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The Rolla Daily News - Rolla, MO
  • Nature's Advocate: Creating the right environment for rhododendrons in Missouri

  • I have many fond memories of hiking the Ramsey Cascade trail in the Great Smokey Mountains National Park of Tennessee, admiring massive 300-plus-year-old tulip poplar trees towering 100 feet over the dense undergrowth of great rosebay rhododendrons (Rhododendron maximum).
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  • I have many fond memories of hiking the Ramsey Cascade trail in the Great Smokey Mountains National Park of Tennessee, admiring massive 300-plus-year-old tulip poplar trees towering 100 feet over the dense undergrowth of great rosebay rhododendrons (Rhododendron maximum).
    When in full bloom the Great Smokey Mountains National Park becomes covered in a magical display of this native shrub. Anyone who has seen this display cannot help but wish that such beautiful plants could be successfully grown in the Missouri climate.
    Many gardeners in Missouri can attest to having tried to grow rhododendrons with little success. Growing rhododendrons in Missouri can be a difficult task, but with a little pre-planning, it is not impossible to include beautiful evergreen rhododendrons in your home woodland garden. If given the right spot, soil amendments, drainage and plenty of water, rhododendrons can do well in the Ozark environment.
    It is highly recommended to plan in advance for the incorporation of rhododendrons and azaleas in a planting site. Once a site is chosen and prepared, a cold hardy rhododendron hybrid of your choice, such as rhododendron ‘roseum elegans’ can be purchased.
    First, evergreen hybrid rhododendrons prefer growing in cooler and milder climates than can be found in Missouri. Therefore, the north side of a house or a north or east facing location should be chosen.
    One of the most common problems with growing rhododendrons in Missouri is leaf and stem dieback during the winter due to the plants having a difficult time taking up water during, cold, bright, windy winter months. It is common to provide rhododendrons with a wind break, a house wall or fence to prevent harsh drying winds during the winter.
    When living in St. Louis, I grew rhododendrons underneath an eastern white pine in a cool location facing northeast.
    A wooden fence windbreak was constructed on the southwestern side of the planting to prevent winter dieback of branches. Providing rhododendrons with wind protection during the winter is critical.
    Choose a site that receives direct morning sun only and is well shaded by afternoon. Rhododendrons grow best in very light open shade.
    Site locations facing south or southwest can receive too intense sun both in the summer causing leaf scorch and in the winter causing leaves to dry out eventually leading to branch dieback.
    To successfully grow rhododendrons, the soil must be acidic with a pH of 4.5-5.5. The site chosen should be tested to determine the current pH of the soil indicating a need for sulfur or iron sulfate amendment to lower the pH. Since sulfur can take time to alter the pH of the soil, it is recommended to prepare a rhododendron garden site months before planting to allow time for the pH to be lowered.
    It is beneficial to plant rhododendrons in raised beds or on a slope to provide adequate drainage. Rhododendron roots cannot take heavy clay soils, which when waterlogged can cause root rot.
    Page 2 of 2 - The Missouri Botanical Garden advises adding soil amendments to provide adequate aeration and increase drainage. Their recommended soil composition is 50 percent organic matter, 25 percent sand and 25 percent soil from the site.
    I recommend adding peat moss to the soil to provide a naturally acidic material which will help improve drainage. When adding peat moss to the soil, make sure that the peat moss is either pre-moistened or plenty of time is allowed for the peat moss to soak up water before the rhododendrons are planted.
    From experience, although peat moss holds water well it naturally repels water when initially hydrated. Simply watering the ground after peat moss has been incorporated into the soil is not sufficient to thoroughly saturate the soil.
    Rhododendrons prefer a cool moist root zone and will need supplemental water during the summer due to their shallow root system, and during the first few growing seasons after planting, may need multiple waterings a week to maintain moisture in the top 4 inches of soil.
    As long as the bed is well drained with plenty of organic matter, overwatering should not be a problem. To prevent rhododendrons from suffering from lack of moisture in the winter, supplemental water is important in November. Mulch with slightly acidic oak leaf mold, pine needles or hardwood mulch to keep roots cool and moist.
    For more information on growing rhododendrons and azaleas in Missouri see MU-Extension guide #G6825 at http://extension.missouri.edu/p/G6825.
    With a little site preparation, you can have success in growing rhododendrons in Missouri. Their colorful spring display and evergreen foliage is well worth the extra effort.
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