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Journal American Rhododendron Society

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Dr. Glen Jamieson ars.editor@gmail.com


Volume 21, Number 1
January 1967

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The Fundamentals of Rhododendron and Azalea Culture
J. Harold Clarke

Botanical Classification
        Rhododendrons and azaleas belong to the genus Rhododendron of the family Ericaceae. This is sometimes called the heath family and does include the heaths and heathers, besides blueberries, pernettya, kalmia, andromeda and several other ornamental plants. Most of this group require a rather acid soil and good drainage.
        More than a thousand species have been described within the genus Rhododendron. Because of the extreme complexity of the genus, the species have been grouped into some forty or more series, each including a number of species which have points of similarity. All of the species of Rhododendron known as azaleas are included in one group, the Azaleas Series, which includes some seventy or more different species. These are distinguished by botanical characters but it is not easy to give a simple distinction separating all those rhododendrons which belong to the Azalea Series from those which belong to the other Series. There are evergreen rhododendrons and azaleas and also deciduous rhododendrons and azaleas.
        Used botanically, therefore, the word Rhododendron includes azaleas. Horticulturally we usually speak of rhododendrons and azaleas, the former including all the non-azalea members of the genus, and that is the way the terms are used in the following pages.
        The many rhododendron species have come from a wide range of native habitats. A large number of our cultivated rhododendrons are derived from species coming from Asia, especially the foothills of the Himalaya Mountains, western China, northern India, Burma and Assam. Others come from Japan, some from Europe and some from North America, including R. catawbiense, the source of hardiness of most of the hardy hybrids grown in the eastern United States. At present a rather large group of species is being studied and named in the East Indies, especially New Guinea.
        Up to the present time some 10,000 rhododendron and azalea horticultural varieties have been named, although not all of them are still in existence; but many new varieties are being named each year.

Climate
        Rhododendron species are found in the wild from the Arctic regions to the tropics spanning a wide range of climate. In the United States a large number of varieties and species can be grown in the Pacific Northwest, especially between the Cascades and the Pacific Ocean. Favorable climate extends down the Pacific Coast, the area getting narrower as it goes south, with the San Francisco Bay area, so far, being about the southern limit of easy rhododendron culture. Azaleas, however, can be grown successfully still further south, a situation which holds true across the country. We find azaleas being grown in great numbers along the Gulf Coast, and other places in the South and Southeast, where so far rhododendrons have been considered difficult.
        Rhododendrons and azaleas grow well in the milder parts of the Middle Atlantic States and down into the Carolinas. Further inland, both toward the west and toward the north, where minimum temperatures reach -20° or lower, only the very hardy varieties will thrive. Some of the deciduous azaleas are extremely hardy and are becoming more popular in the North. Throughout most of the Middle West and the Great Plains area, rhododendrons can be grown but only with special care, and attention to the hardiness of the varieties planted.
        The range over which rhododendrons can be successfully grown is being increased fairly rapidly by the breeding and introduction of varieties that are more resistant to low temperature and to high temperature, and the development of cultural methods which tend to offset unfavorable conditions. In general the northern limit to which rhododendrons can be grown is determined by minimum temperature. It has sometimes been assumed that the southern limit is determined by maximum temperature. More recently it seems that temperature is only one factor and that the southern limit may be determined to a large extent by the prevalence of root rot organisms.
        In general we consider rhododendrons and azaleas to have the same cultural requirements, with azaleas being somewhat "less fussy," hence "easier to grow."

Soils
        Rhododendrons and azaleas prefer rather light soils although they can be grown in heavy soils if special precautions are taken, as will be mentioned later. They do need good drainage, good soil aeration and an ample supply of soil moisture during the summer.
        A high content of organic matter is almost essential in most garden soils. In fact in unfavorable areas rhododendrons are often grown in straight peat moss with a very small amount of soil added to it.
        There may be exceptions with respect to a few odd rhododendron species but practically all species and varieties prefer an acid soil, the optimum degree of acidity being around pH 4.5 to pH 5.5. There are soils, mostly below pH 4, where it may be helpful to add a little lime but this is the exception rather than the rule. In many areas soils are too high in their pH reading and need to be acidified.

What To Start With
        There are occasional advertisements offering rhododendrons, not by name, but in "pink, red and white colors." Obviously, these are seedlings, their quality unknown, and frequently inferior. If you are going to grow rhododendrons it would certainly be advisable to start with named varieties which are clones, grown from cuttings of plants with known qualities. Unfortunately there are a few "group" varieties, that is breeders have given a name to a group of sister seedlings and not to one single plant from which propagation has been by cuttings. This practice of naming group varieties is discouraged by all rhododendron organizations and very few breeders are now doing it.
        Many rhododendron fanciers are growing species and some of them are very satisfactory ornamental plants. The beginner, however, would probably do well to start with named horticultural varieties whose behavior is relatively well known.
        Rhododendron species are simply the wild types as they exist in their native habitat. Usually they have been collected as seed and come relatively true from seed. However, there is always some, and in some cases extreme, variation within a species. In some species superior individuals have been selected out and grown from cuttings. Such plants would be preferable, unless you are especially interested in growing seedlings, and doing your own selecting.
        Rhododendron nursery plants are available as very small plants without flower buds or as larger sizes with flower buds already formed. In some cases the larger plants survive transplanting better but they are more expensive.

When To Plant
        Rhododendrons should always be moved with a ball of soil, either in a container or with the ball of soil wrapped with burlap. In favorable climates rhododendrons are transplanted almost any time of the year with reasonable success. In most areas, however, it will be better to transplant in the fall or early in the spring. There are places in the south where fall planting is favored because the plant has a chance to make some roots and become established before the long, hot summer. In areas where winters are severe, spring planting is usually favored so that the plant will not be exposed to the drying cold winds of winter before it has made new root growth. Fall planting usually means late September, October or early November; spring planting, as early as the soil can be gotten into good condition.

How To Plant
        The first things is to consider the nature of the soil and prepare or modify it if necessary. This may mean working in peat moss or other organic matter, or possibly acidifying the soil if it is too alkaline. Ordinarily if it is more alkaline than pH 6 it would probably be advisable to add some agricultural sulfur unless the soil is going to be modified by using a large amount of acid peat moss. By large amount I would mean at least half of the planting soil. The amount of sulfur to add would depend on local soil conditions and so it would be advisable to confer with the local Extension Agent. Do not use aluminum sulfate to acidify the soil as the aluminum residue may be harmful to the plants.
        The plants will come burlapped or in cans and should be slipped out of the can or the burlap should be removed. Where plants are heeled in for some time in sawdust bins in the nursery the roots will grow out through the burlap and the burlap eventually decays. Where the roots are going through the burlap it should be left on. Simply open it up around the trunk and lay it back, or pull off the top part that has rotted loose.
        Rhododendrons and azaleas are easily damaged by planting too deeply. The top of the root ball should be at the surface of the ground or a little above it. Never plant "an inch or two deeper than the plant was in the nursery" as is sometimes recommended for other types of plants.
        We used to see directions for digging a large hole and filling it with organic refuse, peat moss, sawdust, and soil, especially where soil conditions are somewhat unfavorable. This would be satisfactory under conditions of very good drainage. However, where the soil is heavy the large hole, filled with porous material, becomes a "tub" holding water to the height of the water table in the surrounding soil which may be high, during rainy weather, or periods of prolonged watering. In very light, sandy, acid soils which are ideal for rhododendrons, they may simply be planted in a hole a little larger than the root ball.
        At the opposite extreme are the areas where the soil is alkaline, heavy, inclined to be waterlogged in winter, and probably deficient in organic matter. Under these latter conditions it is advisable to plant on top of the ground, that is a mound, of a mixture of peat moss or other organic matter and soil or sand, is made and the plant is set in that mound entirely above ground. In some cases it might rest on the normal soil surface, in others it would be desirable to build up two or three inches of gravel above the normal soil surface so that the root ball is completely out of contact with the normal soil of the area. The mound of soil may taper off at the edges, or be confined by planks or logs in the form of a planter. Such raised beds require special attention to watering during the summer.
        In those areas in the south where rhododendrons have usually died a short while after planting there is considerable evidence that this death is due to root rot organisms prevalent in those areas. During warm weather, and with plentiful moisture, the fungi flourish and the plant succumbs. Using a fungicidal drench at planting time in these soils may be helpful, especially one considered to be successful against Phytophthora root rot.
        If a root ball is dry it should be thoroughly soaked in a tub of water before planting. Under normal conditions it is not necessary or desirable to break apart the root ball. Where it is difficult to get rhododendron roots to grow out of the root ball into the surrounding soil it may be helpful to hose off the outer part of the root ball until the ends of the roots are exposed. If the plant had been growing in a can, and there is a mat of roots around the outside, this mat should be broken up, to encourage new roots to "grow out of the root ball." The newly set plant, with the soil carefully filled in around it, should always be well watered in.

Subsequent Care
        Although rhododendrons object to too much soil moisture, they are shallow rooted and the roots may dry out during the summer when deeper rooted plants show no ill effects. That means that they should be well watered during the summer, especially in the case of newly set plants.
        It is also highly desirable to use some kind of a mulch which will help conserve moisture and eliminate the need for cultivation. Normally no cultivating should be done around rhododendrons. Weeds should be pulled, or in extreme cases shaved off with a sharp hoe. A fairly deep mulch of sawdust or shavings will be reasonably effective in keeping down weeds. Leaves are all right although they tend to blow away during the winter. Peat moss as a mulch is not too satisfactory, because it is so difficult to wet once it gets thoroughly dried out, although it would be better than leaving the soil bare. Pine needles, wood chips or any similar material which will prevent the sun's rays from striking the ground directly, and the drying winds from actually hitting the soil surface, may he used.

Fertilization
        In very fertile soils rhododendrons may sometimes be grown well without receiving fertilizers. However if they are mulched with something like sawdust there will be a tie-up of nitrogen and the plants are likely to show yellowish leaves and short growth. Except where soils are very fertile it is my feeling that an application of a complete fertilizer, spread on top of the soil at planting time and before the mulch is applied, would be desirable. The amount to use would depend on the normal fertility of the soil, which the gardener may be able to judge by his experience with other plants. Usually there will be need for some fertilizer and so on soils of what might be called average fertility, about half a teacupful for a small plant. to a teacupful for a large plant, of 510-5, or similar material, might be used.
        The nitrogen in this material, as in all fertilizers used on rhododendrons, should be of the ammonium or urea forms. Most ericaceous plants for some reason do not seem to thrive with nitrate fertilizers. Ammonium sulfate is a very satisfactory material to use on rhododendrons which need n1trogen. The nitrogen is in the form rhododendrons like and the residue has an acidifying effect. Urea is satisfactory but does not have the acidifying effect of ammonium sulfate. In most soils nitrogen is the element most likely to be needed, probably every spring, especially where there is an organic mulch. Need for nitrogen is indicated by leaves which are pale green but otherwise normal. Scatter ammonium sulfate over the mulch and water in, late winter to mid-summer if needed.
        It is understood by most gardeners that heavy applications of nitrogen fertilizers in late summer are likely to stimulate soft growth which will be susceptible to winter injury. Some carry this to the extreme and let plants become semi-starved during fall and winter. Recent research indicates that plants reasonably well supplied with nutrients, including nitrogen, are more resistant to low temperatures than those that are starved.
        All plants require potassium but in many cases there is sufficient potassium in the soil. Your local Extension Agent can probably tell you whether soils in your area are likely to be deficient in this element.
        All plants also require phosphorous and while phosphorous may he present in rather large quantities in the soil it is sometimes fixed in compounds unavailable to the plants. Some recent fertilizer experiments indicate that with young rhododendron plants heavy and early phosphorous fertilization tends to favor early production of flower buds. This would depend on the amount of available phosphorous naturally occurring in your soil. Phosphorous does not readily move down into, or through, the soil, hence it is important to see that there is a reasonable amount in the soil at planting time.
        Magnesium, in the form of Epsom salts, is sometimes recommended for rhododendrons. Magnesium is an essential element and lack of it will cause yellowish areas between the leaf veins. If the leaves are a good dark green the addition of Epsom salts would not be useful.

Protect Against Low Temperature
        First plant varieties hardy enough to do well under your normal winter conditions. The A.R.S. Hardiness Ratings indicate the minimum temperature that a well matured plant can be expected to survive without damage. It is not too difficult to find, from the weather bureau, or your neighbor, or from your own experience, the likely minimum temperature which may be expected in your garden. Even with the hardy varieties it will usually be worthwhile to plant in a reasonably protected place, especially where they will not be subject to cold, drying winds during the winter. In some northern areas gardeners protect rhododendron plants by building a windbreak around them, or covering them with burlap, or other protective material during the worst part of the winter. The average home owner may prefer to occasionally experience a little damage to the plants rather than go to the trouble of winter protection. However, protection can be given if a plant is considered to be particularly valuable.
        Even in the so-called ideal rhododendron areas there may occasionally be damage by early fall frosts or by late spring frosts. In the Pacific Northwest, for instance, there are early varieties which will start blooming in February and others will follow along all spring. These early varieties may be quite hardy in bud but the open flowers will be damaged if there is frost. Such varieties are usually planted in a protected place and may be covered during a frosty night. It is possible, under certain conditions, to keep a garden sprinkler running during the freezing period, and if the flowers are continuously coated with water they will be protected from several degrees of frost. This should not be attempted where very low temperature is expected as heavy ice formation could cause breakage.

Shade
        Shade affects the temperature, and soil and air moisture, and so is an important factor in rhododendron growth. In most cases partial shade is desirable; in hot, dry areas it is essential. In good rhododendron areas the plant may survive quite well in full sun but when it is in bloom the flowers will last much longer in partial shade. There are a few varieties which simply will not tolerate full sun, having quite yellowish leaves under such conditions. There are many others which, in a reasonably favorable climate, will make better growth, a better shaped plant, and will set many more flower buds if grown in almost full sun. The beginner, without definite knowledge as to the requirements of a variety he may have purchased, would do well to plant it where it will receive some shade during the middle part of the day. Nurseries commonly keep their rhododendron plants under lath shade in order to keep the foliage in good condition. Gardeners in warm areas occasionally build a small ornamental lath house for their rhododendrons and certain other plants which require shade.

Pruning
        Normally very little pruning will be needed. If a plant grows out over a walk or needs to be restricted for some reason, it may be pruned back moderately without fear that the plant as a whole will be damaged. It is often possible to do this pruning during the blooming season and have some flowers for the house. Sometimes a gardener has an old, leggy rhododendron plant which has grown out of all relation to its surroundings. Such a plant may be pruned back and rejuvenated but it must be done with some care. Usually about a third of the taller branches might be removed one year, half of the remaining branches removed the next, and the rest of them removed the third year, cutting back to two or three feet from the ground or wherever smaller side branches are available for starting the new framework. Often these old plants are of a poor variety and it would be better to replace them with a smaller plant of a superior type. Moving an old plant is possible provided a large ball of soil is dug with it.

Dead-Heading
        It is desirable, with the large flowered rhododendrons, to remove the withered flower cluster after the blooming season. This is fairly easily done as the central axis of the cluster, or truss as it is usually called, will break free from the plant with a quick snap of the thumb pushing on the side. With the smaller flowered rhododendrons and azaleas, deadheading is hardly feasible and in general unnecessary.
        The reason for dead-heading is to make the bush look more attractive, to prevent mold setting in on the flower petals and going down into the stems, and to prevent a heavy set of seed. Some varieties in some areas will set seed heavily and so really need deadheading. Others practically never set seed and so dead-heading with them is less important. Immediately after the flowers wither will be best from the standpoint of appearance. However, it may be done somewhat later, and removing the developing seed pods may be helpful at that time. If for some reason one finds it impossible to take off the old flowers it will usually not be extremely serious.

Pest Control
        We have mentioned the removal of weeds and their suppression by use of a mulch. Gardeners will usually find no really serious problem with weeds unless they have some of the undesirable perennial sorts such as horse tail, quack grass, Canada thistle, or some forms of morning glory. It would be advisable to eliminate such perennial weeds before planting if at. all possible. In some cases it may be practical to eliminate them by chemical weed killer. However this is a tricky business with perennial weeds and it would be advisable to secure local advice from your County Extension Office before attempting it.
        It may also be advisable to secure local advice as to control of insects and diseases. In general rhododedrons are not particularly subject to pests but in some localities there are rather bad ones which may not be present in another area at all, hence the need for local advice. Probably the most common insect pest in the warmer areas is the lace wing bug which works on the underside of the leaf. The tiny young nymphs move around on the underside of the leaf and suck the juices causing a yellowish spotting, and if there are many insects the whole leaf will turn yellow, then brown and drop. Because they work underneath the leaf they may not be seen until damage has been done. They may be controlled with a contact insecticide such as Malathion used according to the manufacturer's directions. In the warmer regions it would be well to keep a close watch for this insect.
        Another widespread insect pest is the root weevil. This may be one of several different species. The adults chew notches in the edges of the leaves and the larvae go down into the soil and feed on the roots of the plants. They may completely girdle (remove a ring of bark) young rhododendron plants and the plants then die. The girdle is below the surface of the ground, perhaps half an inch to an inch and can be observed by pulling away the soil. On a girdled plant the leaves will droop and become pale and lifeless looking. The control may consist of a poison bait for the adults, or a contact spray directed onto the ground around the base of the plant about the time the adults are beginning to emerge. Aldrin has proven satisfactory for most of these weevils, but there is one called the woods weevil which is not susceptible to Aldrin. Fortunately it can be controlled with DDT. Follow the recommendations on the package.
        In certain areas aphids, scale insects or leaf eating caterpillars may cause some damage. Normally they do not require regular spray schedules but may be controlled with a contact spray if they appear.
        There are relatively few diseases affecting rhododendrons. Considering the whole country, root rots, and especially Phytophthora cinnamomi, are probably the most important. Certainly that is true if we are right in assuming that the main reason that rhododendrons have so often been unsuccessful in the south has been because of root rot. These fungi may also cause the death of young plants in the nursery, in almost any area, unless some effort is made to prevent it. At present there seems to be no way to cure a plant attacked by root rot. The important thing is to set plants in well drained soil and give them moisture conditions as nearly ideal as possible, meaning good drainage but ample moisture during the summer months.
        In the south there are areas where a petal blight disease causes serious damage, especially during humid weather. This has been difficult to control but where it is serious local authorities can suggest sprays which will be reasonably effective in preventing it, or at least in cutting down the damage it causes.
        There are several rust fungi throughout the United States which cause yellow or orange pustules on the underside of the leaves. In general these have been more obvious than harmful although in a few cases, with susceptible varieties, they may disfigure the foliage. No practical spray program is now available.

Propagation
        The beginner will normally secure his plants from a nursery and so the subject of propagation is relatively unimportant here. However rhododendrons usually become a hobby plant and many hobbyists want to do some propagating for themselves.
        As mentioned above, rhododendrons can be propagated by seed, the usual way of propagating species. An easy way for the beginner to raise seedlings is to use a transparent plastic refrigerator dish holding a quart or more. By putting an inch or more of moist peat moss in the bottom of the dish, sowing the seed on top of the peat and then putting on the cover, one has a miniature greenhouse. If the peat was properly moist, and the lid is left on, it should not be necessary to water until after the seeds have sprouted which should be within three to six weeks. The dish should be kept at room temperature, outside of direct sunlight, but in a well lighted spot. If it is available a little dry, ground sphagnum moss may be sifted over the peat and thoroughly moistened before the seeds are sown. Seedlings on sphagnum moss are less likely to be affected by dampening off. The advanced amateur with greenhouse space will use flats and cover them with plastic sheeting. After the young plants are one quarter to one half inch high they should be transplanted into a flat of peat moss for growing on.
        Rhododendrons may be rooted from layers. A low branch may be pegged down into a trench and covered with two or three inches of soil. Usually cutting a tongue on the underside of the branch, cutting away from the plant at the place where it is buried, will hasten rooting. Rooting may require a few months and then the new plant will need to be set in a nursery row to straighten up and develop a desirable form.
        A great many rhododendrons used to be grown commercially by grafting. Plants are now preferred on their own roots because the under stock, usually R. ponticum, is prone to send up suckers which often over-grow and engulf the scion growth so that people sometimes say their rhododendron has changed color after the under stock has taken over.
        Most rhododendrons are now grown from cuttings. Commercially they are rooted, usually in August to October, in peat moss, or peat and sand, or peat and perlite, under mist, with bottom heat, and with the use of root inducing hormones. The amateur can readily set up an arrangement for rooting cuttings in a small frame with a heating cable. It would be well to do a little reading on this subject as space does not permit an extended discussion here.

Use In The Landscape
        Perhaps it is a tribute to the rhododendron that so many people, at least with their first plant, will set it out in the middle of the yard as a specimen to be admired by itself. Actually it is nearly always better to consider the rhododendron as a part of the landscape and plant it in a border or a foundation or island planting with other shrubs. The other plant material may include a magnolia, or some other tree, which will provide some shade, and add height to the planting. The rhododendrons should be grouped to achieve certain effects, considering size, color, season of blooming and other characters. Usually the taller growing plants should be towards the back of the planting, medium sized ones in front, and low growing rhododendrons or other plants around the edges. Some of the tall growing rhododendrons tend to become rather leggy and the lower plants in front of them hide the bare trunks and make for a much more pleasing picture.
        Rhododendrons and azaleas go together very well in a landscape planting. With the deciduous azaleas thought should be given to the fact that there will be bare spots during the winter when the leaves have fallen. Usually it is better to plant evergreen azaleas together in masses and somewhat the same is true for deciduous azaleas and rhododendrons in order to avoid too "spotty" a picture.
        In laying out a planting thought should he given to the ultimate size of the plants. It seems that the size of the plant in ten years is a good criterion to use as most of us are not concerned as to how large a plant will be fifty or a hundred years from now. There is really a great difference in the ultimate size of rhododendron and azalea varieties and if they are not laid out with this in mind the rapid growing ones will soon shade out the slow growing ones. Estimates as to mature size may be obtained from various catalogs or from the A.R.S. Ratings Tables. Rhododendrons are friendly plants and seem to do better if planted in groups than if set out singly. In a group each plant will fill up its own area so that at a little distance the whole border may seem a solid mass of foliage. A too aggressive variety may take more than its alloted space which may mean moving out the ones being crowded. Rhododendrons are relatively easy to move because the root system is quite fibrous and compact, and if the plant is not too large most of the root system can be gotten out in a manageable sized root ball, and the plant can be moved with little likelihood of hurting it. In fact rhododendrons are often dug up, taken to flower shows, and then planted back in the garden where they were growing, with no apparent harm.

Varieties To Plant
        One of the most important requisites to success with rhododendrons is getting the right varieties to start with. In a general article such as this it is difficult to specify particular varieties without going into a long discussion. Lists could be made up, for each growing area, based on color, on hardiness, on season of bloom, on size of plant and the many other variable characteristics.
        For the first plant the advice of a local nurseryman should be as good as any. If one becomes bitten by the rhododendron bug and wants to increase his collection, he should study variety lists and read variety descriptions. It would be well, at least for the first few plants, to stick with those considered to be the old standard popular varieties. Others may have a particularly charming flower or color as you see them in a show. However, the ones usually available on the market have probably attained that status by being good growers, adaptable to a fairly wide range of conditions, reasonably hardy, and perhaps are more able than others to take a little abuse and neglect. Start with them and then go on to the more beautiful but more difficult varieties. There are some real good varieties I would like to mention but if I started then I would have to list varieties for different climates, different soils, different seasons, and would wind up with pages of variety discussions. Study the A.R.S. Ratings and the experiences of your neighbors. There is one consolation, that you're very unlikely to purchase a rhododendron which will not grow into a beautiful plant.


Volume 21, Number 1
January 1967

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