Dandelions symbolize hope for Katniss. Why? Well, because these little flowers (some might say weeds) were the first things she saw after Peeta gave her the bread on that fateful day long ago when her family was starving to death:
I had just turned away from Peeta Mellark's bruised face when I saw the dandelion and I knew hope wasn't lost. I plucked it carefully and hurried home. I grabbed a bucket and Prim's hand and headed to the Meadow and yes, it was dotted with the golden-headed weeds. After we'd harvested those, we scrounged along inside the fence for probably a mile until we filled the bucket with the dandelion greens, stems and flowers. (4.13)
Upon seeing the dandelions, Katniss realizes that she will be able to feed her family with food she can gather from the forest. What other people might consider useless, Katniss turns into her family's sustenance. This is both a lesson in resourcefulness and a reminder that hope springs eternal.