Grafting joins the twigs or buds of one walnut tree to the trunk of another for better fruiting trees. Rootstock is the tree you graft to, and scions are grafts that will be joined to the rootstock.
Black Walnuts vs. English Walnuts
Juglans nigra is the black walnut and Juglans regia is the English or Persian walnut. Black walnuts are notoriously hard to graft.
Traditional grafting is done after the last hard frost but before hot weather. The best walnut grafting window is the six weeks of active spring growth (March to mid-June). Different types of grafting have different optimum times.
Take scion wood as leaf buds begin to swell (February to March depending on climate). Graft shoots as rootstock leaves start to open and colour right before spring growth. Graft till mid-June when wood begins to harden.
Budding joins growth buds (not flower buds) and your rootstock. Begin as new leaves unfurl and stop when new shoots are more than 12 inches long. Bud during winter dormancy if you have a protected greenhouse for your rootstock.
Topworking and Inlay Grafting
Weeping sap in mid to late spring can prevent grafts from taking although this is the best time in the walnut growth cycle for grafting. Cut off the rootstock higher than usual for other fruits and wait for sap bleeding to stop.