A flower is the blossom of any plant, including trees. So in this sense, flower and blossom are the same. However, sometimes we use flower to mean a fully mature blossom, and blossom to mean the first sign of a flower emerging from a bud. So when something is blossoming, it is growing. You could use flowering as well, but we tend to use blossoming.

Blossom - is generally the term used for the flowers of fruit bearing trees such as pears, apples, cherries, plums and the like, especially those that flower in spring. A flower, sometimes known as a bloom or blossom is the reproductive structure found in flowering plants.