Azaleas and rhododendrons for Minnesota
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.
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.
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:
- Northern Lights F1 hybrid seedlings. These azaleas are derived from a cross between Rhododendron x kosteranum and Rhododendron prinophyllum, and became available commercially in 1978. The plants are extremely floriferous, with each seedling flowering in a shade of pink, giving a fantastic floral show in late May. The flowers have a sweet fragrance. Mature height and spread are six to eight feet.
- Northern Lights hybrid azalea cultivars.
- Rhododendron "Pink Lights" (Pink Lights Azalea). Introduced 1984. Pink Lights is a clonal selection from the Northern Lights seedling azaleas. The flower has a light pink color with a sweet floral scent. Mature plants will have a height and spread of about eight feet. The plant is extremely floriferous.
- Rhododendron "Rosy Lights" (Rosy Lights Azalea). Introduced 1984. Rosy Lights is a clonal selection from the Northern Lights seedling azaleas. The flower color is a deep rosy pink and plants are extremely floriferous. Plant height and spread is about eight feet.
- Rhododendron "White Lights" (White Lights Azalea). Introduced White Lights is a hybrid of Rhododendron prinophyllum and a white flowered Exbury hybrid. The flower buds have a delicate pale pink cast but open to a white flower with a slight yellow blotch. This cultivar is extremely floriferous and has a flower bud hardiness rating of -35 degrees F. Plant height and spread is about five feet.
- Rhododendron "Spicy Lights" (Spicy Lights Azalea). Introduced Spicy Lights is a selection from hybrids having Rhododendron prinophyllum in their background. The flower has a salmon color with a slight fragrance. Flower bud hardiness is rated at -35 degrees F. Plant height is about six feet and spread is about eight feet.
- Rhododendron "Orchid Lights" (Orchid Lights Azalea). Introduced 1986 Orchid Lights Azalea is a hybrid of Rhododendron canadense and Rhododendron x kosteranum. The orchid-colored flowers are 1-1/2 inches across and are sterile, so seed capsules are not produced. Flower bud hardiness is rated at -45 degrees F. The compact plants of Orchid Lights will mature at an average height of three feet and a spread of three to four feet.
- Rhododendron "Golden Lights" (Golden Lights Azalea). Introduced 1986 Golden Lights Azalea is a hybrid of an Exbury seedling and an unidentified azalea seedling. The golden flowers are 1-1/2 to 2 inches across and have a cold hardiness rating of -30 degrees F. The mature plants reach an average height and spread of four feet. Golden Lights has the added advantage of greater resistance to mildew than some other hybrid azalea cultivars.
- Rhododendron "Northern Hi-Lights" (Northern Hi-Lights Azalea). Introduced 1994. Northern Hi-Lights Azalea is a hybrid of an Exbury seedling and an unidentified azalea seedling. It is a hybrid with the same parents as "Golden Lights". The flowers are creamy white with a bright yellow upper petal and have a cold hardiness rating of -30 degrees F. Plants grow relatively slowly to four feet high and four to five feet wide. The dark green foliage has some resistance to mildew.
- Rhododendron "P.J.M." P.J.M. rhododendron is a hybrid resulting from crossing Rhododendron carolinianum and Rhododendron dauricum. It is an evergreen rhododendron that has very attractive small dark green leaves and lavender pink flowers. This cultivar is very hardy (-35 degrees F) and prefers a sandy soil to a clay soil. P.J.M. are the initials of P.J. Mezitt, the hybridizer of this cultivar. There are other promising selections in this group of evergreen rhododendrons that have been named. Currently they are being evaluated in Arboretum research trials.
- Rhododendron prinophyllum (Roseshell azalea). This species is synonymous with Rhododendron roseum. This azalea is native from Maine to Virginia and as far west as Missouri. The flower buds are hardy to -40 degrees F. The plant is small (three feet) with very fragrant white to rose pink flowers that bloom in late May. The plants are heat sensitive and do better in slight shade than other azaleas. Rhododendron prinophyllum is one of the parents of the Northern Lights hybrids.
- Rhododendron vaseyi (Pinkshell azalea). This species blooms from early to mid-May in Minnesota before its foliage emerges. With delicate pale pink flowers and an open form it fits well into naturalistic settings. Rhododendron vaseyi is an extremely hardy species, with flower buds rated at -35 degrees to -40 degrees F.
- Rhododendron Exbury hybrids. These azalea hybrids are not reliably hardy in Minnesota as the flower buds are killed at -15 degrees to -20 degrees F. Other cultivars listed above are more adapted for Minnesota and should be used instead.
Figure 1. Rhododendron "Pink Lights"
Figure 2. Rhododendron "White Lights"
Based on an earlier fact sheet written by Susan Moe, former University of Minnesota Extension Educator