In praise of PJM rhododendrons

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!