Barley can be found on the grasslands in the form of grouped brown stalks.
In order to consume it, you will need a Grinder. Click on the grinder without any tools equipped and a GUI will pop up asking what you want to grind. Click on the Barley and it should produce edible Grain Mush.
When growing Barley with the plant box it takes 90 seconds for it to fully grow.
Barley respawns 160 seconds after being harvested.