Kholm-grass was a plant native to the planet Honoghr.
Darth Vader used this ecological catastrophe as a means to enslave the Noghri people indefinitely. He sent them supplies to endear them to him, but replaced the kholm-grass with a hybrid variant which continued to poison all other native plants. The decon droids the Empire provided were in fact sent there to prolong the economic devastation of the planet, removing the kholm-grass as slowly as possible. The Noghri did notice the difference in the smell of the new kholm-grass, but attributed it to the chemicals released by the Gahenna decades earlier.