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Rhododendron Guide

Rhododendron List 2007

Rhododendrons and Azaleas

To many people, the word “Rhododendron“ is synonymous with a large, dense bush with rather uninteresting green leaves, that flowers in spring or early summer. While some do fit this stereotype, the genus Rhododendron is in fact one of the largest in the plant kingdom. With more than 800 distinct species from four continents, they range from the large tree form up to 80 ft. to the tiny alpines that creep along the ground with leaves ranging from ¼” to 3 ft. in length. Rhododendrons have flowers of almost every imaginable colour, some spotted while others have bold flares in their throats. The flowers are in groups called trusses and some are large and full, some are cone shaped and others are very informal. Some trusses have over 30 flowers in them while others have only one or two. The foliage also has its own distinct shape and size according to the variety, a good aspect for the effects of foliage in the garden.
Azaleas also form a part of the genus Rhododendron. They also come in a variety of shapes, sizes, and colours with many evergreen varieties as well as the Exbury and Knaphill deciduous varieties with their bright yellows and oranges and reds.

Growth and Size

One of the questions most often asked about Rhodos and Azaleas is how tall will they grow. Most people now prefer Rhodos and Azaleas, which tend to have more compact growth habits then those of years past. One of the main things of note when it comes to size is that the location in which the plants are placed plays a large factor in the overall size and growth of the plants. Rhodos and Azaleas tend to grow much larger in shady areas then they will in bright hot sunny areas so this should be taken into account when deciding on the type of Rhodo to plant. Rhodos will tend to grow larger than Azaleas some eventually becoming small trees. Rhodos are usually grouped into three groups in regards to size –
a) Small – up to 2 feet in height
b) Medium – those growing from 2 feet to 4 feet in height
c) Tall – those plants growing 4 feet and taller.

The more vigorous the plant the larger it will grow and the longer it may take to begin flowering. This is the reason why many of the dwarf plants you find in the garden centers are full of flowers but many of the larger growing varieties have very few blooms at the same age.
One other point worth remembering here too, is the fact that Rhodos and Azaleas will tend to spread at least as much as they grow upward, so a five foot tall plant may eventually spread 6 to 8’ wide.

Soil Conditions:

Since Rhodos and Azaleas have extremely long lives one must make sure to provide a proper growing medium in which the plants can grow and thrive. The best plants in the world will do poorly if planted in heavy, poorly drained soil. Since it is much easier to amend the soil before the plants are established one must first recognize what soil type Rhodos and Azaleas require to perform at their peak. These plants are surface rooters for the most part, requiring large amounts of air at their root system plus adequate moisture during the hottest part of the summer. The simplest way to achieve both these requirements is to make sure your soil has plenty of organic matter. This can be achieved by adding compost, well-rotted manures, peat moss or finely ground bark. The addition of any of the above items will help loosen up the soil, allow more air to enter, and will help reduce evaporation during hot weather. Most Rhodos and Azaleas prefer an acidic soil and this can be maintained by adding compost which will tends to be somewhat acidic.
One of the only soil amendments not recommended for these plants is mushroom manure which tends to be high in calcium which will raise soil pH past the point which benefits the plants. Large amounts of calcium will cause black or brown burn marks on the leaves of the affected plants.
If you have inadvertently placed mushroom manure in your planting beds the excess calcium can be made insoluble by adding aluminum sulfate to the soil on a regular basis.
Make sure to raise your planting beds well above the water table since these surface rooters require superb drainage in the winter and early spring months.
Most Rhodos require a pH in the range of 5.0 to 6.0, which is the typical soil pH within the Fraser Valley.

Planting Conditions and Techniques

The transplanting of Rhodos and Azaleas can continue almost throughout the entire year, even when in full flower with a little care. It is always best however to do most transplanting in early spring if possible. When transplanting make sure to provide the proper soil conditions first and then worry about the proper timing later. If the soil in which the plants are to be moved is adequate then the timing of the move is not that critical. When transplanting Rhodos from within ones own garden it is best to dig a wide shallow root ball since the roost system is very fibrous extending down only 8 to 12 inches but extending out to the edge of the drip line sometimes making a root ball up to 5 feet in width. Rhododendrons must not be planted to deeply since they are a shallow rooting plant.
The same rules apply to Azaleas.
When transplanting: make sure to cover the freshly exposed roots with a layer of composted bark one to two inches thick. A light application of phosphorous such as rock phosphate or bone meal will help establish a strong healthy root system.
If Rhodos and Azaleas are planted properly then they will establish quickly and prosper with little or no care. A light application of fertilizer during the spring months will also benefit the plants providing extra nutrients to help encourage growth and flower production. Fertilizer used should be a compound fertilizer with not too much nitrogen and feeding should be done only in the spring and midsummer (formula 10-8-6).

Watering

Rhodos and Azaleas all require adequate quantities of water especially during the late spring when most of the growth is taking place. This is the most critical time for transplanting, making sure to maintain adequate water during the first season of active growth. One of the simplest ways of determining dryness is the dropping of leaves, which occurs at the first sign of dryness. Since Rhodos exhibit this easily without long-term damage to the plants it is easy to rectify the situation before it can become lethal. One would think this would occur only during the hot summer months but it can also happen just as easily during the cold dry winter months. If plants are located in areas where there is little rainfall during the winter months it may be necessary to irrigate during the winter months. Make sure to soak the plants thoroughly when watering to ensure that the moisture reaches the entire depth and width of the root system. Deep weekly waterings are always preferable to light daily watering. The most efficient way of keeping the root system moist throughout the year is by regular mulching to help preserve soil moisture.
The fibrous root system of the Rhododendron needs fairly constant moisture in the growing season. The competition of other plants’ deeper root systems prevents the rhododendron from getting the water they need. Mulching is an important factor for the health of the plant, keeping the roots moist and insulated in the winter months, using peat or soil rich in organic matter is ideal for Rhododendrons.

Mulching

The biggest secret to large, happy, healthy plants is adequate mulching to help provide a moist, nutrient rich growing medium in which the plants can thrive. When this organic matter breaks down it releases nutrients to the plants, which not only helps feed and nurture them but also provides a buffer from excess fertilizer, moisture and heat. Almost any Rhodo or Azalea can be perked up by a liberal application of mulch. The larger the plant the thicker the application of mulch can be. Ideally for young plants a layer of approximately 1 to 2 inches is sufficient whereas a layer of up to 3 to 4 inches can be applied for larger three to four foot high tall plants.
Adequate materials suitable for mulching can include composted bark, garden compost, compost shredded leaves or peat moss.

Pruning

Although most gardeners do not prune many of their Rhodos and Azaleas this does not mean that they cannot be trimmed to maintain their shape and flower potential. Rhodos and Azaleas respond well to pruning in much the same way that laurels do. They can be kept compact and bushier with regular pruning, resulting in a bushier plant with greater flower production. Large overgrown plants can be brought down in size by hard pruning. Those Rhodos and Azaleas which have overgrown their usefulness can again retain their usefulness by the simple act of pruning. When reducing down large plants to a more manageable size it is best to do this procedure over several years.
Most pruning should be done in the early spring months of March and April right after blooming has finished. Many Rhodos and Azaleas can be deadheaded right after flowering by simply giving them a light pruning.
It is important to note that when large plants are severely pruned that they may either flower lightly or not at all for several seasons as the root system and foliage again start to balance each other. Normal flowering will return after several seasons.

Deadheading:

Removing dead flowers from Rhododendrons is a time-consuming chore, but worthwhile. After the Rhododendron is pollinated and has finished blooming, its energy is then put into producing seedpods. Some Rhododendrons produce so much seed that very little new growth is produced if they are not deadheaded. If not all flowers can be deadheaded, concentrate on the newest growth.

Pest, Diseases and Disorders:

Fortunately there are few serious pests of Rhododendrons and Azaleas. Aphids, lace bugs and weevils (the most common) are among the few, as well as some infrequent problems with whitefly. Weevil damage appears as irregular, chewed notches at the leaf edges and can be avoided by cleaning up leaf litter at the base of the shrubs to remove hiding places. A product such as Tanglefoot can also be helpful in stopping the nocturnal insects from feeding on the leaves.

Diseases:

Galls: Ugly, green, pink or red swellings generally found at the ends of leaves. More commonly found with Azaleas. Pick off and destroy the galls. Spray with a copper fungicide as a preventive.

Leaf spot: Brown or black spots on foliage caused by too damp conditions. Usually harmless, but using a fungicide such as Benomyl should help severe cases.

Powdery Mildew: The first symptoms are light green or yellowish rings on the upper leaf surface. In a short time these appear on the under side as spots or patches of whitish powder. Spray a fungicide such as Benomyl to help.

Root Rot/Wilt: (Phytophthora): Poor drainage and excess water are the usual causes of this infection. This is a lethal disease causing part of or whole plants to wilt and die. There is no known cure other than good drainage in wet areas and fertilizer to maintain healthy plants.

Chlorosis: Yellowing of leaves usually caused by soil problems such as alkaline soil, excess water, and planting to deeply or a lack of fertilizer. If planted correctly and given good drainage with acidic soil conditions, the cause will most likely be due to a mineral deficiency such as iron or magnesium.

NOTE: As with any disease problems, when the damaged plant material is removed, do not add it to your compost. This will harbor harmful spores and diseases that will be spread to wherever the compost is used in the future.

Rhododendron List

Rhododendron	Available Sizes	Colour	Height in 10yrs	Bloom Time	Plant Characteristics
Anna Rose Whitney	36/42	Pink	6'	Late mid	Upright Habit, flat leaves.
Baden Baden	18/21, 21/24	Red	2'	Mid	Evergreen shrub, round form, alternate leaves, attractive flowers
Besse Howells	21/24	Red	3'	Early/mid	Flower openly funnel-shaped, leaves 4"long, shiny wrinkled, compact
Blitz	18/21	Red	3'	Mid	Flower funnel-campanulate, wavy edges, has inconspicuouos brownish markings on all lobes.
Blue Peter	24/30, 36/42	Lavender	6'	Mid	funnel-shaped, frilly edges, lighter towards center. Round, mounding habit
Cary Ann	24/30	Pink	3'	Mid	Flower openly funnel-shaped, Held in conical trusses of 17 flowers
Christmas Cheer	21/24	Pink	4'	Very Early	Broadleaf evergreen, part shade to full shade, compact
Dora Amateis	15/18	White	3'	Early/mid	Unusual small leaves, heavy blooming, sometimes fragrant, greenish-yellow spotting
Fantastica	21/24	Pink	3'	Mid to late	Well formed hardy plant, white in throat, lightly spotted greenish-yellow
Firestorm	21/24	Red	3'	Late	Spreading habit, openly funnel-shaped, wavy edges
Golden Torch	21/24	Yellow	3'	Late mid	Flower campanulate, very lightly flushed pale greenish -yellow, spotted light yellow on dorsal lobe. Ball shaped trusses of 13-15 flowers.
Goldflimmer	21/24	Lavender	3'	Late mid	Compact, frilly edges, brownish-yellow flecks on a paler blotch.
Haaga	21/24	Pink	5'	Late	Dwarf variety, flowers openly funnel-shaped, wavy edged, inside fuchsine pink, outside with brownish red spots. Upright growth habit
Harvest Moon	21/24	Yellow	4'	Mid	Striking foliage, flowers funnel-shaped, fading to creamy white, with brown-red spotting.
Hellikki	15/18	Purple	5'	Late	Flower openly funnel-shaped, wavy edged, Held in dome-shaped truss of 8-12 flowers.
Helsinki University	15/18	Pink	6'	Late	Flower openly funnel-shaped, wavy edges, waxy appearance,
Henrietta Sargent	21/24, 36/42	Pink	5'		Large trusses, compact, evergreen
Holden	21/24	Red	3'	Mid	Flower openly funnel-shaped, tinged blue, with dark red blotch. Held in conical-shaped truss.
Horizon Monarch	21/24	Yellow	6'	Mid	Flower openly-shaped, small vivid red flare. Ball-shaped truss holds 15 flowers
Hotei	15/18, 21/24	Yellow	3'	Mid	Flower openly campanulate, deeper shading of brilliant yellow. Held in loose, ball-shaped truss of 10-12 flowers
Jean Marie de Montague	21/24	Red	5'	Mid	Flower broadly funnel-campanulate, dark red spotting in the dorsal throat.
Lavender Queen	18/21	Lavender	5'	Late mid	Flower slightly frilly, lavender with purple blotch. Held in ball-shaped truss.
Lee's Dark Purple	24/30, 36/42	Purple	6'	Late mid	Flower broadly funnel-shaped, greenish-brown to ochre marks within.
Lord Roberts	21/24	Red	5'	Mid to late	Flower funnel-shaped, v-shaped black markings on dorsal lobes.
Mardi Gras	18/21, 21/24	Pink	3'	Mid	Flower openly funnel shaped, frilly lobes, edges and reverse strong purplish-pink to purplish-red.
Mikkeli	21/24	Pink	6'		Flower openly funnel-shaped, wavy edges, greenish spots.
Minnetonka	18/21	Purple	3'	Mid	Flower widely funnel-shaped, lighter center and vivid yellow-green spots on upper lobe.
Mrs Furnival	21/24	Pink	4'	Mid	Flower broadly funnel-shaped,paler at center, conspicuous rays of sienna and crimson marking.
Nancy Evans	21/24	Yellow	3'	Mid	Flower openly funnel-shaped, wavy lobes, some early orange shading on lobes and reverse, hose-in-hose.
Normandy	21/24, 36/42	Purple	4'	Late mid	Flower openly funnel-shaped, orange spots.
Peter Behring	21/24	Red	3'	Late	Flower openly funnel-shaped, wavy lobes, greenish-yellow spotting and purplish-red ribs.
PJM	24/30	Purple	4'	Early	Flower openly funnel-shaped, wavy lobes,
Purple Lace	18/21	Purple	5'	Late mid	Flower very frilly, deep purple paling in the center.
Purple Passion	21/24	Purple	4-6'	Late mid	Vigorous, dense and compact, a seedling of purple splendor
Purple Splendour	21/24, 36/42	Purple	5'	Late mid	Flower broadly funnel-shaped, wavy edges, large black blotch.
Royal Pink	21/24	Pink	3'		Tight ball truss
September Song	18/21	Orange	5'	Mid	Flower funnel-campanulate, wavy edges,
Sham's Candy	21/24, 24/30, 36/42	Pink	5'	Late mid	Flower openly funnel-shaped, prominent dark yellowish green blotch.
Solidarity	18/21	Pink	4'	Mid	Flower openly funnel-shaped, wavy edges, light purplish pink in throat with purplish-red spots on dorsal lobe.
Taurus	18/21	Red	6'	Early/mid	Flower campanulate, darker throat and black spots on dorsal lobe.
The General	30/36	N/I	N/I		N/I
Unique	21/24	White	4'	Early/mid	Flower funnel-shaped, spotted crimson. Dome-shaped truss of about 14 flowers.
Vernus	21/24	Pink	5'	Early	Flower star-shaped, Full sun for best performance of flowering.
Very Berry	24/30	Red	6'	Mid	Cone shaped trellis, leaves of equal proportions
Virginia Richards	21/24	Pink,Yellow	4'	Mid	Flower long lasting, openly funnel-shaped, aging to apricot and then yellow, pinkish-red spotting.
Vulcan	21/24	Red	5'	Late mid	Flower openly funnel-shaped, wavy-edges, blood red, unmarked. Several different clones sold under this name.
Wardance	18/21, 36/42	Red	4'	Mid	Flower openly funnel-campanulate, wavy lobes, black blotch and spotting.