What ecological conditions does walnut growth and development require?

  Walnut can grow in many regions of our country, reflecting that plants of the genus Juglans have a strong ability to adapt to natural conditions. However, walnut growth has relatively strict requirements for suitable conditions, and thus several major walnut production areas have formed. When its suitable conditions are exceeded, it may survive, but often grows poorly, has low yield or no yield at all, and produces nuts of poor quality, thus losing its cultivation value.

Pictured: walnut

  I. Light

  Walnut likes light. After entering the fruiting stage, it needs even more sufficient light conditions, and the annual sunshine duration must be more than 2000 hours to ensure the normal growth and development of walnut; if it is less than 1 000 hours, both the shell and the kernel develop poorly. Especially during the annual growth period, sunshine duration and intensity have an important effect on walnut growth, flower bud differentiation, and flowering and fruiting.

  In the early-fruiting-type walnut production areas of Xinjiang, Aksu and Kuqa, the annual sunshine amounts are both more than 2700 hours, and the sunshine duration during the growth period (April to September) is more than 1500 hours. The walnuts here have high yield and good quality, and sufficient light is one of the important factors. Likewise, plants at the edges of walnut orchards all show good growth and bear more fruit; on the same plant, the outer branches also bear more fruit than the inner canopy branches, which is also related to light conditions. For this reason, in cultivation, the issue of light must be considered in aspects such as site selection, planting density, cultivation methods, and training and pruning.

  II. Temperature

  Walnut is a tree species that likes warmth, and its natural producing areas are mostly in relatively warm zones, but suitable temperatures differ among different cultivars.

  1. Common walnut The suitable temperature range and frost-free period for growth are: annual average temperature 9-16°C, extreme minimum temperature -25°C, extreme maximum temperature below 38°C, and frost-free period more than 150 days. During the dormant period, young walnut trees can suffer freezing injury at -20°C. Although adult trees can withstand a low temperature of -30°C, when it is lower than -28 to -26°C, branches, male flower buds, and leaf buds are all prone to freezing injury. In Yining and Urumqi in Xinjiang, when the extreme minimum temperature reaches -37 to -4°C, walnut cannot bear fruit and mostly grows as small trees or in clumps. After leaf expansion, if the temperature drops to -4 to -2°C, new shoots can be frozen and damaged. During the flowering period and young fruit period, when the temperature drops to -2 to -1°C, they are frozen and yield is reduced. In areas with excessively high temperatures, such as above 38-40°C, the fruit is prone to sunscald injury and the kernel cannot develop.

  2. Iron walnut It is only adapted to subtropical climate conditions, tolerates damp heat, and does not tolerate dry cold. In suitable planting areas, the annual average temperature is 12.7-16.9°C, the average temperature in the coldest month is 4-10°C, and the extreme minimum temperature is -5.8°C; if too low, it is difficult to overwinter. For example, when introduced to the Beijing area, if seedlings are sown and grown to 3-4 years old, they can be frozen to death including the roots in winter if not protected against cold.

  Research on Yunnan Yangbi walnut (a cultivated species of iron walnut) shows that the average temperature in May has a significant negative correlation with the yield of bubble walnut, that is, in years when the average temperature in May is high, walnut yield decreases; the average temperature in August also has a negative correlation with walnut yield, because under high temperature conditions walnut fruits are easily scorched into diseased fruits and drop.