By likening any apology Noa might offer to little weeds she is expressing her contempt she has for the woman who took her daughter's life. No matter how contrite or how often she apologizes, her words are just an ugly and annoying nuisance that can't bring Sarah back or put an end to Marlene's pain. And she's right. Apologies don't change facts, and while they can bring some healing to the person making the apology as mariannes states, I don't think the pain Marlene is talking about is Noa's.
But, I don't agree with what Marlene says next, that a genuine apology is supposed to stop the pain. It's not the apology, but the acceptance of the apology, the forgiveness, that can heal the pain of the wounded. And Marlene cannot forgive.