Some herbs will naturally darken when dehydrated, but if your herbs are turning very dark, there are two possible causes. First, herbs can darken if they don’t dry quickly enough, so you may want to adjust the temperature setting on your dehydrator. Second, because herbs can also turn dark if they are dehydrated at high heat, you’ll want to take a look at your temperature setting and turn it down if you think your dehydrator is running hot.