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Journal American Rhododendron Society

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Volume 16, Number 2
April 1962

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Liming Rhododendrons
Alleyne R. Cook
Vancouver, B.C.

        I feel it is time to change the old idea that the addition of agricultural lime to rhododendrons is harmful. The probable reason for thinking that lime, which to me is agricultural lime or ground limestone or marl - is harmful to ericaceous plants arises from the fact that large areas of England are pure chalk with a high pH. These areas crop up in odd places. For instance Windsor Castle is built on a chalk hill. Three miles away on acid soil are the world's finest gardens. The Hastings Ridge - along which are the gardens of Nymans, Leonardslee, South Lodge, Wakehurst Place and Sheffield Park - is acid between the North and South Downs both of which are chalk ridges. Everyone knows that rhododendrons will not grow in chalk. Thus the English and the writers of that country without bothering to check if added lime to acid soil would hurt rhododendrons always proclaimed that lime could not be given.
        Bunk! They don't know what they are talking about simply because they have never tried it. If they think that a handful of lime spread on the ground as though it was going on a lawn will change the acidity to any great extent then their knowledge of the soil is very slight. At the same time that lime is going to do good by making elements available to the plants.
        It was the plant collector George Forrest who started the greatest controversy concerning rhododendrons on limestone. On the slopes of the limestone Lichiang Range in China he found them for miles. The observations he made stirred English growers to try growing rhododendrons on English chalk country. One man even published a series of experiments he had conducted to determine which species were less tolerant than others. Finally they found out what they had actually known all along-that rhododendrons will not grow on the English chalk. But growing them on chalk which is obviously alkaline and adding lime to acid soil to improve growing conditions are two completely different aspects.
        Actually Forrest's observations pointed out how unobservant were the English growers. In the botanical garden in the Swiss town of St. Gallen are two rock gardens, one made out of limestone, the other of granite. All the plants growing there are in simulated natural conditions. R. ferrugineum and R. hirsutum are not planted on the latter as one would assume but on the limestone. In the Alps I did see two hillsides covered with R. ferrugineum but omitted to note what rock formation was present. I have color slides of R. hirsutum in the Austrian Alps surrounded by limestone chips.
        Then there is the argument that these plants are growing in the humus that is above the limestone and this must be acid. Why it should be acid no one ever explains. I have seen R. albiflorum in such conditions on the Teton Pass above Jackson Hole in Wyoming. Forrest however had repeatedly stated that plants grew in the crevices in direct contact with limestone and these were the same species that he also found in the decaying humus of the forest and meadows. It must therefore be assumed that the limestone was insoluble and was not effecting the plants.
        Experiments carried out in England by J. P. Fanning and published in the 'Gardeners Chronicle" showed that the amount of calcium in the foliage of such trees as oak and beech growing on chalk was far greater than when they grew on acid soil, and that for gardeners to attempt to use the humus derived from such trees to build raised beds in which to grow their rhododendrons would lead to the death of the plants. But again no one bothered to carry this experiment to its logical conclusion and see what would happen if this humus were used as a mulch around rhododendrons growing in acid soil. It was taken for granted that they would die.
        I repeat that I think it is time to change all these ideas. I have had personal experiences with the following examples, all of them were carried out more than two years ago which has given time for any ill effects to show.

No. 1. In the New Zealand nursery where I first worked plants used for layering had as rough a life as could be imagined. First the lower branches that would bend easily to the ground were layered. After one or two seasons-depending on how fast the layers rooted - these layers were removed. A hole, sometimes feet deep, would then be dug and into this the root would be dropped. This would bring the next tier of branches to a level where they could be layered. When these had been removed the plant would be dug up and tipped on its side so that the upper branches would be within easy reach of the ground. A plant of R. 'Goldsworth Yellow' had undergone part of this treatment and it was quite obviously about to die. As nothing would be lost we covered the area around its roots with such a coating of lime that the earth was not visible. Instead of dying the leaves improved in color and the following season's growth was back to normal. The layers which were down at that time all rooted and when I left the plant was still growing well.

No. 2. The success of this experiment led to the application of 2 tons of lime per acre over the 6 acres of stock plants. These plants were the mainstay of the nursery. They were used for layering, for cuttings, for bud and grafting wood. Beside these uses, customers could see what their plants would be like when grown in their own gardens. This is probably the most comprehensive collection of commercial shrubs to be found in any nursery, certainly I have never seen its equal anywhere in the world. There were rhododendrons such as R. arboreum and its variety keriiiesinum, members of the maddenii series, the cinnabarinum hybrids and numerous other modern and hardy hybrids. In all these would probably cover an acre. Then there were the Magnolias which are also acid loving. There were 76 stools of M. grandiflora and the same number of stools of M. campbellii each group yielding a yearly crop of about 2000 layers. There were also rows of stools of most of the other Magnolias.
        Other ericaceous plants included a complete selection of the Pieris some 30 species of the Cape Heathers, which are in the main small shrubs, and a few of the heathers that are grown here on the Coast. There were numerous varieties of deciduous and evergreen azaleas - the former stooled, the latter for cuttings.
        The collection of South African and Australian natives was far more comprehensive than any I have seen anywhere. These were constantly being changed for if a plant had no commercial use out it went.
        There were bushes of more than 50 varieties of Camellias and in a lower section where the soil remained damp some 50,000 hardwood Camellia cuttings were rooted every year with no attention and just a burlap cover in the summer.
        Even if the rhododendrons were tolerant of lime it is very likely that among this collection there were plants that would be affected to a greater extent and would show it. As long as I was there nothing suffered any ill effects. It was perfectly obvious that all these thousands of plants benefited from the dressing of lime.
        For years stable manure had been dug into the ground which is naturally light and porous. It is quite likely that the acidity of the naturally acid soil had increased until the elements needed for the successful growth of the plants were not becoming available, and the resultant increase in growth after the lime had been put on was caused by release of these elements. It is certain that such a collection would not have been risked if there was any likelihood of wholesale deaths for upon these plants depended the livelihood of the nursery.

No. 3. This is a single plant of R. 'Sappho.' The garden is a thin layer of soil over very hard hard-pan. To the original soil copious quantities of peat had been added (and are still being added) in the belief that this will supply everything the plant needs. The foliage of this R. 'Sappho' was in poorer condition than other plants and on the understanding that if it died I would replace it with two plants the owner screwed up enough courage to allow me to give it a light dressing of lime. The anemic foliage promptly turned green and remained that way for about 2 years. Although it has not returned to its former color and the foliage is still too pale for this variety to be in keeping with the remainder of the rhododendrons in the garden. The entire garden could stand a dressing of lime similar to that which is put on a lawn, but the owner having used his reserve of boldness is unable to face what for him would be a drastic step.

No. 4. The nursery of L. C. Living is on the peat soil of Lulu Island close to the peat farms which supply most of the Canadian peat used in Coast gardens. To his land he applied 7 tons of lime per acre. It is known that peat has the ability of locking lime and releasing it slowly but even so that amount should have been enough to alter the pH from very acid to alkaline. It should also-if one is foolish enough to listen to the 'experts' who have never tried such things-have killed all the plants in the nursery.
        Besides the rhododendrons there were Azaleas, Pieris, Kalmias, Leucothoes, Andromedas all of which are supposed to hate lime. Also in the nursery but probably not so susceptible to the lime was his collection of Japanese Maples which he had imported from New Zealand.
        Along side the treated area was another on which he was growing similar plants, usually the same varieties but different sizes. It was interesting to see how green the foliage of the limed plants became compared to the others. The important thing however was that even this heavy dressing caused no losses among the plants. This was 5 years ago and there have never been any ill effects.

No. 5. A single plant of R. 'General Eisenhower' which was growing in the centre of the lawn on the shady side of a house. On someone's advice the owner had spread Epsom Salts around it. Shortly afterwards on my advice he limed it. The result must have been satisfactory because shortly afterwards he stopped complaining about its appearance, and the following year it flowered so well that he was completely happy. It has again budded well.

No. 6. Two bushes of R. ponticum which were growing in the same border, each 4 feet high and about the same through, on both the growths were short, no more than an inch and they had been that way for several years. Around one I spread 12 piled double handfuls of lime working it in so that it would not be noticed. Two months later the border was given a light dressing of bone meal. This was more than two years ago; the treated plant is still alive and healthy. So is the one that was not limed, in fact we think that the foliage of the latter (un-limed) is a little darker, the new growth a little longer, and this winter it is carrying more buds. The probable reason is, the limed plant is competing with a larger Mahonia, itself an acid soil grower, which is constantly sending suckers through the roots of the rhododendron.

No. 7. This is the most interesting of all the plants because at last I thought I had succeeded in killing a rhododendron by liming it.  What variety it is I do not know - one of those nondescript yellow hardy hybrids that have so many different names and all look alike. It was at this time about 3 feet high, the same in width, with annual growth of about 1 inches. The garden 10 years ago was pure gravel. Since then it had been given an annual dressing of 2-4 inches of mushroom manure. With the exception of this one plant everything else in the garden including 7 rhododendrons and 7 azaleas were in perfect condition. Around this unfortunate plant I spread 6 double handfuls of lime. The effect was immediate. The area of each leaf along the main vein, followed quickly by that around the side veins turned yellow. This spread over the whole leaf which then dropped from the bush. Starting with the lower leaves and moving up the plant, one by one they died. At this stage the owner would have liked to lift the bush, remove the soil and try to save it, but knowing my interest in this project he left it where it was.  All the leaves dropped except the small ones on the top of each stem. These turned not yellow but a healthy green. The new foliage the following spring also was green and the new growth had increased from 1 inches to over 4 inches. The greatest surprise came in the flowers. For years the buds had been pale yellow and the open flowers pale cream. This time the buds were deep pink and the flowers on opening pale pink. After a few days the pink faded and the flowers returned to the cream of earlier years. It is the first time I have known the flower of a hardy rhododendron to change to that extent.

No. 8. Three hardy hybrids, each about 5-6 feet high, each looking unhappy probably because of the continual use of artificial fertilizers and the removal of anything from around the roots that could act as a mulch. Each one was given 12 double handfuls of lime which was washed into the soil.  When these plants flowered in 1961 they were all looking well. Since then the condition of two of them has deteriorated. The one nearest the house is in perfect condition, but it gets much more shade than the other two. The plant furthest from the house is most exposed and also during the summer gets considerable reflection from a concrete patio. It looks like "death warmed up." The third has green foliage on the 1961 growth but the remainder looks miserable. Still, considering that these plants were given about 10 times the amount of lime that I would normally give it is remarkable that they are doing as well as they are. If it were not for the fact that the rhododendrons at the L. C. Living nursery showed no difference in their reaction towards lime I would think that some varieties could stand more than others.

There are three more instances where liming has been done but in which I have not personally taken part.

A. Our former nurseryman limed the heather bed. There were no losses among the 4,000 odd plants.

B. When I was a youth the new person limed not only his lawn but all the flower beds as well. Reports say that the lime dripped from the rhododendron bushes. As a result consternation reigned in the Mother's Guild. (Where is there a woman's organization attached to a church that doesn't spend most of its time trying to run the devil-dodger?) There is no knowledge of anything suffering or dying.

C. One plant that had the same treatment as No. 5. The lime was my suggestion and the plant is doing well.

        In all of these instances lime was only applied once. On every occasion the plants lived, only twice have plants suffered a setback, and one of these has recovered and is in better condition than it was. It is my contention that if a plant is healthy the best thing to do is to leave it strictly alone. If unhealthy then something is lacking and if that something is already in the soil but unavailable to the plants then all the fertilizer in the world will not give a lasting relief.
        It is known that the maximum number of elements needed by rhododendrons are available when the soil is slightly acid. Assuming that the original soil was naturally acid then surely the continual use of peat-which has a very low pH and acid fertilizers and all the other junk that nurserymen and 'experts' have convinced the ordinary grower to buy will tend to lower the original pH. When this occurs the rhododendrons are going to be starved for the elements that are actually in the soil but are not available. I can see no way of making them available except by the use of lime which even if it does not raise the pH will release the needed plant foods.
        If as it is claimed, lime has a toxic effect on rhododendrons it should have made its appearance somewhere amongst all these plants. It might have in the last instance. As I stated earlier lime was only applied once to all these plants. At the same time on several occasions it was applied in much heavier dressings than would be normal. Obviously this is a field for further experiment.
        The amount that is usually put on a lawn should be enough. This means that if in a garden there is a border with plants that are known lime lovers and others such as rhododendrons which can stand a fair amount of lime then everything is going to benefit and nothing is going to suffer. At least that is what I maintain and at the present time I am nearly alone in my opinion.


Volume 16, Number 2
April 1962

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