Have you ever noticed one of these rolling into a street in autumn? Your mind might've labeled it an apple, matching it to a more common search image. But this is no heirloom apple – it is the fruit of a Black Walnut (Juglans nigra) tree. Within this green husk lies a thick, tough shell containing a nutrient dense nut. The flavor of Black Walnuts is stronger – and I dare say, more interesting – than that of the commercially grown walnut. Keep your eyes open and scan the streets for large Black Walnuts dropping fruit near you.
Warning: If you end up gathering some nuts, I should warn you that the husks will stain unprotected hands for a week or more. Consider wearing gloves unless you want to call attention to your foraging activities. Read up on Black Walnuts in a credible foraging guide for more details regarding processing and use. Samuel Thayer's book Nature's Garden has an excellent chapter on this wild food.