 'Knock Out' was developed by amateur rose breeder, William Radler in his Milwaukee, Wisconsin basement in 1989.The cultivar was developed from a cross between the floribunda, "Rosa" 'Razzle Dazzle' and shrub rose, "Rosa" 'Carefree Beauty'.Compared to the other new hybrid rose plants in his backyard test garden that year, Radler recalled that new cultivar was exceptional.