels are well filled. Generally, after White Dew, 5~7 days beyond the maturity period is best. Walnuts harvested at this time have full, well-developed kernels, and the green husk is easy to remove. Walnuts harvested too early have incompletely developed embryos, non-filled kernels, a low germination rate, and weak seedling growth. There are two seed collection methods: the picking-up method and the knocking-down method. The former is to wait for the fruits to drop naturally at maturity, picking them up once every 3~5 days; the fruits obtained by this method are of the best quality. The latter is to knock them down manually when 1/3 of the walnut fruits on the tree have cracked green husks, harvesting them all at once. After harvesting, remove the green husks promptly; for fruits whose green husks are hard to remove, the pile-fermentation method can be used, piling them in a cool place for 3~5 days before removing the green husks. Be careful to prevent the pile from heating up too much and damaging the seeds, and avoid covering with plastic sheeting or piling too thickly; generally a pile height of about 50 cm is suitable.

  Walnuts for sowing must not be sun-dried on concrete surfaces, stone slabs, or iron sheets, lest high-temperature damage reduce seed viability. The best method is shade-drying, airing or wind-drying on a ventilated, shaded earthen surface. Take special care that walnuts used as seed must not be bleached; bleached walnuts have a greatly reduced germination rate.

  Previously, to save seedling cultivation costs, it was often advocated to choose smaller walnuts as seed. Production practice has proven that sowing large seeds for seedling cultivation gives better results. Although the amount of seed used per unit area increases, the emergence rate is high, the seedlings grow vigorously, and the proportion reaching grafting thickness in the second year is greatly increased, thereby reducing production costs. Small walnuts will produce seedlings with weaker growth potential in the future, requiring increased fertilizer and water management.

  In most cases, walnuts used as seed are purchased on the market. When selecting and buying, crack open more of them to observe whether the kernels are full; the cut surface of the kernel should be milky white. Be very careful not to use old walnuts from the previous year for sowing; old walnuts that have been through the summer easily become oily and deteriorate, and their germination rate is greatly reduced. Generally, walnuts with exposed kernels are also not used as seed, because the kernels of exposed-kernel walnuts are prone to insect damage or mold during storage, reducing the germination rate. Fresh walnut kernels are milky white and bright in color, while old walnut kernels are yellowed and oily in appearance.

  3. Seed storage. For walnuts sown directly after harvest in autumn (autumn sowing), the seeds need no treatment. Seeds for spring sowing require storage and pre-sowing treatment. Seed storage methods are generally divided into sand storage and dry storage.

  (1) Dry storage. Walnuts have no after-ripening stage; the seeds can germinate normally without after-ripening. Generally, put the air-dried walnuts into gunny sacks and place them in a ventilated, shaded, dry room or basement; during storage, check regularly to prevent rodent damage. Where conditions permit, they can be placed in a 0°C controlled-atmosphere warehouse or cold storage, and a small amount of seed can also be kept in the refrigerator's cold-storage compartment.

  (2) Sand storage. Also called wet storage or stratification. Choose a place with high terrain, no standing water, shaded and sheltered from the wind, and before the ground freezes dig a trench 1 meter deep and 1 meter wide, with the length depending on the total amount of walnuts to be stored. Choose clean river sand, treat it with 0.1% potassium permanganate or a 600~800 times dilution of 50% carbendazim wettable powder, then add water and mix until the moisture content reaches about 60%; the rule of thumb is "it forms a ball in the hand and crumbles at a touch." If the soil in the trench is rather dry, the sand's moisture content can be appropriately higher, or water can be sprinkled in the trench to adjust it. When storing walnuts, first spread 5 cm of wet sand on the bottom of the trench, then spread a layer of walnuts, sprinkle a layer of wet sand, and continue until 10 cm from the ground surface, then spread about 10 cm of wet sand on top. Then mound soil on top into a roof-ridge shape to facilitate drainage; the thickness of the mounded soil should be greater than the local frozen-soil depth, generally at least 20 cm. When the trench is long, insert a bundle of corn stalks 20 cm in diameter as a ventilation hole every 1~2 meters along the trench, and then build drainage ditches around it to prevent standing water. For a small amount of walnuts in sand storage, they can also be placed in a suitable flower pot, with the sand volume being 5~7 times that of the walnuts, and buried in a shaded place. After the weather warms in early spring, check the seeds in the pit to prevent mold and rot; if signs of mold are found, dig them out early, clean and wash them, then store the seeds in a cool home and sow as soon as possible.