The most common and most likely answer to this crossword clue is the 8 plus 8 letter word JAPANESE KNOTWEED
I've seen this crossword clue in The Guardian
J
A
P
A
N
E
S
E
K
N
O
T
W
E
E
D
More clues leading to the result JAPANESE KNOTWEED
Buddy explains!
The straight part
We know that "plant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plant = japanese knotweed.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.