What does spring symbolize? What could this mean for Melinda?

Spring is a time of new life and growth. For Melinda in Speak it certainly is. To a considerable extent the seasons mirror her feelings. The winter months are bleak and cold, and Melinda finds life especially hard during this time. Spring, then, offers some semblance of hope, however remote; the hope that Melinda will finally be able to confront the terrible ordeal she's suffered and begin to live her life once again.
The way she does this is through her art. Melinda can't communicate what she's going through to another living soul except through her painting. Her feelings are symbolized by the picture of the tree that she's been painting for her school art project. It's notable that, over time, the tree in the painting is transformed from a dead thing surrounded by darkness into a strong, sturdy oak whose branches reach up towards the sun, just as Melinda hopes to reach out to the people around her.