Azaleas and Rhododendrons for PLANTING Minnesota and Wisconsin When planting, it is important not to plant too deep. Most rhododendron roots are within the Azaleas and rhododendrons, best known for their upper four to six inches of soil and should not be showy, colorful flowers, can be used in many buried any deeper than they were previously Minnesota landscapes if the proper species and growing. Dig a shallow hole and backfill around cultivars are selected. the plant with acid peat. Botanically, azaleas and rhododendrons belong Once planted, azaleas and rhododendrons can be to the genus Rhododendron. This genus is one of fertilized once or twice a year during spring or the largest genera of woody plants, containing summer with an acid fertilizer. Ammonium over 800 species. The terms azalea and sulfate or special azalea fertilizers are available. rhododendron are general terms used to describe The use of a mulch is extremely beneficial. It subgroups within the genus. Azaleas that survive will help retain water within the soil during hot, in Minnesota and Wisconsin can fit in one of the dry periods, modify the soil temperature, and two categories but, as always, there are inhibit weeds. Deep cultivation and hoeing exceptions. Rhododendron mucronulatum, which disturb the shallow root system, so a mulch is is hardy in Minnesota and Wisconsin, is preferable. considered a rhododendron, although it is deciduous. PESTS In our climate, azaleas and rhododendrons do not SITE SELECTION have many serious pest problems and a The ability to successfully grow azaleas and scheduled spray program is not necessary. rhododendrons depends on site selection and Powdery mildew may need to be controlled in preparation. years with hot, humid summers. Chlorotic For azaleas, an area with full sun to partial sun is foliage may indicate that the soil is too alkaline preferable although they will tolerate slight and needs further acidification. shade. The evergreen rhododendrons will If proper species and cultivars are chosen, no tolerate more shade than azaleas and need winter protection is needed other than that used protection from winter sun to prevent leaf burn. to discourage rabbits and mice. A cylinder of Azaleas and rhodoendrons are also sensitive to hardware cloth prevents feeding by rodents. extreme heat. Try to avoid selection areas such as an exposed south side of a house where heat is RECOMMENDED SPECIES & CULTIVARS reflected and can build up. Also avoid areas The following list of azaleas and rhododendrons where wind and root competition could be are recommended for Minnesota and Wisconsin problems. landscapes. There are other species and cultivars Because of their very shallow, fibrous root that do survive here, but they are not systems, azaleas and rhododendrons tend to dry recommended due to borderline hardiness, poor out rapidly and will require watering during dry form, or lack of availability. Because flower periods, especially during hot summer days. buds are the least cold hardy part of the plant and With this need for irrigation, a site with good flower show is the primary landscape value, cold drainage is a must. Poor drainage can result in hardiness ratings used in this discussion refer to root rot caused by many species of the fungus flower bud hardiness. Most harsh winters that Phytophthora. will kill flower buds will not injure the Azaleas and rhododendrons require an acid soil vegetative part of the plant and will not affect the for best growth. If your soil is not acidic, the ability of the plant to form flower buds for the addition of acid peat, sulfur, or ferrous sulfate next year. can lower the pH. A pH of 4.0 to 5.5 is optimum. Rhododendron x kosterranum (mollis azaleas). Organic matter should also be added liberally to This hybrid species is often labeled the soil. Compost, manure, sawdust, or acid peat Rhododendron mollis in the nursery trade. Mollis moss will provide necessary organic matter. azaleas are extremely showy, blooming in late May with flower colors in shades of yellows, oranges, and reds. Mollis azaleas are flower bud seedlings, is extremely hardy with mature hardy to –20° to –25° F, and will bloom well height and spread of eight feet. ‘Rosy most years in the Twin City area and south. A Lights’ is extremely floriferous. ‘Rosy particular strain of mollis azalea that has been Lights’ was introduced by the University of selected for hardiness by the University of Minnesota Landscape Arboretum in 1984. Minnesota Landscape Arboretum is sold by 4. Rhododendron ‘White Lights’ (‘White Minnesota and Wisconsin nurseries. At maturity, Lights’ azalea). ‘White Lights’ azalea is a mollis azaleas will have a height and spread of hybrid of Rhododendron prinophyllum approximately six to eight feet. background introduced by the University of Rhododendron mucronulatum (Korean Minnesota Landscape Arboretum in 1984. rhododendron). This is a deciduous Its flower bud hardiness is rated at -35°F. rhododendron with excellent cold hardiness. This cultivar is extremely floriferous Rhododendron mucronulatum is native to Korea, forming many trusses of white flowers with China, and Japan. It blooms in early May and a light yellow blotch. The flower buds do can sometimes be hit by a late frost. The flowers have a pink cast ot them but open to give a are magenta colored and appear before the white effect. ‘White Lights’ reaches a height leaves, giving a bright floral show. A pink and spread of five feet. flowered form of this species, equally hardy, is a 5. Rhododendron ‘PJM’. PJM rhododendron is cultivar called ‘Cornell Pink’. a hybrid resulting from crossing Northern Lights Series of hybrid azaleas. This is Rhododendron carolineanum and a series of hybrid azaleas being developed and Rhododendron dauricum. It is an evergreen released by the University of Minnesota rhododendron that has very attractive small Landscape Arboretum. Any azalea released and dark green leaves and lavender pink flowers. included in this series will have flower bud This cultivar is very hardy, (-35°F) and hardiness of -30° to -45° F to withstand prefers a sandy soil to a clay soil. P.J.M. are Minnesota winters. As the azalea breeding the initials of P.J. Mezitt, the hybridizer of program continues, new selections will become this cultivar. available and will be denoted by a cultivar name 6. Rhododendron prinophyllum (Rose-shell that includes ‘lights’. Current named cultivars azalea). This species is synonymous with include: Rhododendron roseum. This azalea is native 1. Northern Lights F1 hybrid seedlings. These from Maine to Virginia and as far west as azaleas are derived from a cross between Missouri. The flower buds are hardy to - Rhododendron x kosteranum and 40°F. The plant is small (three feet) with Rhododendron prinophyllum, and were first very fragrant white to rose pink flowers that commercially available in 1978. The plants bloom in late May. The plants are heat are extremely floriferous, with each seedling sensitive and do better in slight shade than flowering in a shade of pink, giving a other azaleas. Rhododendron prinophyllum fantastic floral show in late May. The is one of the parents of the Northern Lights flowers have a sweet fragrance. Mature hybrids. height and spread are six to eight feet. 7. Rhododendron vaseyi (Pinkshell azalea). 2. Rhododendron ‘Pink Lights’ (‘Pink Lights’ This species blooms from early to mid-May azalea). ‘Pink Lights” azalea is a clonal in Minnesota before its foliage emerges. selection from the Northern Lights seedling With delicate pale pink flowers and an open azaleas, with light pink flower color. ‘Pink form it fits well into naturalistic settings. Lights’, like the northern Lights seedling, is Rhododendron vaseyi is an extremely hardy hardy to -45°F with mature height and species with flower buds rated at -35°to spread of eight feet. ‘Pink Lights’ has a -40°F. sweet floral scent and is extremely 8. Rhododendron Exbury hybrids. These azalea floriferous. ‘Pink Lights’ was introduced by hybrids are not reliably hardy in Minnesota the University of Minnesota Landscape and Wisconsin as the flower buds are killed Arboretum in 1984. at -15°to -20° F. Other cultivars listed above 3. Rhododendron ‘Rosy Lights’ (‘Rosy Lights’ are more adapted for Minnesota and azalea). ‘Rosy Lights’ azalea is a clonal Wisconsin and should be used instead. selection from the Northern Lights seedling azaleas with deep rosy pink flower color. ‘Rosy Lights’, like the Northern Lights