Two varieties of walnuts are most often grown in North America. The English walnut (Juglars regia) is believed to have originated in Persia or other areas close to the Black Sea; the black walnut (Juglans niger) is native to North America. There are two ways of propagating walnut trees, growing them from seed or grafting branches called "scions" onto healthy rootstock.
Planting English Walnuts
Since walnuts are pollinated by the wind, the source of the pollen is always uncertain; English walnuts are not ordinarily planted for trees intended to produce nuts. They are planted from seed to obtain rootstock; scions from productive trees are grafted onto the rootstock.
In the fall, English walnuts can be planted 4 inches in ground in areas where the temperature drops below 40 degrees F for at least two months, According to the "American Horticultural Manual." If the nuts are to be planted in the spring, they should be placed in a container filled with half sand and half sphagnum peat moss and refrigerated at 40 to 55 degrees F for at least two months. This process is called "stratification." They should then be planted before the first frost of autumn and after the last freeze in the spring.
Planting Black Walnuts
Productive black walnut trees can be grown from seed. Black walnuts are collected during the fall, after which the green hull is removed and the nuts are washed. Nuts that float will not germinate well. The nuts are stratified by placing in a 3-inch deep layer of moist sand and refrigerated at 33 to 40 degrees F for 120 days.
Three to five stratified nuts are planted 2 to 3 inches deep at each tree location and fertilized with 1/4 cup of balanced, slow-release fertilizer. They are watered when the soil becomes dry. During the first year of growth the weaker shoots are cut below the ground.
Black Walnut Container Planting
Single stratified black walnuts are planted in bottomless containers placed on a bench of wire screen; this prevents the roots from circling in the container. Container seedlings planted in the spring need to be watered daily and transplanted to the field three to four weeks before the first killing frost in the fall. Container trees planted in the fall for spring transplanting should be protected from freezing.
Scions for Grafting
Scions, the part of the English walnut or black walnut tree to be grafted onto a root stock, are cut in the late winter from trees known to produce nuts. They are selected from the previous year's growth and should have large, plump buds.
When to Graft
When the rootstock begins growing in the spring, the bark can be easily removed from the wood. This period lasts about six weeks. Small trees receive scions when their emerging buds are 1 inch long; larger trees receive scions when their leaves are starting to unfurl.
How to Graft
The scion and the stock should be roughly the same diameter, according to William Reid of Kansas State University's Pecan Experiment Stateion. The scion should be large enough that at least a third of the stock is removed. The stock trees are cut about 1 inch above the spot where the scion is to be grafted. The scion is cut at a 45-degree angle, dipped into rooting hormone, and placed in a slit cut just under the bark of the stock.
The graft is wrapped with grafting tape then aluminum foil to prevent sunburn. The corners of a plastic bag may be taped above and below the graft to help preserve moisture. The grafted scion should begin growing three to four weeks, after which the plastic bag, aluminum foil and grafting tape are removed.