Answer: Unfortunately, I do not know of a way to preserve the green fruits or nuts for decorative purposes, pretty as they may be when fresh. Use caution when handling these because they will stain -- black walnut is a traditional source of a very dark dye.
To get to the nutmeat of a black walnut, you need to remove the green layer and then shell the hard nut inside it. The nutmeat is then inside the hard shell.
To break the shell is a challenge because it is extremely hard and often the strength required to break it causes it to shatter and intermingle with the nutmeat.
The nuts also have a somewhat unique flavor that not everyone appreciates. (Most walnut recipes are for "English" walnuts.) Hence, I suspect, the relative expense at the market and the rarity of black walnut recipes.
To start a tree from seed, according to Michael Dirr's "Manual of Woody Landscape Plants" you will need to put it through a period of cool moist stratification prior to planting. You can achieve this artifically in the refrigerator or by planting several freshly fallen whole fruits or nuts outside in the fall in the location where you want the tree to grow. With luck, one or more will germinate in place next spring.
When siting a black walnut tree, keep in mind that many plants are sensitive to a substance contained in the roots and, to a lesser extent, other parts of the tree. The juglone works to suppress competition for the tree and as a result can make the gardener's job a real challenge within quite a large distance from the tree. Tomatoes and ericaceous plants such as azaleas in particular will not survive in the presence of juglone. The tree's roots may reach twice as far as the tree is tall, and a well grown tree may reach to about 60 feet or more in height and width.
Finally, take care in disposing of nuts, shells and so on from the tree as they will also contain juglone.
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