Spring is usually a season of beginnings, but this tiny poem starts with the word and then turns away from the expected. Instead of fresh buds or new growth, we get an ending: the last white flower fading from a mountain. The dash after "Spring" works like a held breath, a small pause before the quiet loss arrives.
The image is simple and very visual. A white flower sits against the "blue stem of the mountain," which suggests distance, with the mountain seen as a hazy blue shape and the flower as its last bit of pale light. The word "stem" is interesting, because it makes the whole mountain feel like a plant, delicate rather than huge.
The speaker never says how they feel, but the mood is gentle and a little wistful. There is no drama here, just attention to something passing. The poem seems to suggest that even in spring, things are always slipping away, and it asks us to notice that quietly.