The only one that might be understood easily is「毒草」, which sometimes pops up figuratively in modern literature, likely to be tied to a nuance of thought control:
毛澤東 《關於正確處理人民內部矛盾的問題》八： “有錯誤就得批判， 有毒草就得進行鬥爭。”
On the correct manner of dealing with internal strife among the people, Mao Zedong said: "Errors are to be critiqued, and we must enact struggles against「毒草」".
劉心武 《班主任》： “ 張老師 ， 謝惠敏 說我們讀毒草， 這本書能叫毒草嗎？”
Professor Zhang, Xie Huimin told us that we're reading「毒草」, is this book really a kind of「毒草」?
「毒草」here means a work of literature (or other kind of art) that is harmful to the people or society. The more common antonym to「毒草」in this sense is「香花」rather than「仙草」.
More commonly, however,「仙草」and「毒草」refer to the more obvious sense of mythical/poisonous herbs, respectively.「良草」is not something that pops up as a common phrase at all.
The reason why they're not common is possibly because they are outdated sayings tied to the Mao Communist era on the Mainland. I suggest not to use these kinds of words too much.