The phrase 盛开 means to bloom, as in, "池塘里的荷花盛开." Is this pronounced as "shèng kāi", or is it pronounced as "chéng kāi"?
You can guess the pronunciation according to its usage:
"盛("chéng")" in 盛饭 is used as a verb, which means to fill or to contain. "盛("shèng")" in 盛开 is used as a non-verb(adj./adv./noun), which means prosperous(ly), flourishing(ly)... (盛宴,盛装,旺盛,兴盛...)
by the way, 盛"shèng" is also used as a surname, such as this man