This is nonsense. Outdoor growing yields as good or better herbs as indoor. Illinois is a great place to grow outside, keeping in mind the short season.
To the OP, as long as your plant is free of mold let it go. The cold nights will encourage ripening. Last season I had a plant take 13 frosts and one full snow. Some of the best I've ever smoked that plant was.
You can grab a clear shower curtain or plastic drop cloth from the dollar store and cover the plant at night to keep the cold off the plant.
Let us know how it turns out!