Had grown.
"Has grown" is the present perfect tense of the verb grow. The past perfect tense is "had grown". The simple past tense of grow is grew.
Grown is the past participle of grow. It is used with the auxiliary verb have to create the present perfect tense of grow, which is have/has grown.
Existed is the past tense. The past perfect tense is had existed.
The past tense is bled, and the past perfect tense is had bled.
The past tense of sit is sat; the past perfect tense is had sat.
The past perfect tense of hope is had hoped.
The past perfect tense is had set.
The past perfect tense is had submitted.
The past perfect tense is "had confronted".
The past perfect tense is had rung.
The past perfect tense is had burned.
The past perfect tense is had chosen.