AMES, Iowa – Azaleas and rhododendrons are spectacular flowering shrubs that can brighten any yard or garden. Even though there are many species of azaleas and rhododendrons, only a small number can successfully be grown in Iowa.
Here are some tips from horticulturists with Iowa State University Extension and Outreach showing what steps to take to ensure successfully grown azaleas and rhododendrons. To have additional questions answered, contact the ISU Hortline at firstname.lastname@example.org or call 515-294-3108.
Can azaleas and rhododendrons be successfully grown in Iowa?
There are more than 900 species and innumerable cultivars of azaleas and rhododendrons, but only a small number that can perform well in Iowa. To be successful, the Iowa gardener must select cold hardy cultivars, choose suitable planting sites and follow recommended planting procedures.
Azaleas and rhododendrons are members of the genus Rhododendron. Deciduous members of the genus are commonly called “azaleas” while the evergreen species are referred to as “rhododendrons.”
What are suitable planting sites for azaleas and rhododendrons?
Proper site selection is important when planting azaleas and rhododendrons. Azaleas prefer partial to full sun, while rhododendrons perform best in partially shaded sites. Areas that receive morning sun and afternoon shade are usually good sites for rhododendrons. It is best to avoid windy, exposed sites. During the winter months, strong winds and bright sunlight can dry out rhododendron foliage and cause extensive leaf burn.
Azaleas and rhododendrons require well-drained, acidic soils. The preferred soil pH for azaleas and rhododendrons is 4.0 to 5.5. However, the pH of most garden soils in Iowa ranges from 6.5 to 7.5. The soil pH needs to be lowered to successfully grow azaleas and rhododendrons. Planting azaleas and rhododendrons in wet, poorly drained soil usually results in their death. Gardeners with poorly drained sites should build beams or raised beds to insure good drainage.
How do I lower the soil pH for azaleas and rhododendrons?
Home gardeners can lower their soil pH by adding sphagnum peat moss to the soil. The pH of sphagnum peat moss generally ranges from 3.0 to 4.5. To do this, dig a wide, shallow hole and backfill it with a mixture that is half soil and half moistened sphagnum peat moss.
After planting, mulch azaleas and rhododendrons with wood chips, pine needles or shredded oak leaves. These materials are mildly acidic and help maintain the desired soil pH. Additionally, sprinkle a small amount of sulfur around plants on an annual basis.
Which azaleas can be successfully grown in Iowa?
Azaleas in the Northern Lights Series possess excellent cold hardiness and perform well in Iowa. The Northern Lights Series is a group of hybrid azaleas developed and released by the University of Minnesota. All cultivars in the series have flower bud hardiness of -30 to -40 degrees Fahrenheit and the word “lights” in their name.
Available cultivars include: Candy Lights™, light pink flowers with pale yellow streaks, ‘Golden Lights’, gold flowers, ‘Lemon Lights’, light yellow flowers, ‘Mandarin Lights’, mandarin orange flowers, ‘Northern Hi-Lights’, creamy white flowers with yellow upper petal, ‘Orchid Lights’, orchid flowers, ‘Rosy Lights’, deep rose pink flowers, ‘Spicy Lights’, salmon orange flowers, Tri Lights™, soft pink flowers with white overtones and yellow blotch, and ‘White Lights’, white flowers.
Which rhododendrons can be successfully grown in Iowa?
The most widely grown rhododendrons in Iowa are the Weston hybrids. Introduced by the Weston Nursery in Massachusetts, these cultivars possess excellent cold hardiness. Weston hybrids include: ‘PJM’, lavender pink flowers, ‘PJM Compact’, compact plant, lavender pink flowers, ‘PJM Elite’, blossoms are slightly more pink, ‘Algo’, light pink flowers with dark pink throats, and ‘Olga Mezitt’, bright pink flowers.
The Marjatta hybrid rhododendrons are another group of hardy rhododendrons. Introduced by the University of Helsinki in Finland, cultivars include: ‘Elvirra’, bright red flowers, ‘Haaga’, rosy pink flowers, ‘Hellikki’, violet red flowers, ‘Helsinki University’, shell pink flowers, and ‘Mikkeli’, white flowers.