This medium size (18 inches high) cultivar originated as the result of a cross of two unnamed seedlings of H. 'Brim Cup'. It was registered by Paul Aden of New York in 1986 and has leaves that are broadly ovate with thick substance. It bears near white flowers from late June into July. Since it has such a wide variegated margin of white which tends to be thin in substance, this cultivar often suffers from rips along the leaf edge.

According to The Hostapedia by Mark Zilis (2009), "Like 'Brim Cup', 'Lunar Eclipse' and a few others, 'Knockout' makes an attractive, creamy-margined mound of foliage as an immature, juvenile plant. With age, however, the leaf margin tends to "drop out", leaving an unsightly sawtooth edge. Avoid this cultivar."