The most common and most likely answer to this crossword clue is the 3 plus 5 plus 5 letter word OLD MAN’S BEARD
I've seen this crossword clue in Financial Times
O
L
D
M
A
N
â
€
™
S
B
E
A
R
D
More clues leading to the result OLD MAN’S BEARD
Buddy explains!
The straight part
We know that "Plant growth on dad" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plant growth on dad = old man’s beard.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.