Rhododendron 101

Updated: Jul 13, 2021

I began my gardening career over 40 years ago working on a local estate with an extensive rhododendron collection. There was an impressive hedge of purple-flowering Rhododendron ponticum lining the roadside, as well as impeccable specimens of such cultivars as ‘Blue Peter’ and ‘Moonstone’ which are next to impossible to find today. While I hated the tedious (and very sticky) job of deadheading the spent flowers, that still didn’t detract from their eye-catching beauty when in full bloom, which is why most of us have a rhododendron or two in our landscapes.


Small but Beautiful


With the average urban yard diminishing in size due to land costs, so too does the demand for smaller rhododendrons increase. Two species worth looking for here are Rhododendron impeditum (purple 18” tall) and R. campylogynum (dwarf form) with its unique bell-shaped plum-coloured blooms and citronella-scented foliage. ‘Patty Bee’ (2’ tall) brings us pale yellow blooms that literally smother the plant canopy, while ‘Ginny Gee’ (2’) has a similar flowering display, but does so with white blooms edged delicately in pink. ‘Carmen’ (18”) nearly outdoes all of her rivals with waxy deep red blooms that emerge from glossy burgundy buds reminiscent of a pleated lipstick.


Mid-Sized Marvels


Once we move up to the 3-4’ height range, we begin to see a little more diversity in flower colour, such as the pendulous pink-edged orange bells of the aptly named ‘Medusa’. ‘Polarnacht’ (‘Polar Night’) brings us some of the darkest purple blooms available, which almost seem to draw light in like a black hole. ‘Kalinka’ is a R. yakushimanum hybrid that smothers itself in ruffled bright pink trusses that literally envelop the leaf canopy in May. For much earlier blooms, look to ‘PJM Compacta’ (4’), a very hardy rhododendron (zone 4) with lavender-pink blossoms over burgundy foliage (winter colour), often opening in late March or early April. ‘Seaview Sunset’ (3-4’) puts on a dazzling display with reddish-orange buds that open to ruffled yellow blossoms with pink edges that are lightly scented. Last but not least is ‘Bob’s Blue’ (3’), an upright rhododendron with diminutive foliage that bears spherical trusses of small violet-blue flowers that often cover the entirety of the bush.


Tall and Rare


Many rhododendrons are getting nearly impossible to find due to the fact that most of our smaller wholesale growers have folded and those few who continue with this slow crop, cultivate a very limited variety. These tend to be more compact cultivars as the taller forms such as the beautifully purple-blotched ‘Sappho’ (10’) or even the fragrant ‘Polar Bear’ (12’) or ‘Van Nes Sensation’ are not popular enough to warrant production. Even once commonly found rhododendrons such as ‘Grace Seabrook’ (frilled red trusses) or ‘Fantastica’ (red-pink bicolour) are rare finds these days and a few of my personal favourites such as ‘Elizabeth Ostbo Red’ (burgundy new growth) or ‘President Roosevelt’ (gold-inset foliage with two-tone flowers) seem to have become nearly obsolete. So, when shopping for rhododendrons I would advise keeping an open mind instead of endlessly searching for a variety that you once saw in an old gardening book.


Troubleshooting Rhododendrons: Symptoms & Solutions


This is also the time of year when most people come in to ask me about their rhododendron problems, so I thought I’d help you out with a few simple solutions.

 
Bud Blast

Dead Flower Buds / Bud Blast


Flower buds discolor in autumn becoming brown and dry by spring, and are often covered with black bristle-like fungal spores. This damage can be cultural (drought, sunscald or severe cold) or be spread by leafhoppers, who lay their eggs in the buds. Monitor for leafhoppers (using yellow sticky traps) from spring to late summer and spray with an organic pesticide (Trounce) for control. Mitigate the damage by simply removing the affected buds by hand and disposing of them in the municipal green waste (not your compost pile).

 

Notched Leaf Margins / Black Vine Weevil


This is a nocturnal snout-nosed beetle that forages on foliage once they emerge from the soil in June. The larval stage also feeds on roots and the bark at the soil line. This pest forages other broadleaf evergreens such as Viburnum, Azalea, Kalmia, and Salal. Your best control is to apply nematodes (see image) to the soil in May or at the end of summer. Other options for solitary specimens with a single stem are to use a band of Tanglefoot or apply diatomaceous earth.

 

Pale Leaves with Darker Green Veins / Chlorosis


With acid-loving rhododendrons, this is almost always a symptom of a high pH, which makes iron and manganese unavailable, thus yellowing the foliage. This is usually the result of planting too close to fresh concrete (which leaches lime and raises the pH), allowing the lawn lime to be spread under adjacent acid-loving plants or mulching with mushroom manure, which is alkaline. Lower the pH to 4.5-6 using aluminum sulfate and always use a dedicated rhododendron fertilizer which is good for all acid-loving or ericaceous plants.

 
Azalea Leaf Gall

Powdery White Leaf Growths / Azalea Leaf Gall


Although more common in azaleas, this fungus can also be found in some species of rhododendrons and exhibits fleshy or waxy swollen leaves that often turn whitish or pink. These galls can be removed by hand and disposed of in the municipal waste system (not your compost) but you can deter this fungus by avoiding overhead watering (particularly at night) and spraying affected bushes with fixed copper just before spring bud break and 2-3 weeks later.



 
Lace Bug

Speckled Foliage / Lace Bug


The yellow-speckled green leaves are an indicator of insects sucking on the underside of the foliage: either lacebug or leafhopper. Your best control is to monitor for both of these pests starting in late spring and manage by either dislodging with a strong spray of water from the hose or an application of an organic pesticide such as Trounce, but with the latter, you will need to apply to the underside of the foliage for it to be effective.

 
Rhododendron Droop