Question

read the source of the allusion.
in classical mythology, hercules fought the hydra, a monster with nine heads. when one head was severed, two grew in its place.
what is the meaning of the allusion in the sentence below?
the garden was overtaken by invasive, hydra - headed hogweed plants.
easy to cultivate
difficult to overcome or contain

Explanation:

In classical mythology, the Hydra's ability to grow two heads when one was severed made it extremely hard to defeat. Applying this to the garden, the "hydra - headed" hogweed plants imply they are hard to get rid of or control.

Answer:

difficult to overcome or contain