The Chinese walnut is a geographic form of the so-called English walnut. It occurs over a large area of central and northern China, and it is believed that trees from the northernmost range of this species in China are somewhat hardier than the average English walnut from western Europe. The number of trees of this species under observation is very limited, but those that have been seen appear to be promising. The largest and best tree observed grows on the property of Mr. Geo. Corsan at Islington, Ontario. This tree was subjected to -26 degrees F. last winter and was somewhat injured. The growth this spring was delayed longer than normally and some killing back was noted. Eventually the tree started to grow and made a normal amount of growth. Scions from this tree were grafted on two black walnut trees at the Kellogg Farm in 1933 and a vigorous growth was made in that season. These grafts were carefully examined in the spring of 1934 and were found uninjured. Subsequently a very large graft on one medium sized black walnut tree died, but this was due to injury at the point of union rather than to the graft above. The remaining scions made a good growth this season. Seedling trees of another strain of Chinese walnut showed some variation in their hardiness. Some came through in good condition and made a vigorous growth but others were more or less injured. The limited number of trees under observation scarcely justifies definite conclusion, but it would seem as though this form of Juglans regia is worthy of a wider trial in southern Michigan.
Types of Winter Injury
The following forms of winter injury which have been referred to in the preceding notes are given special attention hereunder.
(1) Killing back of branches.
This type was found on every tree except the hardy varieties of Polish and Russian origin. In some cases the large branches were killed outright, but usually the injury was confined to small branches, and the degree of injury varied from slight to very severe killing. Branches so injured were attacked by fungus diseases and some were beginning to decay and fall off when examined in October. Killing back of the branches was also noted on one excellent heartnut at Scotland, Ontario. This tree was subjected to -30 degrees F. but was less severely injured than many of the English walnuts noted above, and when examined in September showed a vigorous new growth throughout most of the top. There were also several vigorous seedlings from this tree growing near by which were only slightly injured in the bark or which were uninjured. It was interesting to observe that the seedlings of the old heartnut tree that were apparently of hybrid origin were not injured in the least and bore good crops of nuts this year, but the seedlings that were pure heartnuts were injured slightly. This point suggests the desirability of crossing the finest heartnuts with the best butternuts to get a combination of the hardiness of the butternut with the good qualities of the heartnut.
Bark Killing
Bark injury is often found on fruit trees following a severe winter and is occasionally found on nut trees. It may be due to bark splitting or to desiccation or both. In severe cases of bark splitting the bark splits vertically and laterally from the ground up for several feet, but in milder cases the bark is only split away for a short distance. Where the bark is loosened for some distance around the tree or vertically it dies shortly thereafter, but where only a small amount of splitting occurs, the tree may recover if given attention. In such cases the bark should be cut back to the living tissue and all particles of dead or injured bark scraped off. The exposed area should then be coated with a good tree paint or asphaltic emulsion.
The severest case of bark splitting observed was on a vigorous young heartnut seedling at Guelph, Ontario. On this tree the bark was completely split away entirely around the trunk from the ground up for several feet and the injury was so great that the tree died early in the summer. Within a short distance of this tree was another tree of the same origin that was quite uninjured, but this tree, however, was a hybrid between the butternut and the heartnut. On this hardy tree there was a heavy crop of nuts that were intermediate in form between the heartnut and the butternut, this indicating its hybrid origin. Practically all of these hybrids escaped injury even though the temperature was -30 degrees F.
Bark injury was also noticed at the Kellogg Farm on several black walnut trees that had been grafted in the nursery and which were planted in 1932 and 1933. On these trees the scion variety was uninjured but the bark on the stock was more or less affected from the ground up to the point of union. All trees thus affected came out into leaf, but shortly afterward the leaves withered and the top died.
Bark injury from splitting or desiccation was more prevalent on young vigorous growing trees, and on older trees that had been stimulated into a strong growth by fertilizers or late cultivation.