Shall we compare thee to a summer's day? Thou art more lovely. How about if we compare you to a weed? A poisonous flower? Er, what about limestone? Oh, no—you're not into that? It's a compliment, we swear!
Here, let us tell you why—Khaled Hosseini uses that same imagery to characterize Mariam in A Thousand Splendid Suns and chart her growth from a poor harami into a loving and selfless woman.
When we are introduced to Mariam, we are given the image of her as "a weed […] something you rip out and toss aside" (1.2.3). These words would hurt coming from anybody, but they hit Mariam especially hard because they come from Mommy Dearest. But it is accurate, in a way: the world treats Mariam like a weed because she is a harami. She doesn't fit into the social order, and people reject her because of it.
At some point, Mariam even begins to believe that she is nothing but a weed. When she's older, she describes love and hope as "twin poisonous flowers" that she uproots whenever they sprout inside her (3.35.26). Mariam learns to uproot love the same way that she was uprooted.
That changes once Laila and Aziza come along. For the first time in her life, Mariam has people who love her unconditionally. That's a powerful thing. Mariam ends up expressing her love for Laila and Aziza in the most profound way, by sacrificing her life for them. There's one thing that's for sure after that: Mariam is a weed no more.
So, when Laila returns to Mariam's childhood home at the close of the novel, she doesn't see Mariam as a weed—she realizes that Mariam is "as hard and unyielding as a block of limestone" (4.50.112). She can't be uprooted, can't be tossed aside, can't be ignored. Finally.