What is the source of the allusion in the sentence below?
The garden was overtaken by invasive, hydra-headed hogweed plants.

 - classical mythology
 - the Bible
The source of the allusion hydra-headed is classical mythology.
In classical mythology, Hercules fought the Hydra, a monster with nine heads. When one head was severed, two grew in its place.
The allusion hydra-headed means difficult to overcome or contain.