R. lapponicum

Small leaves and lavender-purple flowers make most gardeners think this small rhododendron is an evergreen azalea. However it is a rhododendrron found in Alaska and some of the northeastern states, as well as northern Europe and northern Asia, where the climate is very tough. This smaller grower will be 2-3 ft tall and a bit wider in 10 years and can take temperatures to -25 deg F. This puts this plant in zone 4b. Give this one very sharp drainage as it is used to growing in gravelly soil where it is native. Blooms in  April.