Dianthus armeria

Dianthus armeria is an annual or biennial flowering plant commonly known as Deptford pink or grass pink. It is in the family Caryophyllaceae.
It has small pink flowers that bloom from spring to summer followed by an explosive, inedible fruit. The blooms close up in the afternoon. The seeds are small and blackish or brownish. 
It is a native plant in Europe, and widely grown as an ornamental plant in gardens. It has been introduced to New Zealand and much of North America.