I believe a friend who does commercial harvesting in Oregon, harvests about leaf fall.

my understanding is that they should be green and only just starting to get a spot of color on them. My hazelnuts are still too young to produce, so i cannot say for certain.

Thank you. I've had these for years, and they were really loaded this year, but the squirrels got most of them. (The squirrels get the nuts. The slugs get the strawberries. The birds get the cherries, etc.)

Commercially, when a few nuts are on the ground, they shake the trees to hasten the harvest.