During the sleuthing that occurs after Alaska's death, Miles and the Colonel get the sense that white flowers mean something more than what they think. They first get this inkling when they talk to the cop about Alaska's death and find out that she had white tulips in the backseat. The Colonel remembers a time with Alaska near the Smoking Hole to give the situation some context:
"There was this little white daisy on the bank of the creek, and all of a sudden she just jumped waist-deep into the water and waded across and grabbed it. She put it behind her ear, and when I asked her about it, she told me that her parents always put white flowers in her hair when she was little." (13after.31)
Because we don't want to give too much away (it's pretty fun to unpack what symbols mean for yourself), we'll leave you with just a few thoughts and references.
White is commonly used in in literature as having something to do with innocence and purity. Think about why brides wear white and why baptism gowns are white. Now think about why the flowers might be white.
Flowers are things that are alive. They grow and thrive in the right conditions. Turn these thoughts to Alaska and her parents.
So what do the flowers mean when it comes to Alaska, her relationship with her parents (both of them and each of them singly), and her personality? We'll leave it to you to figure it out.