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Journal American Rhododendron Society

Current Editor:
Dr. Glen Jamieson ars.editor@gmail.com


Volume 9, Number 2
April 1955

DLA Ejournal Home | QBARS Home | Table of Contents for this issue | Search JARS and other ejournals

Some Observations on the Culture of Rhododendrons
Clarence Prentice, Seattle, Wash.

        I am writing these notes as a result of my observations of growing and associating myself with rhododendrons over a period of 35 years. As Darwin once said, "To be a good observer, one must be a good theorist." And it is with this thought I have formulated these notes.
        The family of plants called rhododendrons is much freer of disease than we are led to believe. Lord Aberconway was once asked what were the worst diseases of rhododendrons. His answer was, "Rabbits and Sir Stafford Cripps." I am thoroughly convinced that fungus, virus and pests are the result of conditions rather than actual infections. It is true the virus or fungus can be classified and does exist but the causes are due to conditions and if these conditions are changed, the trouble disappears. It can be a lack of certain elements. Plants are like people. They have different personalities, likes and dislikes; but they are very gullible when it comes to diet. They eat most anything put up in a pretty package. Plants do not have a facility of choosing their diet. They take up poison as readily as nitrogen and have no way of knowing how much to take. Plants take overdoses just as readily as the proper amounts.
        As yet we do not have any definite formulas tailored for the rhododendron's needs. We suspect that a lot of things are necessary and then work from the trial and error method. Some times these are beneficial but some times we burn the leaves. So here again, if the cause is removed, the trouble disappears.
        Growing rhododendrons is not merely digging a hole and planting a plant - trusting to luck for success. First we should study the natural habitat of the species rhododendrons. If we can master their culture, the rest is quite simple. Most species are found in Asia, more particularly the Himalaya Mountains. These districts are different from our garden conditions as they are on edge-vertical not horizontal. Doctor Rock has told us from one river bed to another, a distance of 40 miles by air, you have a difference of elevation between the two river beds of 20,000 feet. This means the country is straight up and down and this is the native habitat of the rhododendron.
        The soil under these conditions must be different from our carefully prepared compost piles and certainly there is no peat moss there. The soil must be of volcanic origin, rocky, coarse, decomposed rock with some roots, grasses, needles and some leaves. It is not deep soil, except in the deeper crevices where soil would accumulate. The drainage is naturally complete and allows no stagnation.
        Another big factor is climate. The native climate is in reverse to the climate of the Northwest; the falls and winters are dry while the springs and summers are wet. So all in all it is almost a miracle that these plants adapt themselves to the conditions we put them in in our gardens.
        We now return to the problem of conditions as the cause of most of our troubles with species rhododendrons. First, we are too good to these plants in our impatience in trying co force them in to fast growth. Because of this, it is natural to assume that a great deal of trouble is winter damage or the reversal of climate - our summers at times being rather dry and winters very wet. Our soil is loaded with peat moss and is sticky and gooey in winter. I grant it holds moisture in summer. But it is this winter wet that does the damage and this damage does not show up until summer. Rhododendrons do not show the damage immediately. To prove this, break a branch of a rhododendron and stick it in the ground in the winter. It will stay green and look just like the plant until the warm spring days come. Then it will dry up. The same is true of a plant girdled in the winter by weevil. It looks fine, may even bloom, then all at once it starts to turn brown and dies quite fast. Your conclusion generally is that it has a fungus or virus disease or you think you have over fertilized.
        I am thoroughly convinced that most of the brown spots, die back and bud blast are winter damage from wet feet. Give the plant a change of conditions and the problem is licked. I have done it many times.
        Air drainage is also important. This can cause a lot of trouble where a plant gets pocketed. A difference of 5 feet change in elevation sometimes makes a great difference. The Northwest from Olympia north where many thousands of rhododendrons are grown has soil of purely glacial debris, variable in different sections from sticky blue clay to pure gravel with nearly all the plant food washed out. So in our district we are faced with physical problems of soil first.

The Preparation Of Soil And Beds For Planting Rhododendrons

        If soil and beds are thoroughly prepared, your results will be very gratifying. I have come to the conclusion that peat moss has been over-promoted and used in too generous quantities for good culture of rhododendrons. It is the course of least resistance (the easy way) but in proper proportions it has merit. Saw dust is being used extensively and I have seen some very good results in its use. I have nothing to offer on sawdust as I have never experimented with it for the simple reason I do not like the appearance of a garden covered with sawdust. It looks too much like the country fair or a circus. Then too, I always get my shoes full when we put in a flower show.
        I am thoroughly convinced that for best results the top soil from the woods with decomposed logs, sticks, needles and leaves-the coarser the better-mixed with the soil is the best planting medium. I believe in digging a $5.00 hole for a 50c plant rather than a 50c hole for a $5.00 plant. The more clay in the soil the more preparation of the physical aspects of the soil is necessary. Soil may be likened to a prize fighter; it can get in very bad physical condition. The heavier the soil, the more difficult to condition, and the more care with plant foods should be exercised. A sandy soil leeches out excess fertilizer rather fast but clay will hold excess fertilizer and may become very rancid. The same condition will exist if it does not have proper drainage as it will hold the water and become stagnant. Rhododendrons will not be injured by lots of water in spring and summer as long as it drains well. They will grow near running water and not be injured but may have trouble growing by a still pond or pool.

The Importance Of Location And The Consideration Of Growth Habits In Planting Rhododendrons

        Some varieties do well in full sun and full exposure while others need shade and semi-shade. This is more important than selecting your favorite color for you must have a plant tailored to your conditions.
        In consideration of growth habits, ask yourself, "Are you planning on a ten year garden or a 20 year garden?" You must choose plants of proper growth habit to maintain a constant contour and composition pattern. So as your garden progresses, will it maintain proper proportions? An example of time is the original plant of R. 'Cynthia' in England which is 100 years old. It is 25 feet tall and has a spread of 75 feet. But who wants to plant a 100 year garden? It would be very dull for the first 50 years.

R. venator
Fig. 25.  R. venator growing on rocks
in the Prentice Garden
Thompson photo

        Now we have arrived at the ceremony of planting this plant. Do not plant it in a swale in the yard. Better build a mound or low rockery, or plant on sloping ground particularly if soil is heavy. Keep plants high. Plant rhododendrons in coarse textured soil. I have seen plants do fine just set on top of soil with a wheelbarrow of soil dumped around the ball. This would make the establishing of plants more of a problem but once established, the results are more gratifying.
        Also you must remember that rhododendrons are busy plants-there is something going on all year round. They do not become dormant like deciduous trees, and are very sensitive to climatic changes, preferring to have their roots cool. When it gets cold they curl up their leaves for protection and when the warmth of spring is in the air they burst forth with all the elegance of the aristocrat of the garden.
        I think we have had a lot of advice on soil acidity and feel we need to use acid fertilizers. However, the soils in the Northwest are naturally acid and in most cases do not need acidifying.

The Rhododendron As A Family

        We should now know the word "rhododendron" as a family of 850 species and thousands of hybrids. It is no more stable than a large family of children-you have as many different personalities and likes and dislikes.
        Therefore it is impossible to make fast and set rules for rhododendron culture. Plants that grow in the native state and 14,000 feet of elevation are certain to respond differently from plants growing at lower elevations. Some grow in moist situation, some on dry ridges and slopes, some like 6.5 pH. and some need 4 pH. This is a tremendous variation. To some a small amount of calcium is beneficial and to others this would be deadly.
        A lot of these questions have no definite answers. As yet we are still groping in the fog on a good many factors of culture. Again most of our information so far is from trial and error.

Selection Of Rhododendrons

        In selecting varieties for the garden, I think we have been over sold on some of these debutantes from the promotions of the hybridist. We have jumped at conclusions as to their true values. After all, it is impossible to adjudge the garden value of a plant by passing judgment on a blown up cut truss-making citations to a new hybrid from these facts. I feel that a more definite appraisal should be made of the culture, habit and constitution of the plant before it is acclaimed. I feel first crosses may be very fine and occasionally a good one. But my experience with the first crosses is that they lack vigor and until the breeding has been carried on to attain hybrid vigor, you are very liable to be dealing with a very unstable plant that gives a poor account of itself in the ordinary garden conditions.
        I would like to see a program of discussion on the merits of some of these highly rated debutantes, and have the American Rhododendron Society demerit the over-rated varieties, recognize the plants of real garden merit and establish a definite nomenclature of names of plants-that any named plant must be a clone and not a family of plants. This has not only been very discouraging to the gardeners but also to the growers. Each grower or collector swears his plant is the best form. So what does a name mean? It is like calling a family of boys all James. It really does not make sense.
        At present there is considerable work being done in this line of thought. I think if seedlings are sold, they should be sold purely as seedlings and not as definite crosses and the naming of plants should be a very serious business. If the plant does not have merit over proven varieties, it should be sold as a seedling and not named, making our lists of varieties more complex. I feel there has been too much enthusiasm in exploiting names-tying name tags on every plant that is a little different and ballyhooing them to the trade and public before the merits have been proven. They should be carried as a number until proven.
        So in summary, I would like to impress the fact that 1) all the leaf blemishes are not disease but may be due to soil conditions brought on by improper selection of plants for your particular situation. 2) Only plants of proven qualities should bear names. 3) Get your plants from a reputable source and don't buy the plant mistakes on the wayside stand or market. 4) Get your advice from competent people who are horticulturists. 5) Do not buy a plant and sack of fertilizer at the same time. For this last item, allow your plants to get established before you fertilize.
        So after all it is all quite simple if you do it the proper way but any deviation may spell failure. They always say "practice makes perfect." But if you practice the wrong way, the more you practice the more imperfect you become.

Some Observations on the Culture of Rhododendrons
Clarence Prentice, Seattle, Wash.

        I am writing these notes as a result of my observations of growing and associating myself with rhododendrons over a period of 35 years. As Darwin once said, "To be a good observer, one must be a good theorist." And it is with this thought I have formulated these notes.
        The family of plants called rhododendrons is much freer of disease than we are led to believe. Lord Aberconway was once asked what were the worst diseases of rhododendrons. His answer was, "Rabbits and Sir Stafford Cripps." I am thoroughly convinced that fungus, virus and pests are the result of conditions rather than actual infections. It is true the virus or fungus can be classified and does exist but the causes are due to conditions and if these conditions are changed, the trouble disappears. It can be a lack of certain elements. Plants are like people. They have different personalities, likes and dislikes; but they are very gullible when it comes to diet. They eat most anything put up in a pretty package. Plants do not have a facility of choosing their diet. They take up poison as readily as nitrogen and have no way of knowing how much to take. Plants take overdoses just as readily as the proper amounts.
        As yet we do not have any definite formulas tailored for the rhododendron's needs. We suspect that a lot of things are necessary and then work from the trial and error method. Some times these are beneficial but some times we burn the leaves. So here again, if the cause is removed, the trouble disappears.
        Growing rhododendrons is not merely digging a hole and planting a plant - trusting to luck for success. First we should study the natural habitat of the species rhododendrons. If we can master their culture, the rest is quite simple. Most species are found in Asia, more particularly the Himalaya Mountains. These districts are different from our garden conditions as they are on edge-vertical not horizontal. Doctor Rock has told us from one river bed to another, a distance of 40 miles by air, you have a difference of elevation between the two river beds of 20,000 feet. This means the country is straight up and down and this is the native habitat of the rhododendron.
        The soil under these conditions must be different from our carefully prepared compost piles and certainly there is no peat moss there. The soil must be of volcanic origin, rocky, coarse, decomposed rock with some roots, grasses, needles and some leaves. It is not deep soil, except in the deeper crevices where soil would accumulate. The drainage is naturally complete and allows no stagnation.
        Another big factor is climate. The native climate is in reverse to the climate of the Northwest; the falls and winters are dry while the springs and summers are wet. So all in all it is almost a miracle that these plants adapt themselves to the conditions we put them in in our gardens.
        We now return to the problem of conditions as the cause of most of our troubles with species rhododendrons. First, we are too good to these plants in our impatience in trying co force them in to fast growth. Because of this, it is natural to assume that a great deal of trouble is winter damage or the reversal of climate - our summers at times being rather dry and winters very wet. Our soil is loaded with peat moss and is sticky and gooey in winter. I grant it holds moisture in summer. But it is this winter wet that does the damage and this damage does not show up until summer. Rhododendrons do not show the damage immediately. To prove this, break a branch of a rhododendron and stick it in the ground in the winter. It will stay green and look just like the plant until the warm spring days come. Then it will dry up. The same is true of a plant girdled in the winter by weevil. It looks fine, may even bloom, then all at once it starts to turn brown and dies quite fast. Your conclusion generally is that it has a fungus or virus disease or you think you have over fertilized.
        I am thoroughly convinced that most of the brown spots, die back and bud blast are winter damage from wet feet. Give the plant a change of conditions and the problem is licked. I have done it many times.
        Air drainage is also important. This can cause a lot of trouble where a plant gets pocketed. A difference of 5 feet change in elevation sometimes makes a great difference. The Northwest from Olympia north where many thousands of rhododendrons are grown has soil of purely glacial debris, variable in different sections from sticky blue clay to pure gravel with nearly all the plant food washed out. So in our district we are faced with physical problems of soil first.

The Preparation Of Soil And Beds For Planting Rhododendrons

        If soil and beds are thoroughly prepared, your results will be very gratifying. I have come to the conclusion that peat moss has been over-promoted and used in too generous quantities for good culture of rhododendrons. It is the course of least resistance (the easy way) but in proper proportions it has merit. Saw dust is being used extensively and I have seen some very good results in its use. I have nothing to offer on sawdust as I have never experimented with it for the simple reason I do not like the appearance of a garden covered with sawdust. It looks too much like the country fair or a circus. Then too, I always get my shoes full when we put in a flower show.
        I am thoroughly convinced that for best results the top soil from the woods with decomposed logs, sticks, needles and leaves-the coarser the better-mixed with the soil is the best planting medium. I believe in digging a $5.00 hole for a 50c plant rather than a 50c hole for a $5.00 plant. The more clay in the soil the more preparation of the physical aspects of the soil is necessary. Soil may be likened to a prize fighter; it can get in very bad physical condition. The heavier the soil, the more difficult to condition, and the more care with plant foods should be exercised. A sandy soil leeches out excess fertilizer rather fast but clay will hold excess fertilizer and may become very rancid. The same condition will exist if it does not have proper drainage as it will hold the water and become stagnant. Rhododendrons will not be injured by lots of water in spring and summer as long as it drains well. They will grow near running water and not be injured but may have trouble growing by a still pond or pool.

The Importance Of Location And The Consideration Of Growth Habits In Planting Rhododendrons

        Some varieties do well in full sun and full exposure while others need shade and semi-shade. This is more important than selecting your favorite color for you must have a plant tailored to your conditions.
        In consideration of growth habits, ask yourself, "Are you planning on a ten year garden or a 20 year garden?" You must choose plants of proper growth habit to maintain a constant contour and composition pattern. So as your garden progresses, will it maintain proper proportions? An example of time is the original plant of R. 'Cynthia' in England which is 100 years old. It is 25 feet tall and has a spread of 75 feet. But who wants to plant a 100 year garden? It would be very dull for the first 50 years.

R. venator
Fig. 25.  R. venator growing on rocks
in the Prentice Garden
Thompson photo

        Now we have arrived at the ceremony of planting this plant. Do not plant it in a swale in the yard. Better build a mound or low rockery, or plant on sloping ground particularly if soil is heavy. Keep plants high. Plant rhododendrons in coarse textured soil. I have seen plants do fine just set on top of soil with a wheelbarrow of soil dumped around the ball. This would make the establishing of plants more of a problem but once established, the results are more gratifying.
        Also you must remember that rhododendrons are busy plants-there is something going on all year round. They do not become dormant like deciduous trees, and are very sensitive to climatic changes, preferring to have their roots cool. When it gets cold they curl up their leaves for protection and when the warmth of spring is in the air they burst forth with all the elegance of the aristocrat of the garden.
        I think we have had a lot of advice on soil acidity and feel we need to use acid fertilizers. However, the soils in the Northwest are naturally acid and in most cases do not need acidifying.

The Rhododendron As A Family

        We should now know the word "rhododendron" as a family of 850 species and thousands of hybrids. It is no more stable than a large family of children-you have as many different personalities and likes and dislikes.
        Therefore it is impossible to make fast and set rules for rhododendron culture. Plants that grow in the native state and 14,000 feet of elevation are certain to respond differently from plants growing at lower elevations. Some grow in moist situation, some on dry ridges and slopes, some like 6.5 pH. and some need 4 pH. This is a tremendous variation. To some a small amount of calcium is beneficial and to others this would be deadly.
        A lot of these questions have no definite answers. As yet we are still groping in the fog on a good many factors of culture. Again most of our information so far is from trial and error.

Selection Of Rhododendrons

        In selecting varieties for the garden, I think we have been over sold on some of these debutantes from the promotions of the hybridist. We have jumped at conclusions as to their true values. After all, it is impossible to adjudge the garden value of a plant by passing judgment on a blown up cut truss-making citations to a new hybrid from these facts. I feel that a more definite appraisal should be made of the culture, habit and constitution of the plant before it is acclaimed. I feel first crosses may be very fine and occasionally a good one. But my experience with the first crosses is that they lack vigor and until the breeding has been carried on to attain hybrid vigor, you are very liable to be dealing with a very unstable plant that gives a poor account of itself in the ordinary garden conditions.
        I would like to see a program of discussion on the merits of some of these highly rated debutantes, and have the American Rhododendron Society demerit the over-rated varieties, recognize the plants of real garden merit and establish a definite nomenclature of names of plants-that any named plant must be a clone and not a family of plants. This has not only been very discouraging to the gardeners but also to the growers. Each grower or collector swears his plant is the best form. So what does a name mean? It is like calling a family of boys all James. It really does not make sense.
        At present there is considerable work being done in this line of thought. I think if seedlings are sold, they should be sold purely as seedlings and not as definite crosses and the naming of plants should be a very serious business. If the plant does not have merit over proven varieties, it should be sold as a seedling and not named, making our lists of varieties more complex. I feel there has been too much enthusiasm in exploiting names-tying name tags on every plant that is a little different and ballyhooing them to the trade and public before the merits have been proven. They should be carried as a number until proven.
        So in summary, I would like to impress the fact that 1) all the leaf blemishes are not disease but may be due to soil conditions brought on by improper selection of plants for your particular situation. 2) Only plants of proven qualities should bear names. 3) Get your plants from a reputable source and don't buy the plant mistakes on the wayside stand or market. 4) Get your advice from competent people who are horticulturists. 5) Do not buy a plant and sack of fertilizer at the same time. For this last item, allow your plants to get established before you fertilize.
        So after all it is all quite simple if you do it the proper way but any deviation may spell failure. They always say "practice makes perfect." But if you practice the wrong way, the more you practice the more imperfect you become.


Volume 9, Number 2
April 1955

DLA Ejournal Home | QBARS Home | Table of Contents for this issue | Search JARS and other ejournals