Why Nightshades are called 'Nightshades'?

Nightshades are a group of a variety of flowering plants(family Solanaceae). It includes common and famous plants like tomatoes, potatoes, peppers, eggplants(brinjals) and tobacco. Many plants of medicinal and pharmaceutical value falls into this family as well. This family comprises of about nearly 3000 different species in total.

Origin of word 'Nightshade' is not clear. Lets look into some possible reasons why it is called so.

Historical Background

Some people used to believe that these plants grow over night rather than in daytime (ie. a requirement of 'Shade' to grow). While certain members of this family are popular as food, certain others are poisons. Some of the members even had been used in witchcraft in past. So, various mythical believes are related to them.

The word 'Nightshades' convey a sense of mysticalness which can be explained likewise.

Certain poisonous members of this group had been involved in poisoning and murder cases in history. The name 'nightshades' might have been originally used to refer to these poisonous plants. However, because the family shares many characteristics in common(leading to  misidentifications), usage might have expanded.

The exact reason for the use of the word is still debatable and unclear. Whatever the reason might be, all Nightshades are a not necessarily 'evil and mystical'. Anyway, not all are bad ones.

French fries and Ketchup
(Potato and Tomato)

Further Reading

4 myths about nightshade vegetables
All About Nightshades