Green Horizons

Andy Thomas, research assistant professor, MU Southwest Center, and collaborator with The Center for Agroforestry, is working to determine the feasibility, practicality and profitability of converting a mature ungrafted seedling-derived black walnut orchard to a grafted, productive orchard of improved nut-producing cultivars.

Thomas said researchers believe this technique, often called "top-working," may "rescue" non-productive seedling-based orchards and convert them into productive orchards. Wild black walnuts, on average, have 7 to 10 percent kernel; improved cultivars typically yield about 25 to 30 percent and consistently bear much heavier crops loads.

Grafting onto large, established rootstocks is probably not as costly as starting from scratch with new trees, but still requires a dedicated investment of both money and time, Thomas said. Once grafted, the trees will require diligent care, maintenance, pruning and training until healed (about five years post-grafting). Once healed and well-established, the yearly costs and maintenance should decline as the trees enter nut production.

This technique of black walnut orchard conversion is still experimental and is not yet recommended to producers, Thomas said.

"Probably the chief drawback to top-working is the longterm exposure and slow healing of the graft union, and potential weakness of the tree even after it is healed," he said.

Part of the study includes painting a fungicide onto the exposed rootstock stump to prevent fungus from beginning to decompose the stump until it is healed and sealed over. After just one season, this technique has shown "slightly positive" results.

The study was initiated in May 2009 using a 20-year-old orchard on an excellent site at Hammons’ Sho-Neff plantation in southwest Missouri. Fifty-six trees were grafted at various heights, with an equal number of control (ungrafted) trees maintained. Because the grafts are placed onto large, mature root systems, researchers expected a very high percentage of grafting success, followed by very rapid scion growth. Indeed, initial grafting success in 2009 was 97 percent, followed by an average scion growth of 53 inches in just the first year.

Stay tuned for more data and results, as this long-term project will be followed for many years.