Pick up the nuts after they fall to the ground. Remove the shells and then place the nuts in water. Nuts that float are not viable and can be discarded. Good, viable nuts will sink to the bottom.
To check the quality of the grain, soak the nuts in a bucket of water. Good quality nuts usually sink to the bottom, while poor quality nuts float upwards and should be discarded. Drop the unshelled black walnut into a bucket of water. If it floats, throw it away because it's rotten.
If it sinks, it's a guardian. It would be a shame to go to the trouble of getting black walnuts almost ready, only to open the treasure chest and find a rotten nut inside. Nuts that smell like paint thinner and taste very bitter are stale and should be discarded. Almost all black walnuts come from trees that grow in nature, while English walnuts come from orchards.
You can also test the quality of the grain by soaking a nut in water if the nut shell is dark brown. Usually, only half of the black walnuts germinate in the first year and the rest in subsequent years. Floating black walnuts are likely to be compromised in some way and should be removed from the harvest to be reviewed or completely discarded. Place an unpeeled black nut on a solid surface that you're not afraid to ruin, such as an old board or concrete block.
You can store shelled nuts in a cupboard at room temperature for at least half a year or refrigerate them for more than a year. Although nuts can be eaten as is by removing the outer shell, the best way to eat them is to soak them overnight. The fruit, leaves, and roots of black walnut trees contain a chemical, juglone, that can have a devastating impact on the roots of other plants. If the black nut has a lot of black or brown color, or is soft, then it is rotten and the nut inside may also be rotten.
Black walnut shells contain a strong colorant-like substance that will stain the skin, as well as any hard or porous surface. Nuts without shells are not kept that long, but they should retain their quality for at least three months in the pantry and a year in the fridge. It's good that there are a couple of small things you can do to find out if a black nut is good or bad. Black walnuts produce a chemical called juglone, which is found naturally in all parts of the tree, especially in buds, nut shells, and roots.