Plants labeled as azalea or rhododendron both belong to the botanical genus Rhododendron. In Minnesota, the limiting factor for either shrub is the extent of winter cold, as the state is located in U.S. Department of Agriculture Plant Hardiness Zones 3 and 4. Deciduous species and hybrids offer a wide range of flower colors and improved chances for winter survival.
Best grown in southern Minnesota where winter lows reach no lower than -20 to -25 degrees F, Mollis azaleas (Rhododendron x kosteranum) bloom in mid-May with flowers of red, orange or golden yellow. These deciduous shrubs reach a mature height of 5 to 8 feet.
As the name suggests, this species hails from eastern Asia, so has proven its hardiness in brutal winter cold. Korean azalea (Rhododendron mucronulatum) blooms in early May and its flower buds may be nipped by frosts, especially in central and northeastern Minnesota. The flowers appear before the foliage and are magenta. Selection 'Cornell Pink' bears pink blossoms.
Northern Lights Hybrid Azaleas
These hybrid shrubs were developed by the University of Minnesota, and have demonstrated hardiness to winter low temperatures statewide from -20 to -45 degrees F. All of the varietal names include the name "Lights" to help identify their origins with the university's breeding program and excellent cold hardiness in all counties of the state. 'Rosy Lights' and 'Orchid Lights' are two cultivars in this line.
An evergreen plant with fuzzy oval leaves, the P.J.M. rhododendron (Rhododendron 'P.J.M.') survives best in the southernmost reaches of Minnesota, especially when protected from cold, drying winter winds. As cold deepens, the foliage rolls into cigar-like tubes. It blooms in late April in southern Minnesota counties.
One of the parent plants to create the Northern Lights azaleas, the fragrant-flowering roseshell azalea (Rhododendron prinophyllum) hails from the eastern United States. It survives winter temperatures no lower than -40 degrees F. This shrub blooms in late May.
Lovely in a woodland setting, the pinkshell azalea (Rhododendron vaseyi) prospers when shielded from the Minnesota winter winds. Its flower buds and twigs survive low temperatures down to -35 to -40 degrees F, allowing its garden use across the entire state. In mid-May its pale pink flowers open.