dauricum hybrid 'April Gem'

Parentage: {[(minus var. minus Carolinianum Group) x mucronulatum 'Cornell Pink'] x (dauricum f2, white)} x {(dauricum, white form x dauricum) white form}, G. Mehlquist cross, reg. 1988.
Habit: Broadly, upright plant. Height 70 cm in ten years.
Flowers: Slightly fragrant, double, white flowers. Flower size 3.5 to 4 cm.
Leaves: Deep olive-green leaves, length 4 cm.
Cold Hardiness: Very varying information is available from -15°F, -26°C down to -25°F, -32°C! The truth lies somewhere between? I would use -20°F, -29°C.