Culture of Rhododendrons
Leonard F. Frisbie, President Tacoma Rhododendron Society
There is no mystery about the successful growing of rhododendrons. The whole thing is a matter of factual common sense. Just about anybody can be a good hand with these peerless glamour flowers if they love them enough to learn what the plants want and if they love them enough to be faithful and prompt in supplying those wants.
For rhododendrons cannot be casually planted out and forgotten if one expects them to give a satisfactory performance. They do have a few definite requirements and if these are properly met the plants will not fail to give a superb account of themselves.
This brief article will not be exhaustive and it will not be technical. It is written for the novice and the beginner and it is based entirely on my own experiences, my mistakes and my modest successes.
Most rhododendrons are good growers under proper conditions. They readily respond to correct treatment by displaying a definite appearance of health and well being. They will look well groomed and will be interesting and beautiful as a foliage plant, even without flowers. Most rhododendrons are also free bloomers under proper conditions. These are the two primary reasons that we grow the genus and these are the. reasons for our devotion. And if we can approach maximum results in these two departments the plants will give us deep satisfaction. Both problems involve correct culture.
A first requirement for well grown rhododendrons is acid soil. Nothing should be taken for granted here for the acidity must be definite. Litmus paper will give a positive bluish reaction if wet soil is acid. A light lavender to pink reaction indicates lack of acidity. A weak solution of aluminum sulphate can be used if the soil is not sufficiently acid. The chemical should be thoroughly dissolved in the solution. And this chemical should always be used either in mild solution or in a balanced dry formula. Used otherwise it is quite sure to be extremely harmful to the plants. An acid fertilizer should be used annually, either ready prepared or mixed according to a balanced formula. Citrus rinds are of little value and they give the planting an unkempt appearance. Peat and leaf mold from the woods are dependably acid. These can be mixed with the soil. Baled peat is also good, but the fine, horticultural variety should always be bought.
Soil for rhododendrons should be loose and friable, and it should also be highly nutritive. Loose soil is essential because the feeder roots are tender and they will be slow to penetrate hard soil. Big, husky root balls that are close knit are essential for a well grown rhododendron. This the grower will have if he uses a fifty-fifty mixture of peat and good rich, loose soil. The peat will also retain the moisture sufficiently. The genus abhors drought, but also likes good drainage.
Watering and feeding are two closely related rhododendron problems. Our dry summers require faithful watering. The planting area should be kept damp. Feeding should be done just before the plant flowers. The new growth follows the flowers and the purpose of the fertilizer is to stimulate this growth. Too much watering and too much feeding stimulates too much growth and a second round of new wood instead of lower buds will result. A very light watering daily is good, but a thorough watering once or twice a week will be satisfactory also. I use two ordinary teacups full of acid fertilizer for a plant two feet high and increase it according to the size of the plant. This seems sufficient for most of my plants. Each grower will need to learn by experience just how much is best for his own soil and situation. When I have a plant that is not responding as it should I give it a liberal mulch of well rotted cow manure, an inch or two lightly scratched into the soil. An annual vegetative mulch is very good. It adds humus to your soil and cools the root area in warm weather. It also tends to retain moisture. If acid material is available at all, then use it. Oak leaves are fine. Saw dust is used by some, but I have had no experience with it.
The location of rhododendron plants should be given careful consideration. It is important. Most varieties will do well in a considerable amount of sunshine. A half day of full sun, preferably morning, is ideal. Filtered sunlight that gives half shade will be equally good. Many sorts can be grown in full sun with excellent results, but if colors fade quickly shade should be provided. Half sun will be sufficient for a good crop of flower buds and for maturing the wood. Plants with sufficient light will assume a natural form, but too much shade causes plants to draw toward the light and the result is weak and leggy growth. The plants of R. cinnabarinum I have seen in this country have long leggy growth and few flowers because of too much shade. Lord Aberconway sent me an excellent color slide of the roylei variety of this one. The plant is in an open situation and is compact, bushy, heavily foliated and it is loaded with flowers. Few varieties will bud well without at least half sun. Rhododendrons do not like wind, though many varieties will take quite a lot without damage. However, shelter should always be provided from strong sweeping winds. R. 'Loderi King George' and R. 'Snow Queen' are two varieties that require tempered sun and wind shelter.
Such large leaved sorts as R. falconeri should be tried by more people. A well groomed plant of this one is such a thing of beauty with its rich green spreading foliage, but discolored and ragged leaves are anything but attractive. In winter I give mine overhead glass protection from the frost. It does not seem to mind the cold we have here, but direct frost is devastating. The plant also must have protection from winter winds. In the spring strong feeding is required to mature the size of new leaves. Extra strong watering is also required during the growing season. Under these conditions the foliage reaches large size and there are no stunted, deformed leaves. There is something breathtaking even about a small plant of this variety if it is well grown. I can never pass it by casually, but must always stop for a good look.
If plants are set in the loose soil mentioned above and have good, well knit root balls, then moving can be done without the slightest danger. Moving will be helpful. Breaking the roots at the periphery stimulates more root growth. But plants should not be moved during the growing season which follows the flowering. The warm summer season is not good for moving either. The fall, after mid-September, is a fine time for moving, as is the spring from March on to flowering time. So the novice can move his well rooted plants around without fear until he finds the location the plants like best.
I hope nobody will take any of these remarks as being the last word. Every grower needs to vary any technique until he finds out what is best for his soil and his location. If his plants grow luxuriantly and bloom heavily, then he is a success. But that's the goal. It is easy to attain if one loves his plants sufficiently.