How soon can you eat a walnut after it falls from the tree?

The kernel of the nut is edible as soon as it falls from the tree.

However, the shell of the nut is encased in flesh. Pale green, smooth skinned and about a quarter inch thick it is rather hard. It will cause severe discomfort to the tips of your thumbs if you try to peel it at once by hand.

You can use tools if you are in a big hurry. If not, leaving the nut alone for a few weeks will cause the flesh to soften. The process is marked by steady darkening in colour. It becomes easy to open by hand, and the delay gives the nut a few weeks to "ripen" which improves the flavour.

Be warned. Peeling away the flesh by hand, whether or not you leave it to soften, will stain your fingers dark brown. This wears off in about a week, or you can wear rubber gloves.

Source : personal experience.