Growing rhododendrons in a cold climate

The colour spectrum of my garden widens dramatically when the first rhododendrons begin to flower in various shades of white, pink, orange, red and magenta.

The genus Rhododendron is a huge one with more than 1,000 species and about 28,000 cultivars, but for the majority of Canadians most of these beautiful shrubs are out of reach, due to their lack of cold hardiness and preference for acidic soils. But thanks to the breeding efforts of hybridists in Massachusetts, Minnesota and Finland, even gardeners in Zone 3 can enjoy these vivid springtime blooms by planting cultivars that are designed to withstand our frigid winter conditions.

Only gardeners distinguish between rhododendrons and azaleas — to botanists, they’re all rhododendrons — and in spite of numerous exceptions, by tradition rhododendrons have evergreen leaves and bear flowers with 10 stamens, while azaleas have five stamens and tend to be deciduous. Most cultivars like to be sited in part-shade (or “dappled” shade); leaves often scorch in full sun, and dense shade can result in fewer flowers.

Possibly the most widely grown rhodo in the world — and always the first to flower in my garden — is lavender-pink ‘P.J.M.’ (R. [PJM Group] ‘P.J.M.’). The original cultivar was hybridized at Weston Nurseries, about 50 kilometres northwest of Boston, Massachusetts. Weston Nurseries was founded in 1923 by Latvian immigrant Peter John Mezitt (1885-1968), and ‘P.J.M.’ is the result of the very first rhododendron crosses made by his 24-year-old son Edmund — talk about beginner’s luck! But that was in the summer of 1939, and as ornamentals were moved to “the back 40” to make way for vegetable production during the Second World War, the young rhododendron seedlings were all but forgotten.

Edmund V. Mezitt (1915-86) later recalled a sunny Sunday afternoon toward the end of the war in May, 1945: “My heart still skips a beat when I remember the reaction of our entire family when we saw that ribbon of brilliant pink running across the hill. We named it ‘P.J.M.’ [his father’s initials] right on the spot.”

The result of a cross between the small-leaved Piedmont rhododendron (R. minus var. minus [Carolinianum Group]) and from Siberia, the Dahurian rhodo (R. dauricum [Sempervirens Group]), ‘P.J.M.’ grows about five feet (1.5 m) tall and wide. Hardy to Zone 3, it bears striking magenta-pink flowers above evergreen foliage, and due to its carefree nature, it’s an excellent choice for rhodo newbies.

Following the introduction of ‘P.J.M.’, other plant hybridizers were keen to jump on the hardy rhododendron bandwagon, and in 1957 Albert G. Johnson began a breeding program at the University of Minnesota that would eventually flower into the Northern Lights Series. After Johnson’s death, the program was expanded by Harold Melvin Pellett (1938-2014), and the first Northern Lights cultivars were introduced in 1978. Since then, 15 cultivars have been released, all of them hardy to at least -30°C.

Look for the deciduous azalea ‘Northern Lights’; it produces trusses of up to 12 fragrant pink flowers on leafless stems, and grows about five feet (1.5 m) tall and wide. Hardy to -40°C, it’s derived from a cross between Mollis hybrids (R. ×kosteranum, Belgium 1870s) and our native alkaline-tolerant roseshell azalea (R. prinophyllum).

Another one of my favourites is ‘Mandarin Lights’, a cross between the roseshell azalea and R. ‘Exbury White’ (England, 1920s) which produces fragrant, bright orange flowers and is hardy to -34°C. I scandalized my neighbours by surrounding it with dark purple ‘Negrita’ Triumph tulips. Daring gardeners eager to pull a similar taste-defying stunt should note that rhododendrons have fine, fibrous roots that lie close to the soil surface, so to avoid root damage, complementary — or contrasting! — bulbs should be planted at the same time the rhodos are. Mulch with (in order of preference) pine needles, shredded oak leaves or bark chips to help keep the root run cool and conserve soil moisture.

Meanwhile, across the pond in Finland, a young plant hybridizer named Marjetta Uosukainen initiated a rhododendron breeding program in 1973 at Helsinki University using the Asian R. brachycarpum for cold tolerance. By the time she retired in 2014, Marjatta had introduced a dozen large-leaved cultivars that are now known as the Marjatta (or Finnish) Hybrids.

Look for ‘Helsinki University’ which grows six feet (1.8 m) tall and wide, and bears pink flowers with orange-red flecks that are held above glossy evergreen foliage. Likely the toughest large-leaved rhodo in commerce, it’s hardy to -40°C.

Although PJM Group and Northern Lights Series cultivars are tolerant of slightly alkaline soils, all rhododendrons prefer acidic soils in the range of pH 4.5 to 5.5 (pH 7.0 is neutral). Elemental sulphur pellets and iron-rich, soil-acidifying rhodo fertilizers will help, but where soils are strongly alkaline, I would suggest growing rhodos in raised beds or large containers.