Growing media pH

What is pH, and how do I obtain the proper pH for my rhododendrons?

The term "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 and 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.

The experts indicate rhododendrons prefer an acidic medium.  The preferred pH should be between 5 and 6.5.

It is almost impossible for a layperson to determine the pH of the potting medium they use.  There are pH meters on the market...but in my experience the ones that cost less than $100 are practically worthless.  I have yet to try one that is better than plus or minus 1 pH.

But...all is not lost.  It is actually fairly easy to get your pH in the desired range.  Fir or hemlock bark is almost always in an acceptable range and, therefore, an ideal medium to use.  It is best if the bark has decayed or mulched 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, yes, soil can be too acidic, the plants have difficulty taking in the nitrogen and phosphorous they need for growth.

The foliage will not be the rich green that you expect.  Adding lime to the medium or the soil will raise the pH and help this condition.  Too low pH can occur when soil has been fertilized heavily for years.  The fertilizer frequently increases the acidity of the soil...that lowers the pH.

If the planting medium of soil is too alkaline, i.e. the pH is too high, it usually causes iron and/or manganese deficiencies.  These deficiencies result in chlorosis - a condition where the veins may remain dark green - but the spaces between the veins will be yellow...the leaves are said to be chlorotic.  To remedy this condition sulfur is often applied for a quick fix.  Good mulching will also help in the long run.  A decomposed mix that would not use up the nitrogen in your fertilizer is best.

In summary, pH is important but your plants will tell you if you have a problem.  Generally, it is always best to use bark in pots and bark or pine needle mulch as an additive for your soil...and you will rarely have a problem.

Companion plant: Viburnum bodnantense

The winter-flowering Bodnant viburnums are tall, fairly narrow shrubs which have bunches of tubular pink flowers on leafless stems. They bloom in late winter and have frost-resistant blooms which stay for weeks and are quite fragrant.

The species name 'bodnantense' refers to Bodnant Gardens, North Wales, where the hybrid was raised in 1935. The Bodnant vibumum grex (a grex denotes all the offspring of a particular cross) is a cross of the Chinese viburmum farreri with the Himalayan viburnum grandiflorum and, includes the cultivars 'Dawn', 'Charles Lamont', and 'Debian'.

'Dawn'...often called 'Pink Dawn'... is commonly grown and is widely available. 'Charles Lamont' has dark pink flowers which are somewhat larger than 'Dawn'. 'Debian' is tall and has a stiff habit with slightly fragrant whitish flowers which turn red with age.

'Dawn' is a deciduous upright shrub, 2-4 m. (6-12 ft) in height. It blooms in Fall and Winter after the leaves have fallen. Before they fall, the leaves turn a burnished bronze color. Red buds open to fragrant pink flowers that fade to white flushed with pink as they age. Wet weather and frosts may limit flowering display. Branches can be forced inside for a winter bouquet. The fragrance indoor may be somewhat overpowering.

Photo by Giraffenigel

'Dawn' is not particular as to its location, liking sun, but also doing well in partial shade. It enjoys acid, well-drained soil, but does well enough in other soils. It is fairly cold-hardy...but requires a protected placement where the chilliest winds won't hit it at temperatures below zero degrees F. Overall, this hybrid is very adaptable.

Since the plants are fragrant, grow them near a path you are like to use frequently. You will be rewarded! Otherwise, you won't appreciate the scent at a time of year when you're less inclined to go down a damp cold garden to smell it. The flowers have a pleasing perfume. The new leaves get so large that the blooms occurring the rest of the year can pass unnoticed. The leaves smell like citrus if crushed or bruised.

Fertilizer Nutrient Elements

A bag of fertilizer has three numbers prominently featured on the label...something like...5-2-0. It's important to know what those numbers mean because the wrong combination can do more harm than good. The numbers indicates the amount of three major plant nutrients: nitrogen, phosphorus, and potassium, abbreviated NPK.

One way to remember what those things do, and keep them straight is the phrase "up, down, and all around." Nitrogen is needed for green, leafy growth...the up. Phosphorus helps produce healthy roots...the down. Potassium is important for overall plant health and resistance to water or insect stresses, so it's...the all around.

Plants need anywhere from 14 to 18 plant nutrients for best health and production. Magnesium and sulfur comprise the macronutrients along with NPK. The micronutrients include: iron, manganese, boron, zinc, copper, molybdenum, and maybe chlorine. Testing your soil is the way to determine what is needed.

Success With Heathers

The singular species, Calluna vulgaris, inhabits large portions of Europe and northern Africa, from Siberia to Morocco, in vast tracts of open land and hillsides. The predominant flower color is purple...but it sports out in pink, red, and white. For well over 100 years, avid plants men have made over 500 selections of form, bloom, and foliage variations, and some of them are real eye-poppers!

Now, if you are one of those people that think that they always look so ratty, I am about to show you the error of your ways. Site selection is probably the most important factor. These are ericaceous the same family as rhododendrons...and like them, we should take our lessons from Mother Nature. Full sun is an absolute must and some slope to the ground is very beneficial. Soil should drain well, but retain some moisture, especially in summer during bloom time, anywhere from July to November depending on cultivars. An acid, humus-rich soil with no manures or strong fertilizers is necessary...a lean media keeps the growth compact. If you cannot engineer even a bit of a slope, then be sure that the soil is deep and loose. And then...prepare to PRUNE!

Ok, ok, and ok! I know no busy band of elves prunes on the moors, but truth be told, if you took aside only one or two of those million of plants, they probably would be ratty...but there's safety in numbers and distance. We do not have that advantage in our get out the hedge clippers. March is a good time to do this...trim just to the bottom of last year's flower spikes to make a somewhat mounded shape. Now, was that so hard? Once a year is all it takes, and a spring pruning still lets you enjoy all that wonderful winter foliage on the colored forms.

Choosing varieties for your garden will be by personal preference. With that many to pick from, it is like roses or rhodos...grow the ones you like to look at. I can only give guidelines of the forms that are available. The flowers can be single or double on long or short spikes. I have taken full open spikes of 'Peter Sparkes' and 'H.E. Beale' and hung them to dry, and they make wonderful bouquets...unfortunately, not too durable...but so easy you can do new ones every year. Colors range from white and pale pink to bright pink, crimson, and purple. There are varieties like 'Marleen' that seem to bloom forever since the blossoms don't ever actually open, just show outer color.

The foliage on some is so spectacular that even if they didn't bloom, no one would notice. Red-gold ones like 'Firefly', 'Blazeaway', 'Boskoop', and 'Sir John Charrington' are bright all year round. Silver ones like 'Jan Dekker', 'Silver Knight', and 'Grey Carpet' will gleam against dark backgrounds. Some like 'Spring Torch' and 'Spring Cream', have colored foliage on new growth only...almost like an early bloom...and then the proper flowers come later. Very dwarf ones can be used in troughs or rockeries, as 'Foxii Nana', 'Dainty Bess', 'Humpty Dumpty' and 'J. H. Hamilton'.

The list is endless...but this should give you an idea of the lovely ones there are to choose from. Visit gardens and plant centers in summer when they are in bloom. I only wish I had more room in my garden to have an entire bank devoted to drifts of all year color.

Happy planting!

Rhododendron williamsianum

Rhododendron williamsianum, introduced to horticulturists of the western world in 1908 by Ernest "Chinese" Wilson is at once one of the most recognizable and one of the most enchanting rhododendrons. Its unique combination of almost orbicular, mid-green leaves, spreading and somewhat rounded or dome-shaped habit, and disproportionately large, candy-pink, campanulate flowers make it easy to recognize and well-adapted to gardens both large and small. Its densely mounded shape often has a somewhat brooding appearance, a bit like having a very large toadstool in the garden...but its solid presence makes a good anchor to a plant grouping, and can provide an effective screen, if needed.

Although never thought of as a large rhododendron, it can in time become quite massive, spreading up and out in large billowy curves that are certainly beyond the reach of vertically-challenged gardeners.

R. williamsianum flower
Photo by Chris Klapwijk

It was a species that has so captured the imagination of rhododendron fanciers that a veritable frenzy of hybridizing ensured. After all, there were so many positive characteristics to try to attach to other rhododendrons: small, tidy, glabrous leaves, with lovely bronze new growth, well-shaped, pretty pink blossoms that were large in relation to the leaf size, and a spreading compact habit that keep the blossom down where people could easily admire them.

Looking through Salley and Greer's book Rhododendron Hybrids, I gave up counting at the end of the C's, having reached something over 55 hybrids. Each decade seemed to bring on a new wave of williamsianum crosses:

  • Rothschild in the early 30s, including the iconic hybrid 'Bow Bells'.
  • Lord Aberconway in the late 30s and early 40s, with a whole series of hybrids beginning with "A" ('Adrastia', 'Adrean', 'Amata') as if he were trying to get in first in the Yellow Pages, but also developing the wonderful 'Cowslip', a williamsianum x wardii cross.
  • Hobbie in the mid 40s, with his 'Gartendirektor(s) 'Glocker' and 'Reiger'
  • all the way up to Weldon Delp and Hans Hachman's more recent hybrids.
  • Even J. C . Williams of Caerhays Castle used this namesake rhodo in his inspired 'Hummingbird' cross with R. haematodes.

R. williamsianum is truly endemic to Sichuan, China, being found only there, and even within that province its distribution is very limited. However, it is now in constant and widespread production and cultivation all over the world.

Lepidote Rhododendrons

Rhododendron enthusiasts are often asked what they mean when they refer to a rhododendron as a "lepidote". The confusion is compounded when one sees quite a bit of space devoted to lepidotes in flower shows.

A nurseryman may tell you casually that it refers to the small-leaved evergreen rhododendron. This can be used as a generality...but not as a hard and fast there is a fair amount of overlap between lepidotes and the "large-leaved" elepidotes.

Real traits that make it a useful distinction is lepidote rhododendrons have scales on the underside of the leaf which protect the plant's stomata (leaf pores) through which oxygen, carbon dioxide, and water vapor pass. The scales evolved originally to regulate moisture, to help keep water in the cells in dry times, and help shed it in times of surfeit . This allowed plants evolving in the tropics to live in the quick-drying forest duff on the very thin soils of the tropics, or even epiphytically on rocks or tree trunks. Turn a leaf of the lepidote over and look for the tiny scales; some are big enough to see with the naked eye, but a hand lens will reveal a world of otherwise hidden detail. Elepidotes, on the other hand, are without scales to cover their stomata.

But winter brings many of the same demands as the tropics on a plant...encasing it in wet snow or desiccating it with cold dry winds, and the scales evolved to become adept at dealing with harsh winters as well. As a result, lepidote rhododendrons have adopted and spread to nearly all environments, from tropical jungles and Siberian woodlands to mountain meadows and alpine tundra. Because of this wide tolerance of soils, temperatures and exposures, they are especially useful to gardeners in the Northeast United States. Other adaptations, such as fast regrowth after predation from grazing animals, and early bloom to deal with short growing seasons in cold climates, give us a plant that is easily pruned to shape and early to give the gardener a boost after a long winter.

Most lepidotes have axillary buds...extra flower buds under the terminal flower bud or along the branch...and in bloom often smother the foliage until all you can see are the flowers. Lepidote species range from tiny creeping alpines suitable for the rock garden to tall forest and meadow plants for woodland wildflower gardens and formal borders.

Best of all, the hybrids developed by plant breeders bring hybrid vigor to the party, giving us a huge range of plants, among them some of the easiest and the hardiest rhododendrons to grow in cold climates. A commonly grown lepidote cultivar is "Rhododendron 'PJM', which was hybridized by the Mezitts, and is now grown and admired all around the gardening world.

The large-leaved fancy varieties of rhododendrons may get all the big press...but for vigor, hardiness, adaptability, easy of culture, and sheer traffic-stopping, eye-popping show of flowers in the money is on the lepidotes!

 From the Massachusetts Chapter Newsletter, September 2006.