Black Walnut: Guidance for Seed Transfer Within the Eastern United States

Abstract

Black walnut (Juglans nigra L.) is a shade-intolerant hardwood tree common in riparian forests across the Central United States. Commercial values for top-quality black walnut logs are the highest for any tree in North America. Production of juglone from walnuts may have phytotoxic effects on neighboring plant communities. Genetic diversity of this species is high due to seed dispersal and pollen flow characteristics, while population structure is low. Common garden studies revealed relatively weak clines for growth traits but strong latitudinal gradients for cold tolerance. Seed transfer distances of 200 mi (322 km) from south to north, or roughly 2 degrees latitude northward, is considered a safe recommendation to maintain growth. At the northern edge of the species’ range, such as Minnesota, local sources are best. Leaf anthracnose is an important pest, and thousand cankers disease can be a concern, especially in drought-prone areas. Black walnut is likely to expand northward with climate change, but its migration may require human assistance, and such expansion may be limited by soil conditions, site availability, deer browse, or drought.

Citation

Pike, Carolyn C.; Woeste, Keith. 2022. Black Walnut: Guidance for Seed Transfer Within the Eastern United States. Tree Planters' Notes. 65(2): 29-36.