Grafting mature unproductive black walnut trees to improved cultivars demonstrates horticultural and economic feasibility in Missouri, USA

Abstract

Black walnut (Juglans nigra) is a specialty nut crop produced in the central USA. Most nuts are harvested from unimproved wild trees and are of poor quality; however, a growing number of nuts are coming from grafted orchards with superior quality. Our objective was to study the feasibility of converting mature unimproved black walnut trees to improved cultivars by grafting or top-working. A 20-year-old black walnut planting in southwest Missouri was used. 108 trees were divided into blocks and randomly designated as ungrafted, or grafted to one of three cultivars ('Sparrow', 'Kwik-Krop', 'Emma K') in May 2009. Fungicide treatments applied to rootstock trunks were also studied. Initial grafting success was 94%. Seven grafted trees had broken or died within five years leaving an overall success rate of 81%. By 2014, scion height averaged 5.5 m with mean girth 13.2 cm. As expected, increase in trunk diameter at breast height (137 cm) was greater in ungrafted trees (27.1%) compared with grafted trees (21.8%). Also as expected, nut production was reduced among grafted trees during the healing process; ungrafted trees yielded a cumulative average of 298 nuts tree-1 over five years, whereas grafted trees yielded 28 nuts. At the common market price of US $ 0.22 kg-1 for unimproved black walnuts, that represents an economic loss of only $ 88 over five years from 54 grafted trees. While top-working established black walnut trees is labor intensive, and may result in loss of some trees, the process is horticulturally feasible and the economic trade-off appears justifiable.