This is the promotional phrase for Disney's ad campaign for Mulan. I don't know if it's an actual Chinese saying or can be directly credited to somebody, but it does have some basis. The pine, the bamboo, and the plum blossom are featured in a lot of Chinese artwork and are very symbolic. The pine and bamboo remain green when it snows and the plum blossom begins blooming when it's still winter. "Mulan" (magnolia) is not a flower in the trilogy, but it does bloom early in the spring, before the leaves form. More information here.