Loveliest of trees, the cherry now
Eastertide timing
Cherry blossoms peak in early April in England—exactly Easter season. The white blooms double as resurrection imagery.
Psalm 90 reference
"Threescore years and ten" comes from Psalm 90:10—the biblical human lifespan of seventy years. He's doing mortality math.
Springs, not years
He counts in springs (bloom seasons) rather than years. Each missed spring is a permanent loss of beauty, not just time.
Double meaning
"Hung with snow" describes both the white blossoms now and the actual snow that will cover these trees after he's dead.