The original text reads:
Small herbs have grace; great weeds do grow apace.
This conveys that small herbs grow with grace, while big weeds grow quickly.
Perhaps recalling the context will enable you to manipulate this quote more effectively. We know that the archbishop announces that the party bringing the young prince will arrive within approximately one day, which triggers a conversation about the appearance of the prince, whose mother, grandmother and younger brother are eager to meet. The queen hopes that he has grown. However, his grandmother heard that the 9-year old duke has almost overtaken him. The boy hopes that this is not the case, since his uncle Gloucester told him the quote in question. Subsequently, the duchess states that, since Richard grew slowly, he should be full of grace, about which she ironically expresses her doubts.
Based on the above, a metaphor is a potential technique, being the implied comparison of Richard and the small herbs that grow slowly. Additionally, irony is also a potential technique since the old duchess is expressing irony when establishing the connection between Richard's slow growth and that of small herbs, which according to this logic, suggests that Richard should be full of grace, as explained above.
I hope this helps!