In the Prologue, Henrik Vanger, and Detective Inspector Gustaf Morell are given a clue that Harriet now lives in Australia. Just like every year since Harriet's been gone, Henrik gets a flower in the mail. This one is a Leptospermum (Myrtaceae) rubinette, your basic white flower. Here's our clue.
The plant was native to the Australian bush and uplands, where it was to be found among tussocks of grass. There it was called Desert Snow. (Prologue.13)
Morell and Vanger have too much of the wrong information, and too little of the right. They can't force meaning from what is obviously a clue to her disappearance. Partly, this is because they're utterly convinced she's dead.
The flower doesn't clue Blomkvist and Salander in to Harriet's whereabouts either. They don't even bother with the flowers. Eventually, they find Harriet the old-fashioned way: by scaring Anita Vanger into calling Harriet, and tracing the call. The flower functions more as a symbolic clue than a practical one.
The flower is a symbol of miscommunication. Harriet intends to communicate that she's alive and well through her annual gifts, in the tradition she'd followed since she was eight. But, because Henrik is certain she's dead, he thinks the flowers are sent by her killer to remind him that something awful happened to her. They become a horrible symbol of an unsolved murder. When he finally learns the truth, the flowers take on a whole new meaning.
The Desert Snow flower, which Henrik gets on November 1, 2002, can also be seen as a symbol of Harriet herself. She is a Swedish woman (snow) living in Central Australia (desert). Pretty cute, right?