Tag: rhododendrons

  • Growing rhododendrons in a cold climate

    Growing rhododendrons in a cold climate

    P.J.M.'-rhodos photo by Stephen Westcott-Gratton
    Widely grown lavender-pink  ‘P.J.M.’ rhododendron (R. [PJM Group] ‘P.J.M.’) (Photo by Stephen Westcott-Gratton)
    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.

  • PJM rhododendrons make gorgeous display

    PJM rhododendrons make gorgeous display

     

    Two PJM rhododendrons blooming in the front garden are making a gorgeous display, with hundreds of lavender-pink flowers stopping traffic on my road. Each spring, people ask what these shrubs are, and I’m pleased to say almost every garden can have some. The small-leaved lepidote rhododendrons in the PJM group are the easiest rhodos to grow. (For a list of cultivars, see my “Making a Garden” blog entry on March 30, 2011.) The secret to their success is their ability to grow in the alkaline soil found in southern Ontario.

    Rhododendrons are ericaceous plants and require acid soil, usually found where granite forms the underlying bedrock. The bedrock in southern Ontario is limestone, and that causes our soil to be alkaline, quite the opposite to what rhododendrons need. Soil acidity or alkalinity is measured on a scale of one to 14, with neutral at seven. Soil registering below seven is acidic, and soil above seven is alkaline. The measurements are referred to as pH (parts hydrogen), and the measurement for my garden soil is pH 7.3, or slightly alkaline. Getting my head around the fine points of pH science would require a degree in soil chemistry, but it’s not hard to understand in simple terms. As a demonstration, distilled water measures neutral (pH 7), baking soda is strongly alkaline (pH 9), and bleach is drastically alkaline (pH 13). Going the other way (below neutral pH 7), black coffee is strongly acidic (pH 5), tomato juice is very acidic (pH 4), and lemon juice is extremely acidic (pH 2).

    As it happens, rhododendrons grow best in soil with acidity in the range of black coffee (pH 5 to pH 6). Acidic soil amendments (like sulphur in pellet or powder form) can be dug into alkaline soil, but their acidity is short-lived and quickly overwhelmed by the surrounding alkaline environment. Consequently, I often see declining rhododendrons— particularly  large leaf elepidote cultivars—with sparse, yellowed foliage and few flowers. They just can’t cope with alkaline soil. Fertilizer might perk them up for a few weeks, but then they return to their sickly state.

    Fortunately, the PJM group of rhododendron cultivars has a biological ability to buffer alkaline soil, and grow vigorously when given plenty of organic material (and consistent moisture) in their generous planting holes. I add lots of peat moss, composted manure and leaves when planting. I also place a three-inch (8-cm) layer of leaves over the root zone in autumn. In spring, they get a drink of water-soluble fertilizer (using a nearly balanced analysis) after they finish flowering to help them grow new twigs and buds for next year’s blooms.

    The PJM shrubs have been in full bloom for almost two weeks, and now two shrubs of ‘Olga Mezitt’ are about to open. These will have vibrant pink flowers, and likely add another two weeks to the display. The shiny foliage continues to be attractive after the flowers are finished, and takes on a deep burgundy colour in cold weather. The two PJM shrubs grow in light shade, and have become leggy (four feet/1.2 m) as they reach for light. This has been an advantage, as one climbs into a long-needled pine, and the other reaches up to window height, allowing the flowers to be visible from indoors. The ‘Olga Mezitt’ shrubs are in direct sunlight and are more compact, with deeper colour in their winter foliage.

     

    Other posts by Judith:

  • In praise of PJM rhododendrons

    PJM rhododendron (Photo by CC-BY-SA-3.0-2.5-2.0-1.0  www.creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons)
    PJM rhododendron (Photo by CC-BY-SA-3.0-2.5-2.0-1.0  www.creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons)

    I’ve fallen into the classic, over-eager gardener’s trap — caught in mid-stride by a surprise snowfall. I was off to an early and industrious start, beginning with the small garden area outside my north-facing front window, where I trimmed a small boxwood hedge (more about that, below) and limbed up a tree to bring in better light for the rhododendrons.

    Rhododendrons are fussy about their soil — they like acidic soil (about pH 5.5), lots of humusy organic materials such as chunky peat moss and leaves, and consistent moisture. I can provide the organic materials and moisture, but my soil is alkaline.

    I’ve solved the problem by growing rhododendron cultivars from the PJM group (officially hardy to Zone 5, but also flowering in Zone 4, which includes Calgary, Alta.), originally developed in Massachusetts at the Weston Nursery by Peter J. Mezitt. They’re a cross between Rhododendron carolinianum and R. dauricum var. sempervirens. The small-leaved evergreen shrubs (with foliage shades of mahogany and plum in autumn) have smaller flowers than standard rhododendrons. They’re midway between azaleas and the largest rhododendrons in size, and most importantly, they’re more tolerant of alkaline soil. When given lavish amounts of organic material in their planting holes (up to 50 percent chunky peat moss) and an annual leaf mulch, they’re healthy and attractive year round. PJM hybrids produce little or no seed, and consequently most energy is directed toward making a great many flowers. My earliest blooming hybrid is ‘P.J.M. Elite’, a tall shrub growing to 48 inches (1.2 m), with a generous show of vivid mauve flowers in mid April. (Some gardeners say ‘P.J.M. Elite’ is garish, but I think it’s gorgeous!) The clear pink flowers on my ‘Olga Mezitt’ shrubs bloom two weeks later, nicely extending the season of display. I’ve also had ‘Aglo’, a more compact shrub with beautiful, warm pink flowers.

    This spring I’ll be prowling nurseries looking for ‘Thunder’, with pink-purple flowers and mahogany winter foliage. There are many PJM hybrids, and others you might see in garden centres this spring are ‘Black Satin’, dark rose-pink blooms, mahogany-black foliage, semi-upright; ‘Checkmate’, lavender-pink, compact and mound-like (the most dwarf form of PJM shrubs); PJM Compact Form, tetraploid flowers and foliage, large, upright and spreading; ‘P.J.M. Regal’, lavender-pink, vigorous, wider spreading and slightly lower than ‘Elite’; and ‘Weston’s Pink Diamond’, double pink flowers, frilled petals, semi-evergreen foliage, upright and spreading. Cultivars with white flowers include ‘April Snow’, ‘P.J.M. White Form’, ‘Molly Fordham’, ‘Weston’s Innocence’ and ‘White Angel’.

    Bags of chunky peat moss — not the commonly found finely milled peat compressed into bales — are sometimes hard to find. Chunky peat is slower to degrade and lasts longer in the planting hole. Rhododendron societies often supply chunky peat to their members or know where to purchase it. It’s fine to substitute baled peat moss if that’s what you’ve got. The only fertilizer I’ve given my rhododendrons is composted manure, applied over the root zone (and under the leaf mulch).

    Societies such as the Rhododendron Society of Canada Niagara Region are wonderful online sources of information about growing these plants, and offer good guidance in soil preparation and fertilizer. Of course, becoming a member of your regional rhododendron society is the best way to learn about growing rhododendrons and getting access to some really terrific plants!

    Boxwood psyllid

    I have a small boxwood hedge under the front window, and over the past two summers the leaves have been off colour and speckled with yellow. Foliage at the tips has been cupping and curling inward, looking almost like little cabbages or rose buds. When I brushed my hand over the hedge, small green flies with transparent wings fluttered out and then resettled. Eventually the hedge began losing foliage.

    The little flies are boxwood psyllids (Psylla buxi), sucking insects that cause the tender new leaves to curl into a cup form. The psyllids overwinter as eggs deposited near the tips of branches. When boxwood buds open in May, the eggs hatch into sucking nymphs and feed on juices from the fresh young leaves. The psyllids reach flying adult stage in late May through early June, and mate to begin the cycle again by laying eggs.

    There is only one generation of boxwood psyllid each year, and that makes it easier to know their schedule and interrupt the cycle. Examine the plants in April for cupped leaves at the tips, which tells you they’ve been infested the previous summer. Trim the plants, removing all the cupped foliage (where psyllid eggs may be deposited), shortening the top and side branches by about two inches (5 cm). Just to be safe, collect and dispose of all the trimmings.

    If later in summer you notice cupped foliage on boxwood tips and see small green flies with transparent wings hiding inside the plants, spray with insecticidal soap or horticultural oil to eliminate the adult psyllid flies, and then trim the hedge lightly againto remove all the cupped sections where eggs may have been laid.

    Thanks for visiting with me at Making a Garden, and hope to see you again next week!

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