Tips For Growing Healthy Rhododendrons

By Willem Morsink

This section

is an ongoing effort by our society to provide basic growing information for the urban rhododendron gardener. Traditionally we provide TIPS at the Annual Sale, held on the last weekend of April, as well as at our end of September-Member’s Plant Sale and information meeting. At the April sale you will be able to find scores of the advanced members, scattered throughout the Floral Hall giving tips to rhododendron shoppers and newly joined members. “Tips for growing healthy rhododendrons” will be an on-going attempt to bring basic information on growing rhododendrons to the new member.

First of all, rhododendrons and azaleas, which are rhododendrons, are long-lived plants providing that the site selection and soil preparation recreate the conditions that exist where they are found growing in the wild. Rhododendrons do well in the shady well drained locations where you are unable to grow roses. Roses and annual flower beds are best in the sunny well drained locations where the rhododendrons would be unhappy. Rhododendron beds complement your sun-plant-beds, making it possible to utilize most areas and nooks of your garden.

Site Requirements

Super soil drainage is critical; if your soil is heavy (clays and heavy loams) and has poor drainage, you must raise the entire root system above grade by creating a raised mound or planting bed (15 cms or more in depth) using retaining structures such as logs, timbers , granite rocks (not lime stone). A layer of coarse gravel below the organic materials of the raised bed would provide good drainage. Natural slopes facing northerly or north easterly would be ideal.

They need a moist soil medium which is largely organic; a minimum of 50 % acid-peat derived from sphagnum moss, together with organic compost from oak leaves, shredded pine bark, pine needles, decaying wood chunks etc. The organic medium, with a high percentage of humus should hold enough water between rains, but watering once a week, during droughts may be necessary. But never over water; that would encourage soil fungi attacks.

An acid (Ph 5.5) soil medium is a must; soil-acid testing kits are available from the bigger garden centers; iron sulfate and sulfur powder are used to lower the acidity of alkaline (limy) soil medium and the underlying soil strata. Consult with an advanced member grower. Do not use aluminum sulphate; it is toxic.

Light requirements

Shade from the winter sun, from December 15 till April 15, is critical in the Toronto Region, and north; this is especially important when the ground is frozen, and when the sun can scorch the leaves. John Weagle from Halifax, noted that “with careful siting you can locate your rhododendron where it gets spring, summer, and fall sunshine, but misses the winter sun (January – April) entirely, utilizing the shade of buildings, fences, and/or conifers”. Another way is to place coniferous branches over low growing rhodos, or stick branches upright between the rhodos, or use a burlap sun screen, but never cover them totally with burlap. Several members hit the streets in early January, to collect abandoned Christmas trees; these rootless trees are placed between the south and southwest winter sun and the rhododendron plants.

They need some sun to encourage bud formation and compact growth; again” with careful siting you can locate your rhododendron where it gets spring, summer, and fall sunshine, but misses the winter sun (January – April) entirely, utilizing the shade of buildings, fences, and/or conifers” The height of the screen near the plant must be a minimum of 2 and a half times the height of the rhododendron from December till April, in the Toronto region.

Do not plant in the heavy shade below deciduous trees, or below the branches of conifers; heavy shade in the summer deters flower bud growth and results in leggy, stretched branching. Rhododendrons planted below deciduous trees will be exposed to the sun in the winter, and will scorch in the Toronto region. In milder climates of the eastern United States, winter exposure to sun below a deciduous canopy does not cause a problem, because of the milder temperatures and because the soil rarely freezes.

Note that on heavy soils such as clays and clay loams, all trees including pines and oaks are shallow-rooted in the one to two feet soil layers. Roots cannot penetrate the compacted soils below the surface layers. Only on deep sandy soils do oaks and pines grow deep root systems.

Shelter the rhododendrons from prevailing winds, to prevent desiccation, and to prevent snow blowing away from the plants. Evergreen rhododendrons may suffer from winter desiccation that causes symptoms similar to freeze/thaw injuries. Winter desiccation first causes dehydration of the leaves, which may be followed by death of originally green leaf tissue (necrosis), and result in the horrible browning-off of green rhododendron leaves in spring. Winter desiccation may occur over the duration of several winter months. It may also occur after a sunny period when the leaf temperature can be raised 15 degrees C above freezing air temperatures.

On larger country properties opened-up pine plantations on sandy soils provide dappled light for most of the day; care must be taking to provide winter sun shelter for rhododendrons situated on the south and south west boundaries of such plantations.




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The latest newsletter was sent out on Sept 30th, 2018.

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Schedule – 2018

General Meeting

Location: Toronto Botanical Garden
Date: Sunday November 4th, 2018
Time: 1 pm - 4 pm
(free admission, free parking)