What is pH? How do I obtain a proper pH for my rhododendrons?

pH refers to the acidity of a material. Technically, it is a measurement of the hydrogen ion content. The pH scale ranges from 0 to 14, pHs of 0 to 7 are acidic, pHs of 7 to 14 are referred to as being basic or alkaline. A pH of 7 means the material is neutral. For rhododendrons the preferred pH should be between 5 and 6.5.

It is difficult for a layperson to determine the pH of the potting medium they use. There are pH meters on the market, but the ones that cost less than $100 are practically worthless. However, it is fairly easy to get your growing medium pH in the desired range. Fir or hemlock bark is almost always in an acceptable range and, therefore, an ideal growing medium to use. It is best if the bark has decayed for six months prior to use.

The reason pH is important for plants has to do with the intake of minerals and nutrients. If the pH is too low, rhododendrons have difficulty taking in the nitrogen and phosphorous they need for growth. If the planting medium of soil is too alkaline, i.e. the pH is too high, it usually causes iron and/or manganese deficiencies.

In summary, pH is important for growing healthy rhododendrons. Generally, it is advisable to use fir or hemlock bark in pots and bark or pine needle mulch as an additive for plants grown in the ground.

R. catawbiense was collected in the wild in 1803 in the Eastern U.S. and was introduced to Britain in 1809. When hybridized with other rhododendrons it introduced cold and heat hardiness into rhododendron hybrids. The first group of rhododendrons to become popular in gardens was the hybrids called the "iron-clads." There were a group of early British hybrids involving crosses of R. caucasicum, R. catawbiense, R. ponticum and R. maximum that survived the coldest winters at Arnold Arboretum in Boston. In 1917, Ernest Henry Wilson (1876-1930) published a list of what he termed the "Iron Clad" rhododendrons that for many years were successfully growing in the Arnold Arboretum.

 Wilson's Dozen Iron-Clad Rhododendrons

 1. Album Elegans, pale purplish pink hybrid or selection.
 2. Album Grandiflorum, pinkish white hybrid or selection.
 3. Atrosanguineum, bright red with purple markings hybrid.
 4. Catawbiense Album, pinkish white hybrid or selection.
 5. Charles Dickens, crimson red with purplish markings hybrid.
 6. Everestianum, purplish-pink with green markings hybrid.
 7. Henrietta Sargent, a deep pink hybrid.
 8. Lady Armstrong, deep purplish pink, pale center hybrid.
 9. Mrs. Charles S. Sargent, rose pink hybrid.
10. Purpureum Elegans, pinkish purple hybrid.
11. Purpureum Grandiflorum, violet with green flecks hybrid.
12. Roseum Elegans, lavender pink with green markings hybrid.

Rhododendron decorum was first described by Adrien Rene Franchet in 1886 from a plant collected in Moupin, western Sichuan. It was introduced to cultivation in 1887 by Pere Jean Marie Delavay. In the wild it can be found throughout west and southwest Sichuan, southeast Tibet, northwest Yunnan and northeast upper Burma. It grows in pine, spruce, and open deciduous forests, and on grassy mountains and rocky scrub. It is found at elevations of 6000 to 15,000 feet.

The foliage is variable but typically shaped oblanceolate to elliptic, up to 7" long, smooth green on both surfaces, with rounded ends. The funnel-campanulate flowers are white to pink to pale lavender, variously marked and quite fragrant. Held in an open-top truss with 7-12 flowers.

There are two subspecies, ssp. decorum and ssp. diaprepes, with the former having smaller leaves and corolla and fewer stamens.

R. decorum is quite variable in hardiness. Typical cold hardiness is 0°F (-18°C).

Plants in cultivation are easily grown and generally bloom at an early age. With a May/June bloom time and a pleasant fragrance, R. decorum is worth growing in your gardens.

R, decorum has found considerable use with hybridizers and many fine hybrids have been produced, including R. 'Caroline', 'Lackamas Spice', 'Newcomb's Sweetheart', 'Apricot Sherbet', and 'September Song'.

Here in the Pacific Northwest, we've had a cold, wet and windy spring, so plants are late blooming this year. However, I've finally got a bit of good colour showing now in early April.

R. 'California Gold' is a R. maddenii hybrid, and tender for us so I keep this beauty in a container and bring it in whenever frost is predicted. It's a bit of a nuisance to do, but whenever I see it blooming, I'm glad I took the trouble. The large flowers are soft yellow with brown anthers that show up nicely against the petals. The outer edges of the petals are just a bit frilly, so really nice and, they have the added bonus of excellent fragrance. When in bloom, 'California Gold' perfumes the sunroom and adjacent dining room. One other attractive attribute is the dark red shedding bark.

R. 'California Gold'

R. 'Shamrock' is another pale yellow flowered hybrid in bloom today. Among my acquaintances, I find some people aren't impressed with 'Shamrock'. They think it's too pale with a bit too much green in the flower colour to stand out well. Well, that's just crazy talk! I think this is a sweet, tough little plant and I value it for its early blooms. It is often out in mid-March although this year it's a couple of weeks later. The parents are a dwarf form of R. keiskei and R. hanceanum nanum and 'Shamrock' has inherited their small stature. And, for those who wish it had showier flowers, put it next to something blue like 'Muscari' as the colour contrast is very attractive.

R. 'Shamrock'

My third "bloomer of the day" is lovely R. dendrocharis. This species rhododendron has flat-faced, open flowers of bright pink. The leaves are tiny and very dark green. In the wild, R. dendrocharis usually grows as an epiphyte, so it needs excellent drainage. Other than that, I've found it easy to grow. I keep it where it gets dappled shade and protection from the noonday sun during the summer.

R. dendrocharis

There are several reasons for rhododendrons to have few or no flowers:

• Application of nitrogen-rich fertilizer can cause excessive vegetative plant growth and suppression of flower bud formation.

• Plants pruned in the late summer or fall will have few blooms due to removal of flower buds. The proper time to prune is in the spring immediately after flowering has finished.

• Cold weather can kill flower buds. Buds change color from green to brown. Select varieties cold hardy for your garden location.

• Some hybrid rhododendron varieties do not bloom profusely every year.

• Light is the primary factor that stimulates flower bud development.  Rhododendrons should be planted where they receive sufficient light so that they set flower buds, but not in too sunny a location that leaf damage occurs.

Until recently, there were very few variegated evergreen azaleas available in the United States. These include the Satsuki varieties:

• ‘Keigetsu’. One of the earliest blooming Satsukis with light pink flowers and a red margin. The green leaves are flecked with white on a low spreading plant.

• ‘Uki-nishiki’. White flowers with many variegations of stripes, flecks, etc. of a dark purple. The leaves are flecked with yellow.

• ‘Shira-fuji’. The flowers are variable, generally white with purple variations. The leaves have an attractive distinct white margin on a dwarf plant.

• ‘Shinyo-no-tsuki’ sport. Very large white flowers with crimson blotches that bloom in June. The leaves sport cream stripes longitudinally throughout.

• ‘Fuji-no-mine’. A dwarf cultivar admired for its unusual foliage, the leaves are variegated with many flecks of yellow. Pink flowers with a darker circle that bloom in June.

American growers began introducing variegated azaleas in the late 1980s and early 1990s. One of the most popular and now widely available is ‘Silver Sword’, a sport of ‘Girard’s Rose’ with a distinct white margin bordering a dark green leaf. Other Girard plants with white margined foliage included ‘Girard’s Variegated Gem’*, a sport of pink-flowered ‘Girard’s Border Gem’*, and ‘Hot Shot Variegated’* with bright orange-red flowers. These varieties tend to have a lighter green color in their leaves.

Two southern varieties of the white-margined types discovered in the 1980s are ‘Southern Belle’, a sport of ‘Pink Ruffles’, and ‘Red Ruffles Variegated’. Another lovely choice is ‘Silver Streak’, a Greenwood hybrid with white-margined foliage and purple flowers; it is a sport of ‘Deep Purple’.