It's the promise of warmth, not the actuality. But it's also freshness without qualification; when it's young and hungry, when its juices flow freely.
To catch a whiff of a freshly mown lawn on a brisk spring day is to imagine all the delights that lie in store. But it's also to imagine how quickly they can wither.
It is the seasonal reverse but the poetic equivalent of what Gerard Manley Hopkins describes in Spring and Fall:
It is the blight man was born for,
It is Margaret you mourn for.