On February 28, 2001, a black walnut cultivar - rootstock experiment was planted in the Spring River bottom at the Southwest Center. This large, multi-location study, conducted under the auspices of the University of Missouri Center for Agroforestry, is the first of its kind anywhere. The objective is to document and quantify the long-term effects of rootstock source on the performance of selected black walnut cultivars established in two geographically-distinct plantations in Missouri, and a third, smaller site in Arkansas.

While most Missourians are familiar with black walnuts and have at one time or another either eaten them or picked them up off the ground for cash, very few have seen or tasted the improved horticultural-quality nuts that are now being grown and studied. Improved black walnut trees are much more productive than ordinary wild trees, and produce nuts that are larger, easier to crack, better-tasting, lighter in color, and much higher in percent kernel. These improved black walnut cultivars must presently be propagated by grafting scionwood (dormant cuttings) of the improved germplasm onto seedling rootstocks, essentially "cloning" or identically copying the genetics of an original superior tree.

Rootstock science in many crops, such as apples, is very sophisticated in that a particular rootstock is known to perform better in a particular soil and climate, and to possess specific desirable traits such as disease resistance, strong and vigorous roots, a more compatible graft union, or growth-modifying characteristics such as dwarfing. Virtually nothing is known about black walnut rootstocks and rootstock / scion combinations. Because of the notable difference in rootstock performance in other tree crops, including Persian (English) walnuts, we assume that significant improvements can be made in black walnut cultivation by studying the performance of various rootstock sources under different soil and climatic conditions.

Black walnut seeds from four different sources (Sparrow, Thomas, Kwik-Krop, and unimproved nursery bed-run) were sown in the greenhouse in fall, 1998. In spring, 2000, five different black walnut cultivars (Sparrow, Thomas, Emma K, Surprise, and Kwik-Krop) were grafted onto each of the seedling groups. A year later, 120 of these trees were planted on 4.2 acres at the Southwest Center. The second replication, consisting of 180 trees, was installed at the University's Horticulture and Agroforestry Research Center (HARC) at New Franklin. An additional, smaller rootstock evaluation, containing 48 trees, was planted at the Arkansas Agricultural Research and Extension Center in Fayetteville. At this third site, hydrogeologists have drilled more than 60 wells to document and monitor the water table and fluctuations thereof - important information in understanding rootstock performance.

We may eventually subdivide both the HARC and SW Center plantings to allow for the addition of a cover crop study which would help us elucidate the importance of cover crops on long-term productivity in grafted black walnut orchards. This is indeed a very long-term experiment. We are collecting horticultural data each year, but will probably not be able to make many conclusions for several years. We anticipate that data will continue to be collected for 50 years or more as differences in the various rootstocks are manifested over the long life of the three orchards.