snow-in-summer

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A woolly, white, mat-forming perennial herb (Cerastium tomentosum) native to Italy and widely cultivated in rock gardens for its showy white flowers with notched petals.

Century Dictionary and Cyclopedia

1. n. A garden name of Cerastium tomentosum. See snow-in-harvest.

WordNet 3.0

1. n. chickweed with hairy silver-grey leaves and rather large white flowers
2. n. annual spurge of western United States having showy white-bracted flower clusters and very poisonous milk