Wintersweet is one of the various colloquial names for the Asian shrubs in the prunus family, all of which are commonly mistranslated as plum tree. It flowers in the winter, sometimes in the snow. Thus, "plum" blossoms represent beauty and strength, and perseverance in adversity, in Chinese poetry, art, and literature. It's this very trait of endurance that makes the "plum" blossom less popular than the more ephemeral sakura/cherry blossom in Japan.

When you see a plum blossom in a Chinese or Japanese painting or referred in a poem, it may either pragmatically indicate late winter, or stand for the metaphoric meanings above. Under Japanese death poetry, there's a poem using a plum blossom motif. Cherry blossoms were pretty common as death imagery, because of their transience--thus the description of the falling plum blossom is just that much more poignant.