In a few words, the most important enemy of English and black walnut trees is night frost during late spring or early fall. Areas with frequent night frosts during the spring should definitely be avoided. In general, walnut tree cannot tolerate very low and very high temperatures as well as very strong winds. High summer temperatures cause damage to the fruit and may result in the shrinking of nuts. Some small damage can occur roughly at 100°F (38°C), but greater damage occurs when temperatures greater than 108°F (42°C) on average persist. However, among the different varieties, there is a significant difference in terms of their tolerance to high temperatures. Most walnut varieties are sensitive to extremely low winter temperatures. When in dormancy, they can tolerate on average temperatures up to 14°F (-10°C) without suffering serious damage.