Rhododendrons are amenable shrubs. They’ll grow for any one of us, whether we are skilled gardeners or rank beginners, whether we have a thousand acres or the smallest of foundation plantings.
But to grow them well, we must understand their requirements. Their native haunts are a clue to the kind of treatment they need in our gardens.
Features Of Rhododendron Species
The flowers of the choicer rhododendron species and varieties, grouped in great, brilliantly-colored clusters, are beautiful beyond comparison.
The foliage is of the richest green, glossy, attractively formed, and elegantly arranged northern Pacific region has awakened to on the stems.
As a whole, the plant is a great many more beautiful shapes without being stiff or formal, aristocratic without being haughty.
It is dance and variety that nurseries are the one shrub that comes close to perfecting with each other to see who can bring it on.
Abundant Rhododendrons In Various Places
To most of us in the East, unfortunately, the word rhododendron merely means Rhododendron maximum, and to most gardeners along the West Coast, it signifies only Rhododendrons californium.
So easily and cheaply can these wild shrubs be collected that immense quantities are used everywhere except in the extreme South, limestone country, and desert areas?
Within the past few years, however, the Northern Pacific region has awakened to the fact that many more beautiful rhododendrons exist in such abundance.
The variety that nurseries are vying with each other to see who can bring out the longest list or plant the most acres.
King of Flowering Shrubs
But the rhododendrons are already kings of flowering shrubs, particularly in the State of Oregon.
These new rhododendrons come from Asia or from the hands of European hybridists, where every year sees superior forms and colors.
- Rhododendrons sinogrande, with leaves the size of canoe paddles and huge heads of yellow flowers
- loderi, blooming like a giant lily and sweetly fragrant
- Earl of Athlone, truly blood-red
- Rhododendrons augustini, a mass of blue
And a thousand others give outstanding and lavish displays.
Native Species In The East
And what about the East? We can’t grow these particular forms, but others far superior to the native species have been with us, some for nearly a century, and may be seen here and there in the choicer gardens.
We have nurseries offering 30 or 40 fine varieties in many shades and with better growth habits.
Why haven’t these become more popular? Because most of us don’t know how to grow them. Left in total neglect, they thrive magnificently unless trees, vines, or fires invade them.
The ingenuity of the average gardener—even an experienced gardener—destroys them within a few years.
A Permanent Mulch
To begin with, all rhododendrons require a permanent mulch above the roots. Most gardeners, most of the public, and some experts refuse to believe this, but it is true.
Deprived of mulch, the lower leaves soon drop off, the plants eventually becoming mere sticks with small clusters of foliage at the top.
With proper mulch, these same plants would have filled out, covering themselves from ground to summit with rich foliage and thriving for a century.
Mulch After Transplanting
The mulch must be spread on the ground’s surface within a few months after transplanting, preferably at once.
It must never be dug into, removed, or covered up with anything except more mulch.
What To Use
What is mulch? Specifically, it is a layer at least 2” inches thick of oak leaves, pine needles, or similar material.
Other kinds of leaves will do, or weed straw may be mixed into an advantage. Soil is not mulch. However, peat moss is not mulch, and sawdust is not good.
The purposes of a mulch are to insulate the feeding roots against heat and cold and to retain moisture.
As its lower layers eventually decay, it furnishes the only fertilizer rhododendrons require. Therefore, insulation must have air spaces.
Any material which packs down is not mulch. The roots immediately find their way up through such material and demand a mulch above it.
If oak leaves are used, they must be anchored against the wind by scattering twigs and small branches over them, which also should never be removed.
Year by year, the mulch should be added because the decaying lower layers become soil.
The ground under the mulch must never be disturbed, nor must any material except mulch be placed on top of the mulch.
If acidification is necessary, dry peat moss may be scattered over it. Never much at a time, and allowed to sift down through. If the peat moss stays on top, it ruins the mulch.
All these rules are continually violated in nurseries because nurseries are not interested in having the plants established.
Transplant Twice A Year
Transplanted once or twice a year, nursery plants can be kept more or less healthy for two or three years. But once the plants are left to root in one place, they must be mulched or fail.
After 4 or 5 years of proper mulching, you are rewarded, not only by a shapely, healthy plant but by not having to mulch it anymore, for it mulches itself.
For the lower branches, reaching down to the ground, form a close screen. Leaves that fall there or blow in are held in place by the foliage.
Fertilization Of Rhododendrons
Fertilizing rhododendrons is unnecessary. Rhododendrons do not want chemical fertilizer; they want decaying leaves.
The soil in which rhododendrons are planted must be acidic, but merely making it acid does not fit these shrubs.
It must also be full of organic matter, and if it is, the acidity usually takes care of itself. Leaf mold is ideal.
Good Mulch Maintains Acidity
Adding peat or peat moss, quantities of it thoroughly spaded in, will make most soils suitable except in alkali country or on limestone. A good mulch maintains acidity.
No chemical acidifier should ever be used, except that in an emergency, a thin sprinkling of sulfur (no other chemical) may be scattered over the surface of the mulch, about as much per square foot as you would put salt on a steak.
Cottonseed meal, carefully scattered dry so that it sifts down through the mulch, makes a satisfactory fertilizer for rhododendrons, especially in the first years, before their roots become well established.
But generally speaking, woody plants should not be fertilized.
On Alkaline Soil
Recent experiments have shown that rhododendrons can be grown on (not in) alkaline soils by anyone willing to take the trouble.
Beds of leaf mold and peat should be spread over the surface of the ground a foot deep, in a place where water from elsewhere cannot wash over them to neutralize their acidity.
They should be sprinkled with collected rainwater only. Mulching is, of course, obligatory.
Rhododendrons come mainly from regions of heavy rainfall, sub-irrigated by the flow from melting mountain snowfields, enveloped by frequent fogs, and drenched with generous dews.
Liberal Sprinkling
Liberal sprinkling helps even the hardiest kinds, and all appreciate heavy soakings through the first three weeks of May as the buds expand.
The flowers on a wet rhododendron will sometimes reach nearly twice the size of those on a dry one.
An important office of the mulch is to sop up and retain all possible moisture.
Care In Watering
Watering anytime after August 15 and until the onset of winter in late November.
However, it should be reduced to a minimum or avoided altogether when possible, for the wood can be properly hardened by keeping the plants rather dry.
Dangerous frost injury often results from fall watering. Also, wet plants usually fail to set flower buds and cannot bloom the following year.
About Thanksgiving time, hardly earlier, a drenching should be given to the plants.
Spraying of Bordeaux Mixture
Mild sprays of Bordeaux mixture, arsenate of lead, and nicotine sulfate may sometimes be necessary for all rhododendrons.
Bordeaux stops fungus spotting and tip blight; arsenate of lead controls inch worms and katydids; nicotine sulfate sprayed on the undersurface of the leaves will kill young lace bugs.
These sprays should be used at about half the recommended strength, and other or stronger sprays should be avoided.
It is particularly important not to use any spray that will injure spiders, for little jumping spiders control other pests that would seriously injure the plants.
Early Slimmer Pruning
Rhododendrons will stand moderate pruning well but do not need it. The best time is in early summer after the flowers have faded but before new buds begin to form.
They will bloom best if all old flower heads are removed as the flowers fade. They can stand heavy shade or nearly full sun, but the moderate shade is best for them.
The shade of deep-rooted trees such as oaks, tulips, and magnolias is much better for them.
While maples, elms, and lindens have surface roots that compete with rhododendrons.
Long-Established Hybrid Varieties
Long-established hybrid varieties that withstand the climate around Boston, New York City, Cleveland, and Chicago are in the approximate order of their hardiness:
- Charles Dickens—Red
- Catawbiense Album—White
- Boule de Neige—White
- Kettledrum—Red
- Purpureum Elegans— Purple
- Purpureum Grandiflorum—Purple
- Atrosanguineum—Red
- Caractacus—Red
- Roseum Elegans—Pink (There are inferior forms sold under this name)
- Mrs. C. S. Sargent—Rose
- Catawbiense Grandielorum—Pale lavender
New Hardy Varieties
Many others only slightly less hardy may he tried in less frigid sections and more protected spots.
New hardy varieties are appearing, but they still require years of testing.
On Long Island and Cape Cod, the most favored localities in the East, numerous spectacular varieties will thrive if well cared for.
The waterside climate, with abundant moisture in the air at all times, favors rhododendrons.
The presence of a large body of water moderates both heat and cold for them. However, the coastal climate south of the Chesapeake is too hot for them.
44659 by G. G. Nearing