Rhododendrons are shrubs with an astonishing diversity of colors and sizes that we’d do well to use in our landscaping.
Azaleas and rhododendrons belong to the same family, Ericaceae. In the 19th century, the two genera were merged into a single one: Rhododendron.
Aperçu
Rhododendrons offer breathtaking blooms that dazzle us every year. By selecting a hardy rhododendron and a location suited to its needs, this shrub will be maintenance-free and is sure to become a garden favorite.
There is great variety in the genus, with variations in flower color, size and either deciduous or evergreen leaves.
Today, there are tens of thousands of rhododendron varieties. However, for Quebec and Canada, it’s important to choose rhododendrons that have demonstrated their hardiness, to fully appreciate their cultivation.
Le saviez-vous?
- Rhododendrons range in height from 20 cm (8″) to several meters (over 15 feet), depending on the variety selected;
- Hardy rhododendrons for northern climates (zone 4 and up, 3 USDA) are winter hardy without winter protection;
- Rhododendrons can provide dazzling spring blooms in May and June;
- The flowers of some rhododendrons fill the garden with an extraordinary fragrance.
Types of Hardy Rhododendrons in Northern Climates
Most rhododendrons on the market today are almost exclusively cultivars derived from complex hybridizations of several species.
For hardy rhododendrons, you can buy deciduous rhododendrons called azaleas and rhododendrons with small or large evergreen leaves.
Deciduous Rhododendrons or Azaleas
The history of hybridization has been marked by a number of figures, including Dr. Harold Pellett and his team at the University of Minnesota, who developed the Lights series of azaleas, which includes hardy and very hardy cultivars.
Rhododendrons à petites ou grandes feuilles persistantes
To obtain beautiful rhododendrons with small or large evergreen leaves, the planting site must be protected from winter winds. The presence of a conifer hedge to the west, to cut off prevailing winds, is an undeniable advantage.
Several cultivars in the well-known and much-loved ‘PJM’ (Peter John Mezitt) series are undoubtedly the best-sellers in North America. In addition to withstanding harsh winters in zone 4 (zone 3 USDA), they flower early.
As for the large-leaf evergreen rhododendrons in the Finnish hybrid series, they are also very hardy and can withstand temperatures approaching -30°C (-22°F).
The Secrets of Success
- an acid soil with a pH ranging from 4.5 to 6;
- organic soil that drains well but remains cool;
- a location protected from prevailing winds and the midday sun.
Rhododendrons don’t like heavy, clayey or excessively draining, dry soils. In these cases, as their root system is not very extensive, this makes it possible to create raised beds with soil suited to their growth.
Using a potting soil for acid-loving plants at planting helps to correct soil acidity.
It takes three to four years for a rhododendron to establish itself, and it must not run out of water during this period. Mulches keep the soil cool and ensure a good level of humidity, which it needs for its first few years, as well as limiting weed growth.
To encourage abundant flowering, it is beneficial to place rhododendrons in morning sun under light shade.
Hardy rhododendrons are long-lived shrubs that bloom more and more over the years, and are generally maintenance-free.
Insects and Diseases
Rhododendrons don’t have many enemies, although white-tailed deer sometimes eat the flower buds in winter.
At leaf-out, caterpillars can nibble new shoots on deciduous rhododendrons, but manual control will usually resolve the situation.
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