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Extension > Garden > Yard and Garden > Flowers > Azaleas and rhododendrons for Minnesota

Azaleas and rhododendrons for Minnesota

Michael Zins; Assistant Professor, University of Minnesota Landscape Arboretum

Azaleas and rhododendrons, best known for their showy, colorful flowers, can be used in many Minnesota landscapes if the proper species and cultivars are selected.

Botanically, azaleas and rhododendrons belong to the genus Rhododendron. This genus is one of the largest genera of woody plants, containing over 800 species. The terms azalea and rhododendron are general terms used to describe subgroups within the genus. Azaleas that survive in Minnesota belong to the sub-genus Pentanthera and are characterized by being deciduous and having five stamens. Rhododendron is a term referring to the plants within the genus having evergreen leaves and usually ten stamens within the flower. Most species found in Minnesota can fit in one of the two categories but, as always, there are exceptions. Rhododendron mucronulatum, which is hardy in Minnesota, is considered a rhododendron, although it is deciduous.

Site selection and soil preparation

The ability to grow azaleas and rhododendrons successfully depends on site selection and soil preparation.

For azaleas, an area with full sun to partial sun is preferable although they will tolerate slight shade. The evergreen rhododendrons will tolerate more shade than azaleas and need protection from winter sun to prevent leaf burn.

Azaleas and rhododendrons are also sensitive to extreme heat. Try to avoid selecting areas such as an exposed south side of a house where heat is reflected and can build up. Also avoid areas where wind and root competition could be problems.

Because of their very shallow, fibrous root systems, azaleas and rhododendrons tend to dry out rapidly and will require watering during dry periods, especially during hot summer days. With this need for irrigation, a site with good drainage is a must. Poor drainage can result in root rot caused by many species of the fungus Phytophthora.

Azaleas and rhododendrons require an acid soil for best growth. If your soil is not acidic, the addition of acid peat, sulfur, or ferrous sulfate can lower the pH. A pH of 4.0 to 5.5 is optimum.

Organic matter should also be added liberally to the soil. Compost, manure, sawdust, or acid peat moss will provide necessary organic matter.

Planting

When planting, it is important not to plant too deep. Most Rhododendron roots are within the upper four to six inches of soil and should not be buried any deeper than they were previously growing. Dig a shallow hole and backfill around the plant with a mixture of acid peat and existing soil.

Once planted, azaleas and rhododendrons can be fertilized once or twice a year during spring or summer with an acid fertilizer. Ammonium sulfate or special azalea fertilizers are available.

The use of an organic mulch is extremely beneficial. It will help retain water within the soil during hot, dry periods, modify the soil temperature, and inhibit weeds. Deep cultivation and hoeing disturb the shallow root system, so a mulch is preferable.

Pests

In the Minnesota climate, azaleas and rhododendrons do not have many serious pest problems and pesticides are usually not necessary. Powdery mildew may need to be controlled in years with hot, humid summers. Chlorotic foliage may indicate that the soil is too alkaline and needs further acidification.

If proper species and cultivars are chosen, no winter protection is needed other than that used to discourage rabbits, mice, and deer. A cylinder of hardware cloth prevents feeding by rodents.

Recommended species and cultivars

The following list of azaleas and rhododendrons are recommended for Minnesota landscapes. There are other species and cultivars that do survive here, but they are not recommended due to borderline hardiness, poor form, or lack of availability. Because flower buds are the least cold hardy part of the plant and flower show is the primary landscape value, cold hardiness ratings used in this discussion refer to flower bud hardiness. Most harsh winters that will kill flower buds will not injure the vegetative part of the plant and will not affect the ability of the plant to form flower buds for the next year.

Rhododendron x kosteranum (mollis azaleas). This hybrid species is often labeled Rhododendron mollis in the nursery trade. Mollis azaleas are extremely showy, blooming in late May with flower colors in shades of yellows, oranges, and reds. Mollis azaleas are flower bud hardy to -20 degrees to -25 degrees F, and will bloom well most years in the Twin City area and southern Minnesota. A particular strain of mollis azalea that has been selected for hardiness by the University of Minnesota Landscape Arboretum is sold by Minnesota nurseries. At maturity, mollis azaleas will have a height and spread of approximately six to eight feet.

Rhododendron mucronulatum (Korean rhododendron). This is a deciduous rhododendron with excellent cold hardiness. Rhododendron mucronulatum is native to Korea, China, and Japan. It blooms in early May and can sometimes be hit by a late frost. The flowers are magenta colored and appear before the leaves, giving a bright floral show. A pink flowered form of this species, equally hardy, is a cultivar called "Cornell Pink".

Northern Lights Series of hybrid azaleas. This is a series of hybrid azaleas being developed and released by the University of Minnesota Landscape Arboretum. Any azalea released and included in this series will have flower bud hardiness of -30 degrees to -45 degrees F to withstand Minnesota winters. As the azalea breeding program continues, new selections will become available and will be denoted by a cultivar name that includes "lights". Current named cultivars include:

Based on an earlier fact sheet written by Susan Moe, former University of Minnesota Extension Educator

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