What is Yoshon?
The source for the laws pertaining to Yoshon are found in Vayikra (23:9-14). These laws are applicable to the Chamishas HaMinim of barley, oats, rye, spelt and wheat.
These grains are Yoshon if they took root prior to Pesach. If the grains did not take root prior to Pesach then they are chadash until the subsequent Pesach. There is a halachic dispute if the hashrashah (grain taking root) must be 3 days or two weeks prior to Pesach. The laws of chadash do not apply to other grains such as rice, corn and buckwheat.