The most common and most likely answer to this crossword clue is the 5 letter word CANNA
I've seen this crossword clue in The Times
C
A
N
N
A
Buddy explains!
The straight part
We know that "Grow this plant in the topics/Scots cannot" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case grow this plant in the topics/scots cannot = canna.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 CANNA mean?
CANNA noun- Any member of the genus Canna of tropical plants with large leaves and often showy flowers.
- A measure of length in Italy, varying from six to seven feet.