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All About Rhododendrons

Introduction
Rhododendrons are spectacular garden shrubs grown for their clusters of Spring blooming, bell-shaped flowers and attractive evergreen foliage. Rhododendrons are members of the Ericaceae (Heath) family of plants and as such are related to Azaleas, Heathers, Blueberries, Arbutus trees and more. There are hundreds of species and thousands of Rhododendron cultivars available in a diverse range of sizes, shapes and colours. With this much selection, there is at least one rhododendron ideal for every garden!

Rhododendron Classifications
Rhododendrons are broadly classified into 6 main categories. These are Elipidote Rhododendrons, Lepidote Rhododendrons, Species Rhododendrons, Vireya Rhododendrons, Evergreen Azaleas and Deciduous Azaleas. Azaleas are discussed in detail in a seperate document. Vireya Rhododendrons are more tropical and are not usually found in the Pacific Northwest. Species Rhododendrons are prized by collectors and make great additions to a garden. Elipidotes and Lepidotes are both hybrid Rhododendrons and are commonly available.

Elipidote & Lepidote Rhododendrons
Elipidote Rhododendrons are generally referred to as “Large-Leaf Rhododendrons” and are identified by their large, broad, leathery leaves with blunt tips. A key characteristic of Elipidotes is that they do not have scales on the undersides of their leaves. However, the leaf bottoms may be covered in a “wooly” or “hairy” covering called an indumentum. Lepidote Rhododendrons or “Small leaf Rhododendrons”, are identified by their smaller leaf size and tiny scales on the underside of the leaves. Both Elipidote and Lepidote Rhododendrons are evergreen in our area.

Rhododendron Bloom Times
Rhododendrons bloom at different times during the Spring. These times are divided into 5 categories.

  • VE – Very Early Blooming
  • E – Early Blooming
  • M – Mid-Season Blooming
  • LM – Late Mid-Season Blooming
  • L – Late Blooming

    Stagger the bloom times of Rhododendrons in your garden in order to lengthen the display of flowers in your garden.

    Landscape Uses
    The spectacular display of Spring flowers and attractive evergreen leaves make Rhododendrons popular, versatile shrubs that offer multiple seasons of interest. They are excellent for use in shrub borders, foundation plantings and with smaller varieties, rock gardens. Rhododendrons truly shine when planted in groups or in woodland areas that offer filtered shade. Excellent companions for Rhododendrons include spring blooming bulbs, hellebores, hostas, bleeding Hearts, pieris, heathers and japanese maples.

    Cultivation
    Rhododendrons are easy to grow provided you plant them in a good location. While Rhododendrons will tolerate shade as well as the Pacific Northwest sun, they thrive in areas that receive partial or filtered light. Locations near high branched trees at the edge of a woodland, or an area that receives morning sun and afternoon shade are ideal. As Rhododendrons vary greatly in shape and size, make sure you take this into account before selecting a planting location.

    Rhododendrons have shallow, delicate roots that are unable to penetrate rocky or heavy soils. Plant in soils that are porous and coarse enough to allow oxygen to reach the roots. Select a site with moist, well drained, acidic soil that is rich in organic matter. Rhododendrons like an ample supply of moisture but should not be allowed to sit in stagnant water as the submerged roots will rot and die. Good drainage is the key. Test the drainage in an area by digging a hole and filling it with water. If the water drains away relatively quickly then you can plant safely. If the water does not drain away, or drains away very slowly, refill the hole with soil and plant in a mound above the surrounding soil level. You may want to plant in an alternate location or contact a Landscape Professional to help you with an overall drainage plant. Acidic soil with a PH between 5.0-6.0 is a must for Rhododendrons. While most B.C. soils are naturally acidic, you can increase acidity adding agricultural sulfur or ferrous (iron) sulfate to the soil. Another way to ensure that your soil is both acidic and humus rich is to amend the soil with a high quality peat-moss.

    Planting
    Once you have choosen a site, dig a hole twice the width of the Rhododendrons rootball or container. Make this hole the same depth or even slightly shallower than the rootball. For balled and burlapped plants, cut the strings that hold the burlap in place, but do not remove the burlap itself. Place the plant in the hole and refill the hole with soil. For plants in pots or containers, remove the pot, place the plant in the hole and refill the hole with soil.

    In both cases, ensure that the soil level of the plant is the same or even slightly higher than the surrounding soil level. Planting too deep is one of the major causes of plant loss. Firm the soil around the roots. Water thoroughly after planting to soak the soil and assure rapid root growth into the surrounding soil. Do not use bonemeal or mushroom manure with Rhododendrons. Use a liquid transplant fertilizer instead.

    Mulching
    Rhododendrons have delicate, shallow roots that benefit greatly from a generous application of mulch to prevent weeds, preserve moisture and protect them from the Summer heat and Winter cold. Apply a 2-4 inch layer of mulch, but keep it several inches away from the trunk. This protects the plant from insects and disease that may inhabit the mulch. Mound the mulch in a way that enables water to run in towards the center of the plant, not away from it.

    Fertilizing
    Rhododendrons can grow without a lot of added nutrients. During the initial planting, you may wish to use a liquid transplant fertilizer to help jumpstart root growth. Avoid using bonemeal or mushroom manure during planting as they “sweeten” or reduce the acidity of the soil. Thereafter, fertilize the plant at least twice a year: once before they flower in Late Winter / Early Spring, and once after they flower in May / June. Do not fertilize in fall as this will create new growth that may not have time to harden off before freezing temperatures set in.

    Avoid using a general purpose fertilizers on your Rhododendrons. Rather, use a product that has been specifically formulated for acid-loving plants such as Miracid or a Rhododendron granular fertilizer.

    Watering
    Rhododendrons thrive in moist, but well drained conditions. Water twice a week after planting and more often in case of hot weather. Do not allow them to dry out, but do not over-water either. Rhododendrons should never sit in pools of stagnant water.

    Deadheading
    “Deadhead”, or remove spent flowers after they have finished blooming.This redirects the plants energy into producing next years flowers rather than producing seed. Deadheading also reduces the chances of disease or pest infestations and improves the plants general appearance. To deadhead, simply snap off old flowers with your hands or use a pruner.

    Pruning
    Rhododendrons can be pruned as required as long as you understand the way they grow. Rhododendrons produce flowers on last years growth. That is, flower buds for next Spring’s blooms develop during the Summer and Fall. If you prune during this time, you risk removing next year’s flowers. Therefore, the best time to prune Rhododendrons is immediately after they flower. For best appearance and health, prune out weak inside limbs, any limbs that cross or rub and any dead or damaged branches. Rhododendrons can tolerate severe pruning at any time of year as long as you are willing to wait a year or two for them to re-flower.

    Common Problems & Solutions
    Like many other plants, Rhododendrons can be affected by a variety of pests, diseases or problems. Some of the common ones include:

    Light Yellow Leaves with Brownish Burned Areas
    Sunburn. Protect the plant from the sun by giving it more shade or move it to a more protected site

    Leaf Drying or Browning
    Freeze burn from winter temperatures or winds. Give more protection to the plant. Plant behind other plants, buildings or structures to provide shelter. Ensure the plant has a good mulch.

    Green Leaf Veins with Yellow Areas In Between
    This is called Chlorosis and is commonly caused by a lack of iron or soils that are not sufficiently acidic. Increase acidity or add iron to the soil.

    Light Yellow Leaves
    Commonly caused by a lack of nitrogen. Apply a fertilizer designed for acid loving plants.

    Split Bark
    Commonly caused by freezing temperatures while the plant is not dormant.

    Rolled Leaves During the Fall/Winter
    Commonly caused by extreme cold temperatures. The leaves naturally roll to minimize the amount of surface area exposed to the cold. Leaves will return to normal when the temperature rises and moisture returns to the leaves.

    Edges of Leaves have notches
    Small black beetles called Root Weevils eat notch shaped areas on the edges of leaves. Being nocturnal creatures, weevils come out at night to feed. Destroy them by hand at night or use organic nematodes or chemical pesticides to control the problem.

    Rhododendron Shopping List
    To ensure you have everything you need to succeed with Rhododendrons make sure you pick up the following during your visit to Art’s.

     

  • Several Rhododendrons
  • Mulch
  • Fertilizer for Acidic Loving Plants
  • Peat Moss
  • Nematodes to Control Root Weevils
  • Pruners
  • Companion Plants like spring blooming bulbs, hellebores, pieris, japanese maples, bleeding hearts and more!

     

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