The Evil Sith plants are evil mutated plants that often ensnare or capture escapees or intruders for being capture by the bad guys. They serve as defenses for many places that sith deemed important. They were created by Sith Alchemy and evil sith sorcery not to mention darth grievous's powers mutated them into being powerful. Also they sometimes wrap victims into cocoons to mutate them into cape wearing villains or spray pollen or spores that knock foes unconsious to make em easy prey.
After Darth Grievous dies the rest of these evil plants were exterminated forever in hope to never see those corrupted plants again.
The Sith plants that capture, ensnare and attack its foes are similar to the Cobra-la trees in the GI Joe A real american hero Movie and other evil plants in other shows and movies like the Heylin plant as well as UnderGrowth plants.