Just in case anyone wasn’t sure on this point, they do. This myth came from the 1971 book Diet For a Small Planet by Francis Moore Lappe, who stated that plants are deficient in certain essential amino acids and therefore must be eaten together to form a ‘complete’ protein. She formally retracted her statement as false 10 years later, but the damage was done, and the myth is even printed in some medical and health textbooks to this day!
The important thing to remember is that while some plants do have less of a given amino acid or two, they all contain some of every essential amino acid so combining/complementing proteins is completely unnecessary. As long as you are taking in enough calories from whole foods you are guaranteed to meet your daily requirements for protein from plants alone – and that includes all 9 essential amino acids!
Fun fact: the only truly incomplete protein that we’ve ever read about is gelatin – an animal-based protein!