The word зелье comes from Proto-Slavic *zelьje "grass, vegetation" which kept its meaning in many Slavic languages, including Church Slavonic.
It shares the root with the word зелёный "green", and the latter actually descends from the former.
The meaning of "potion" is a later development.
The word зелье in its original sense is used in Russian every now and then:
- Кто бы мог подумать, что заморская выдумка ― мороженое и это табачное зелье, которое еще так недавно русский народ называл чертовой травою, найдут покупщиков у самых дверей питейного дома? // Who could have thought that this foreign concoction, the ice-cream, and this tobacco herb, which Russian people had been calling "the devil's weed" so recently, would find buyers at the very door of a drinking establishment?
- Въ прочемъ извѣстно, что къ ней примѣшивается ваниль, пряное зелье и сахаръ, а не рѣдко бываетъ и безъ двухъ первыхъ примѣсей. // It is known, nevertheless, that vanilla, spicy herbs and sugar are being mixed into it, and it's not rare for it to occur without the first two additives.
- «… Чего нам ждать от него? Злое зелье вырывают с корнем. Вот тебе награда, изменник!» ― воскликнул он и острым кинжалом поразил предателя. // "What shall we expect from him? A pernicious weed is to be eradicated. Here's your reward, traitor"!, exclaimed he and smote the renegade with his sharp dagger.
This word, in my opinion, is a good way to translate "weed" in a Tolkien book.