The most common and most likely answer to this crossword clue is the 9 letter word MANDRAKES
I've seen this crossword clue in The Guardian
M
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The straight part
We know that "noxious plants" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case noxious plants = mandrakes.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does MANDRAKES mean?
MANDRAKES noun- A mandragora, a kind of tiny demon immune to fire.
- Any plant of the genus Mandragora, certain of which are said to have medicinal properties; the root of these plants often resembles the shape of a small person, hence occasioning various mythic, magical, or occult uses.
- A root of a mandrake plant that resembled human form, especially one kept or used for magic or occult purposes.
- The drug methaqualone.