Seeds Sulphur Trumpet or Polemonium pauciflorum is an extremely cold hardy selection with handsome silvery fern like foliage. Sulfur yellow trumpet flowers are up to 2 inches long and flushed pink on the underside. This hardy perennial will survive minus 50 degrees F. Growing 18 high and 12 across this plant prefers part shade. Will bloom from May to June. Zones 6 to 10.