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Rhododendrons and Azaleas

Rhododendron and Azalea

General Description 

When you hear rhododendrons and azaleas what probably comes to mind is visions of Butchart Gardens in balmy BC.  But did you know that here in Haliburton we have the right conditions for growing them?  We have acidic soil, lots of pine needles for mulch, lakes to provide plenty of water and many trees to produce the dappled shade or 4 - 6 hours of sun that they need.  Growers have now developed Zone 4 hardy varieties.  Rhododendrons and azaleas are mainstays of late spring because of their spectacular clusters of mauve, pink, purple, yellow and orange showy blooms and large green leaves that often last through the winter.   The flowers are usually tubular, funnel or bell shaped.  But they are fussy.  Leaves of the smaller azaleas are usually pointed and narrow while leaves of the rhododendrons are usually large and leathery.  A better distinction is that rhododendrons are broadleaf evergreens while most azaleas are deciduous, losing their leaves each fall.

Rhododendrons are part of the family Ericacea and the genus Rhododendrons.  The genus Rhododendron is divided into the subgenera categories of rhododendron and azalea, of which there are more than 1,000 species. The genus is spelled with a capital R and the subgenus is spelled with a small r.  The azaleas are distinguished from rhododendrons by having only 2 small anthers per flower.  The genus Rhododendron is characterized by shrubs and small to (rarely) large trees.  The smallest grows to 100 cm and the largest to 30 m.  Leaves are spirally arranged and leaf size can range from 1 cm to over 50 cm and exceptionally to 100 cm.  They may be either evergreen or deciduous.  In some species of rhododendrons the undersides are covered with scales (lepidote) or hair (indumentum).  These scales are unique to rhododendron.  Species of the genus Rhododendron are widely distributed - in Asia, Europe, North America.  


The genus Rhododendron and the family of Ericacea appeared about 65 million years ago in North America.  This is interesting because it indicates they did not originate in the mountains of South East China and the archipelago of islands between Asia and Australia, where they proliferate now. Species of the genus Rhododendron are widely distributed – in North America, Europe, Russia, Australia and the Solomon Islands.

There have been many attempts historically to group the various types of Rhododendrons.  In 1996,  Chamberlain brought the species together, dividing 1,025 into 8 subgenera.  Now this genus has been progressively subdivided into a hierarchy of subgenus and species.  Rhododendrons have 8 subgenera based on morphology—namely the presence of scales, deciduous leaves and the floral and vegetative branching patterns.  The first two subgenera (rhododendron and hymenanthus) represent the species commonly considered as rhododendrons.  The rhododendron subgenus is the largest, containing nearly half of all known species and all of the lepidote species.  The next two smaller subgenera (pentathera and tsutsusi) represent the azaleas.  There are 16 species of azaleas native to North America.

Rhododendrons for Cold Climates

Hybrids of both rhododendrons and azaleas have been developed to live in Zone 4.    However they may need winter protection in our area.  The azaleas include the Northern Lights series bred at the University of Minnesota and have names like Orchid Lights, Rosy Lights, Mandarin Lights, Lemon Lights, Spicy Lights, etc (hand out).  Rosy Lights is said to be the hardiest azalea, followed by Orchid Lights.   In general, azaleas grow very well if they receive 4 hours or more of bright sun per day.  Although most azaleas like some sun or dappled shade in order to bloom, there are some, like the “Maid in the Shade” series that will grow in dense shade.  On the other hand, if shade is too dense, it can cause fewer blooms and spindly growth.

The hardiest rhododendrons are the PJM series, an introduction from Weston Nurseries in Massachusetts. The PJM series include Elite, Victor, Compact (which we have ), etc. and do well in zone 4a and with extra winter care Zone 3. Gardeners in Haliburton County report that the PJM series thrive here. 

A new series of very hardy evergreen rhododendrons have also been developed in Finland at the University of Helsinki.  The Finnish plants should have some sun at midday and include Catawba and Aglo . 

Planting and Care

Rhododendrons and azaleas must be planted in acidic soil that is highly organic and supremely well drained.  The organic soil requirement is based on their origin in the forest.  They require regular watering but must be well drained. So don’t plant them in clay soil.  Dig a hole 1.5 to 2 feet deep and fill with water.  If it takes longer than 10 or 15 minutes to drain, the drainage is insufficient.  They also despise excessive dryness.  The roots require lots of oxygen. Sound fussy?  I told you so.

Formula for soil is 2 parts peat moss, 1 part composted manure and 1 part topsoil.  You must dig out a fairly large hole, remove the existing soil, (don’t put it back in the hole) and replace it with the new mixture.  Be sure you don’t plant it too deep or the roots will rot.  It should be even with the soil level or slightly above.  Then add your mulch generously, but don’t let it touch the base of the plant. 

In cold zones, like ours, follow a general rule of planting in enough sun (minimum of 4 hours, preferably in the afternoon) to increase flowers and avoid mildew.  Plant on the lee side of a wind break.  Some growers advocate a site sloping to the north or east, protected from wind.  They perform best with moisture and shelter under trees.  Some breeders suggest that large leaved varieties particularly need dappled sun.  Dense
 shade and poor air circulation can be deadly to Rhododendrons.  The combination frequently results in fungus which attacks new growth and eats away at the plant branch by branch.  Do not plant them near the lake, as they do not like cold winds.  Try a more protected area.

The American Rhododendron Society warns do not fertilize after June 30.  Water until the first hard frost.  Block from drying winds with burlap wraps, pine boughs, or build a snow shelter.   

Azaleas and rhododendrons have shallow root systems and need moisture and mulch to keep them from drying out. Be careful when digging around the roots.  Buy plants that are deep green, not yellowed or wilted and well watered.  To prune, just gently pinch off their faded flowers so as not to remove next years emerging buds.  But Rhododendrons do not really require pruning.

Causes of Death of Rhododendrons

  1. Excessive water. These plants need good drainage and excessive clay in the soil holds the water.
  2. Lack of water.  Rhododendrons need to be watered frequently since the roots are near the surface.  Water established plants twice a week and new plantings more often.  Fertilize sparingly and usually just before blooms appear.  Use compost rather than manure.
  3. Excessive application of fertilizer particularly at the base of the trunk of the plant.
  4. Planting too deep.  The plant needs to be placed at soil level or slightly above.
  5. Cold winter temperatures.  Begin your garden of Rhododendrons with the hardiest plants.
  6. Fungus diseases cause die back.

Companion Planting 

Although we suggest that rhododendrons be planted in groups of specially prepared soil, there are a number of companion plants that like similar conditions.  Some of these are: heaths, heathers, pieris, Mountain Laurel, Bearberry, Wintergreen, and Blueberries.  One local gardener plants her rhododendrons with hostas, astilbes, ferns, and then uses heucheras for edging.  It makes for a marvellous scene.

Possible companion trees include pine and red oak.  Do not plant near birches or firs or any other tree that has spreading roots, rather than a tap root.  The surface roots of Rhododendrons and these trees cause competition and weakens the Rhododendrons.


Some Rhododendrons are poisonous to grazing animals.  Honey from bees that feed on some types of flowers is also toxic.  However, pharmacological studies have found some evidence that rhododendrons may have anti-inflammatory value.  Some historical evidence suggests that azalea consumption has hallucinatory and laxative effects.  Lastly, Labrador tea can be made from the Rhododendron groenlandicum plant or Labrador tea plant which is native. 


Rhododendrons and azaleas can be purchased from Johnston’s Greenhouses, Humber Nurseries, Sheridan Nurseries, Corn Hill Nurseries, and locally at Carey’s, Pine Reflections, and Country Rose. Most of the local nurseries will order particular Rhododendrons for you.  And, if you are lucky, grocery stores like the Independent, and Canadian Tire or Home Depot, may have them for sale.  

A list of some of the hardy Rhododendrons and Azaleas can be downloaded below

Rosy Lights Azalea     

Rosy Lights Azalea                                                                                                        Orchid Lights Azalea


Helsinki University Rhododendron                                                                           Mandarin Lights Azalea


Dinah Wilson,
Sep 10, 2014, 10:06 AM