American hazelnuts are typically ripe in September or October. The nuts should start to turn brown, but the leaves around the nut should still be green. If you pick the nuts before this time, they won't be ripe enough and won't taste good. The leaves around the nuts are called involucres.

How do you know when hazelnuts are ripe?

They should be harvested when the husks begin to turn yellow around late September. Store the nuts in a dry, airy room or shed in slatted boxes in shallow layers or in net bags hung up. They need to be turned regularly to ensure they are drying evenly. Once they are dry remove the husks and store as before.

Can hazelnuts be picked early?

Picking some nuts that are unripe is an occupational hazard, but if you wait until at least the end of July to pick (in California) you should be OK.