Most varieties of rhododendron are not winter hardy in northern climates, but several varieties being tested at the Landscape Arboretum of the University of Minnesota are showing promise for successful growing in higher latitudes.
Azaleas, a deciduous variety of rhododendron, are somewhat hardier, and are the variety of choice for northern growers. Rhododendrons and azaleas are both considered subsets of the rhododendron genus.
Despite cold winter temperatures, rhododendrons thrive planted on the east side of buildings. This protects them from hot summer temperatures or winter sun scald, which occurs when the bark of trees or shrubs is suddenly cast into a shadow when air temperatures are below freezing. The sudden cold after the warmth of the bright winter sun heating up the bark on the tree is what causes the damage.
Plant rhododendrons in soil that is very well-drained and slightly acidic, the same type of soil in which evergreens grow well. Rhododendrons make good companion plants for evergreen shrubs and ground covers.
Amend the soil by adding peat moss, compost and well-rotted manure. This will improve the soil’s fertility and moisture holding abilities. The acidic peat moss will also help lower the pH of the garden bed. You can also add sulfur or ferrus sulfate to lower the pH to an optimum level of 4.0 to 5.5 on the pH scale.
Apply a thick mulch to hold in moisture and prevent the growth of weeds. Rhododendrons are shallow-rooted, and their roots can be harmed by cultivation. They are also thirsty and will benefit from a water-retaining mulch. Water them well during dry summer weather.
Korean Rhododendron (Rhododendron mucronulatum) – Native to Korea, China and Japan, this hardy, deciduous variety blooms in early May with magenta colored flowers. The flowers appear before the leaves. A pink cultivar, called “Cornell Pink” is as hardy as the magenta-flowered variety.
Rhododendron P.J.M. – A cross between Rhododendron carolinianumand Rhododendron dauricum, P.J.M. is an evergreen rhododendron with small leaves and lavender pink flowers. Hardy to -35 degrees Fahrenheit, it prefers sandy soil. This is a promising group of hardy hybrid rhododendrons and is currently being tested at the University of Minnesota Landscape Arboretum.
Mollis Azaleas Series – With flower buds hardy to -25 degrees Fahrenheit, the mollis series of hybrid rhododendrons (Rhododendron X kosteranum) bloom in late May in shades of red, orange and yellow. They grow about 6 to 8 feet high and wide at maturity.
Rododendron Pinkshell Azalea – Extremely hardy, Pinkshell azalea (Rhododendron vaseyi) has flower buds hardy to -35 to -40 degrees. Its delicate, pale pink flowers bloom before the leaves unfurl. It grows into a shrub with an open form, fitting well into natural gardens.
Northern Lights Azalea Series – A series of F1 hybrids, Northern Lights azaleas are a cross between Rhododendron X kosteranum and Rhododendron prinophyllum. Released commercially in 1978, the flowers are both prolific bloomers and extremely fragrant. Named cultivars–all featuring “Lights” as part of their name–are available with flowers in rose, pink, white, salmon, orchid, golden yellow and white with a golden upper petal. Developed and released by the University of Minnesota Landscape Arboretum, the flower buds of Northern Lights azaleas are hardy to -30 to -45 degrees.
Don’t forgo planting these spring-blooming beauties if you live in the far north. There are plenty of varieties of hardy rhododendrons to choose from. And don’t forget about azaleas, their deciduous cousins.
Before ending this article I’d like to mention that I am going to start talking about home improvement and home services such as plumbing, hot water, pool renovations and designs and much more. I’m excited to expand my blog into these areas.