Walnut is a deep-rooted tree species. The taproot is deep, the lateral root is wide, and the fibrous root is slender and dense. The concentrated layer of root system is 20~60cm underground, accounting for more than 80% of the total root system. Walnut 1~2-year-old seedlings showed that the growth rate of taproot was higher than that of aboveground parts. After three years, the lateral roots grew faster and the number increased. With the increase of tree age, the horizontal roots expand faster, the nutrient accumulation increases, and the growth rate of aboveground branches exceeds that of roots (see table).

Growth of Walnut Branches and Roots (Hebei Agricultural University)

The growth of mature walnut root system is closely related to soil type, soil thickness and groundwater level. The soil conditions and environment are good, and the root system is deeply and widely distributed. When the soil layer is thin and dry or the groundwater level is high, the depth and extension of the root system are small. Therefore, walnut should be cultivated in a place with deep soil, excellent texture and sufficient water, which is conducive to root development, thus accelerating the growth of branches and shoots and achieving the goal of early, high quality and high yield.

(2) Bud

According to the properties and characteristics of walnut buds, they can be divided into four types: mixed buds (mixed flower buds), leaf buds (vegetative buds), male flower buds and dormant buds (latent buds).

1. mixed bud

A bud containing branches, leaves and female flower primordia. After germination, the mixed buds can grow fruiting branches, leaves and top female flowers. Generally, mixed buds are single buds and double buds. Late-fruiting walnuts usually form mixed buds of 1~2 buds at the top and below the fruiting branches, which can be solitary or overlap with leaf buds and male flower buds in the axils of compound leaves. Early-bearing walnuts can bear mixed buds except the terminal buds, and can bear mixed buds between the axils of the following 3-5 leaves (at most 20 leaves). The mixed buds are semi-circular, full and hypertrophy, generally 5.6 mm long and 5.5 mm thick, covered with 5~7 pairs of scales.

2. Young leaves

It is planted between the top of the vegetative branch and the axil of the lower leaf. Most lateral leaf buds are solitary or overlap with male flower buds. Judging from the ratio of mixed buds to leaf buds, the number of leaf buds of late-bearing walnuts is more, while the number of leaf buds of early-bearing walnuts is less. The leaf buds on the same branch gradually increase from bottom to top, and the scion is often dominated by middle and upper leaf buds. The vegetative buds at the top are usually wide triangles, and the lateral leaf buds are mostly semicircular and small. After the leaf buds germinate, only the branches and leaves grow, which is the basis of tree growth.

3. Male flower buds

Bare buds are actually shortened male inflorescences. Male flower buds are mainly planted in the middle or lower part of annual branches, and single or double male flower buds overlap, or male flower buds overlap with mixed buds. The male flower buds are short and conical, and the scales are too small to cover the buds. Male flower buds elongate to form male inflorescences. The number of male flower buds and the number of male flowers per male inflorescence are related to varieties and tree potential.

4. Dormant buds

According to its nature, it should belong to a kind of leaf bud, which generally grows at the lower part and base of branches and does not germinate under normal circumstances. With the stop of branch growth and the increase of branch age, the bud body falls off and the bud primordium is embedded in the skin. Its life span can reach decades or more than a hundred years. Latent buds are flat and thin, which can germinate branches when stimulated by the outside world, which is conducive to the renewal and rejuvenation of branches.

(3) branches

Walnut branches are generally divided into vegetative branches, fruiting branches and male flower branches, which are the basis of crown formation, flowering and fruiting.

1. Nutrition Department

Also known as growth branches, according to the growth potential of branches, they can be divided into three types: development branches, overgrowth branches and secondary branches.

2. Fruiting branches

Branches with mixed buds are called fruiting branches, and branches that grow from mixed buds and have female flowers are called fruiting branches. Strong fruiting branches can grow short branches at the top, and most of them can form mixed buds in the same year. Early-bearing walnuts can also germinate in the same year, bloom in the same year, and bear fruit in the same year, which is called secondary flower and secondary branch. Mixed buds, leaf buds (vegetative buds), dormant buds and male flower buds are borne on the fruiting branches, but sometimes there are no leaf buds or male flower buds.

3. Male flower branches

Refers to a thin and short branch with male flower buds at all nodes except the top leaf buds. The terminal buds of male flower branches are not easy to differentiate into mixed buds. The number of male flower branches is a sign of weak tree vigor and poor variety, which consumes a lot of nutrients and is mostly thinned when pruning.

(4) Leaves

Walnut leaves are odd pinnate compound leaves, and the number of compound leaves is related to tree age and branch type. A few days after the mixed bud or vegetative bud cracks, the compound leaf primordium with gray fluff can be seen. In about 5 days, with the appearance and elongation of new branches, the compound leaves gradually spread out. After10 ~15 days, most of the compound leaves spread out and grew rapidly from bottom to top. After about 40 days, with the formation and capping of new branches, compound leaves grow and take shape. At the end of 10, the leaves turn yellow and fall off, and the leaves fall off earlier in areas with lower temperature.