Soft as the massacre of Suns — Sundrop

Blossoms of the evening primrose, or called sundrop, look like swords in the evening. The beams of light from the sun vanish in the evening as if being slain by the sundrop's blossoming.

Soft as the massacre of Suns
By Evening's Sabres slain

(F.1146/J.1127)

[1] Suns:: sunbeams. massacre of Suns:: sunbeams vanished in the evening.
[2] Evening:: evening primrose, also called sundrop, will blossom in the evening like swords. Sabres:: swords, blossoms of the sundrop.
[1, 2] Suns, Evening:: Suns can mean the canons. Evening has the definition of the closing or declining period of a person's life _{(OED 2b)}.

Evening Primrose. Is so called, because its large yellow blossoms are only open on cloudy days, in the evening and at night. . . . Sundrops, or Perennial Evening Primrose. Common in open places, one to three feet high, with yellow flowers. Blossoms June to August. ─ The Farmer's Promotion Book (1856)

SUNDROP. Blossoms in a kind of spike; yellow. They open about eleven o'clock, and, commonly, not more than one on the same day. . . . PRIMROSE. Leaves oblong; serrated. Blossoms in a large spike; terminating; yellow. ─ Bulletin of the Lloyd Library of Botany (1787)