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Journal American Rhododendron Society

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Volume 10, Number 1
January 1956

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Mulches and Fertilizers
By Warren Baldsiefen, Rochelle Park, N. J.

        These comments following Guy Nearing's article in tile October Bulletin, "Rhododendrons are Vanishing," may seem to be in opposition. This is not the case for I am in complete accord with his basic point that the rake and fertilizers are largely responsible. However, my opinion on the value of mulches and fertilizers differs somewhat and is presented here.
        A decade and a half of experience and careful consideration have taught me that for the most beneficial all around mulch we must turn to the conifers. Both plantations and those indigenous to almost any region provide a variety of species from which to choose. Because of their abundance in our section, Red and Eastern White Pines are utilized the most, although Jack, Pitch, Virginia, Austrian, and Scotch Pines have been used with equal success. Offhand I can think of no region where Rhododendrons are grown that conifers will not also thrive, and any, reasonable effort expended in gathering needles will be well worth the time.
        The needles of Red Pine (Pinus resinosa), being coarser and somewhat larger, tend to resist decomposition longer. We use these on larger plants. The soft delicate texture of White Pine (Pinus strobus) needles makes them extremely well suited for mulching either small alpines or one or two year old large leaved sorts where the mulch, in order to be most effective, must be worked closely around the stems. I have seen pebbles used as a mulch on alpines but did not notice any superior results over those mulched with Pine needles. Moreover in commercial growing where every few years transplanting is necessary the ease and convenience with which Pine needles are removed makes them, in my experience far superior to pebbles. The leaves of Spruce and Tamarack being both short and slender, are exceptionally good mulching for dwarf or very young plants. These, or any other small needles, are applied by being gently broadcast over the plants to the desired thickness. Any needles which adhere to the plants may easily be removed by gently brushing over the rows with a light small stick.
        Beneath plants three years or older which are lined out either in the field in full sun or under lath shade, depending on the parentage, we spread a mulch of Red Pine needles approximately 3 to 4 inches deep over the entire area. After settling which takes several months the depth is reduced to about 2 inches. If undisturbed such a mulch will serve the plants 3 to 4 years. If the plants are removed for any reason, the mulch, if scraped away before digging, can be replaced and reused. Neat in appearance, a needle mulch settles to an even, tidy cover that never can mat down or become sodden. The criss-crossing, interwoven needles allow good aeration yet never under any conditions become scattered by gusts of wind. No unsightly weights in the form of sticks or lengths of board that might entangle spray equipment or otherwise interfere with day to day working around the plants are necessary to hold the mulch in place. Both the insulating and moisture holding capacity are superb.
        Young plants out of pots or out of the rooting frames and older stock up to the time they are lined in the field or lath house are mulched with either Spruce, Tamarack, White Pine or shredded needles of the more coarser pines. This mulch need not be 3 to 4 inches in depth since the smaller needles apparently insulate equally well when spread to only half that thickness. At the height of the summer during the hottest days of the season when the temperature soared into the low hundreds and high nineties the temperature 2 to 3 inches under such a mulch hovers in the mid-seventies.
        Oak leaves are a good mulch, and for that matter I know of no deciduous leaves that are not satisfactory. Shredded they are less apt to be caught by the wind and will cling better to the ground but as this entails considerable extra work it is not usually practiced commercially for plants lined in the fields. It is being done occasionally for small liners. Large leaves in such a situation almost seem to smother little plants. However the old belief that the use of Maple and Elm leaves invites disaster because of their alkaline residue upon decomposition has not in my experience proved true. I have seen many rhododendrons mulched with such a covering without the plants suffering any ill effects. The rapidity with which leaves decompose has a large influence on the acidity or alkalinity of their residue. The more rapid the rate the more alkaline. And so it might be that even oak leaves rapidly decayed, would tend to be neutral. The chief objection to a leaf mulch is their blowing about, accumulating deep in some areas and leaving other sections bare. Especially around plants where the mulching is left to nature's hand, the thick saturated layers matted together form an almost impervious layer, defeating one of the prime requisites of a mulch: to be light, airy and allow free air circulation.
        Peat moss can be used for mulching for lack of a more suitable material. It is not one of the better mulches. I have never seen it used commercially on any scale. Around the home it gives the beds a somewhat manicured appearance, perhaps too much so for rhododendrons. A probable victim of high winds it must be replaced every year, or nearly every year. An outstanding fault and major drawback of a peat mulch is that once dry it sheds water like a duck. Therefore, to serve the purpose of a mulch it must be kept moist with periodic sprinklings during the hot dry summer, perhaps once a week. The thicker the layer of peat the less apt it is to dry. I have never observed the fine rootlets grow to the surface of the mulch and be destroyed though in theory it seems plausible. From past experience in plunging potted rhododendrons into peat I can recall numerous instances when the roots reached into this medium just below the surface, without any apparent harm to the plants. If some roots did rest on the top or near the top of the peat for a brief time before being killed it was not discernible. My theory is that the roots will shun any hot dry area and will reach for conditions advantageous to their development.
        The words "sawdust mulch" are becoming about as controversial as the word "hardiness." There always seems to be someone who will dispute any statement that is made. In the last few years I have become thoroughly convinced that some sorts of sawdust have their useful place in certain phases of rhododendron culture. As a permanent mulch sawdust can be used, and with success. However the material must be coarse enough to allow ample air and water circulation. Sawdust from a craftsman's shop usually is too fine and will pack and cake on drying. Sawdust from the mill where the rough logs are cut is coarser and more satisfactory. Of the many hardwoods and softwoods native to the East, the only sawdust that has decidedly resulted in utter failure for me was White pine (Pines strobus). In instances enough to prove conclusive, a mulch of this material, when fresh, stunted and even killed plants. Death was not confined to shrubby ericaceous plants but to many annuals and herbaceous perennials as well. There seemed to be some toxic effect on these plants and death had no connection with a lack of nitrogen, for nitrogen was present in abundance. This is the only sawdust I have ever known to be so disastrous. I have seen plants thriving under a cooling cover of sawdust. It is a first rate insulator and temperature readings under equal depths of sawdust as compared with other valuable mulch materials, left no doubt but that those plants under sawdust had cooler roots by a few degrees.
        We know that sawdust (mainly cellulose and lignin) in its decomposition to humus will borrow nitrogen from the soil. This is brought about by the cellulose bacteria which demand for their existence more nitrogen than can be supplied by the wood. They therefore take nitrogen from the soil, nitrogen that would normally be used in the growth of plants. And this nitrogen is then later returned to the soil upon the death of the bacteria. In this temporary process of decay therefore, we must supply small amounts of nitrogen to the plants (inorganic being more readily available) to supplement that which h been borrowed from the soil. This mount has been established at 2 pounds per hundred square feet of surface for every inch depth of sawdust mulch applied. It may vary with the kind of sawdust but probably ever so slightly that the above rule will still apply. After the mulch has been in place a year the annual spring feeding should cover the needs of the plants. If not, a trace of additional inorganic nitrogen can be added.
        The commercial use of sawdust, which resulted from accessibility and low cost, and the ease of application, has from my observations of its use from Oregon to New Jersey, resulted in many failures. In commercial growing a crop is removed from an area every few years either to be re-spaced or to be sold. And it is here the trouble begins, for it is when these plants are lifted that some mulch, left from digging, falls into the hole and the sawdust becomes an ingredient of the soil. Perhaps meticulous care might prevent this but I have never seen it so, and in every instance where plants were removed, sawdust, only partially decomposed, was mixed with the soil and the next plants placed there had to compete with it for nitrogen. Many sorts of woods (as sawdust) take a dozen years or more to break down and in this time several crops of rhododendrons can be taken from a given area. Each time, more sawdust enters the soil and each time the supplemental nitrogen feeding becomes more grotesquely complicated. I have seen it become so hopelessly disastrous that as a consequence, fields were left fallow for years. The theory that periodic feeding will eliminate this fault is weak. If in the late summer with leaves showing deficiency symptoms we could ascertain with any degree of accuracy precisely how much nitrogen is needed, a safe application might be made. However this is impossible, for we have sawdust both as a mulch and as a soil amendment in various stages of decay, possibly complicated by several different kinds of wood, and to try to supply the proper amount with out over-stimulating, even to the slightest degree, at such a critical time in the plants' growth cycle just before cold weather is, in my opinion, next to impossible. In sections which are subject to extremes of climate both in summer and winter, any measures which alter the plants' natural metabolism, even ever so slightly, can be fatal at these sustained periods of high or low temperatures. I have also seen the quick death of plants from the effect of sawdust in the soil. In other instances growth was retarded for a year before returning to normal. Here is an account from a close friend of mine and his unfortunate experiences with sawdust:

  1. Used as a soil amendment and also as a mulch for a group of Mollis hybrid azaleas. Initial growth slow, no chlorosis; in a couple of years plants resumed regular growth rate.
  2. Used as a soil amendment and also as a mulch for a small group of catawbiense hybrid rhododendrons. Chlorosis and restricted growth evident to such an extent that it was necessary to move plants out of the sawdust plot to save them.
  3. Used as a soil amendment and also as a mulch for large group of R. calendulaceum and other species of the Luteum Subseries. Some initial chlorosis but no growth restrictions and results generally satisfactory.
  4. Used as a soil amendment and as a mulch for a large group of hybrid seedlings from my own crosses. Results all but disastrous. Immediate chlorosis followed by an epidemic of all sorts of leaf-spotting fungi, and then a virulent epidemic of Phyopthera cinnamomi. Growth sharply restricted. About 35% of the plants in the plot were lost and an occasional plant still drops out more than a year later."

        There have been other instances when the use of sawdust was followed by an epidemic of Phytopthera with such suspicious promptness that some pathologists believe it is quite possible for the sawdust to act as a carrier of this pathogen.
        In innumerable cases where sawdust has been used as a constituent of the soil I have seen the disparaging evidence of failure and heard the complaints against it by those who rogued out the dead plants. I have tried to keep abreast of all advances and progress in the use of sawdust as a soil amendment and I have yet to receive any reliable recommendations based on critical experiments over a period of years that prove beyond the shadow of doubt the safety of the material. In my view, its use should be confined to mulching of rhododendrons which are permanently planted.
        Wood chips make an excellent permanent mulch. The only exception I know of is eastern White Pine which has the same effect on the plants as pine sawdust. Today huge shredders which grind up limbs pruned in line clearance and street shade tree maintenance are a source of large quantities of good mulch. Chips the size of a quarter and smaller and twigs and pieces up to 4 to 6 inches long, infixed with chopped leaves, provide a material that will prove a valuable acquisition to rhododendron culture. Being coarse and of random sizes, in their settling there is always sufficient air space. From my observation over a period of several years there is absolutely no nitrogen deficiency problem as there is with sawdust. This may be due to the fact that chips by comparison, being so much larger than sawdust, have infinitely less surface area and in their decomposition engage relatively fewer microorganisms. This is just an assumption. Chips make a neat covering, by no means unsightly and seem to "belong" with rhododendrons. I think they have a great future as a mulch. Commercially I have seen them used without any problems and where they were plowed into the field after a few years they had no noxious effect on the next crop.
        There are many other satisfactory materials used for mulching and those herein described are only the most popular kinds. As the years go by we will include in the present list of mulches, new and possibly better substances, perhaps replacing what we now consider to be the best.
        Conclusive experience and experiments over many years have taught us that a mulch is mandatory for successful Rhododendron growing. With the exception of an odd plant here and there, I have never seen any large leaved rhododendron successfully grown for any number of years without a mulch even though the utmost care was taken in planting. Those that needed mulching showed all the signs of declining vigor: foliage smaller than normal; growth stunted or reduced to tiny spurs; leaves dull and pallid and limbs dying from disease or prolonged insect infestation. Death invariably follows after a period of years. The main reason for the decline and death of these plants was that the soil without a mulch became dry and hot, killing the fine feeding roots and the uppermost roots were chopped off when the ground was cultivated. Without a mulch and where no cultivation takes place the soil immediately above the root area becomes hard and smooth. Although this does not act as a complete seal, it does, with the aid of soot from cars, factories and homes, form a thin layer through which the passage of oxygen to the soil atmosphere is impeded to its detriment.
        However I have seen rhododendrons flourishing without a mulch over a great enough period of time to accept the fact that under certain conditions, which are strictly local, combined with proper handling by experts, large leaved rhododendrons can be raised commercially without a mulch. In the vicinity of Cleveland, Ohio, nurserymen do not use a mulch for plants lined out in the field and many use none for their young plants in the frames or beds. Plants prosper, in that area under those conditions largely, I believe, due to the high water table, excellent soil and proximity to Lake Erie. Rhododendrons in the Cleveland region that I have seen do not merely exist. They thrive. Growers there with whom I have talked agree that, were it practicable to mulch older plants in the large fields, especially where no irrigation is provided, it would eliminate the occasional leaf burn on some few plants during the dry rainless summer weeks. But the fact remains that with irrigation. or with rain about once a week, this burn is eliminated without mulching.
        Anthony Shammarello of Euclid, Ohio, grows luxuriant plants. His one year liners are mulched with ground sugar cane and when they are 3 years old the plants are lined out in the field. At that time they have a root system established in peat moss and soil approximately 4 to 6 inches wide and 6 to 12 inches deep. Once planted in the field, they receive no mulch. Mr. Shammarello's soil is neither light nor heavy and drainage is perfect. In preparing the soil for planting he first ploughs under, in the autumn, a cover crop of Sudan grass 5 to 6 feet high, which lays all winter and is disked in the spring. Then in the rows in which he is later to plant, 2 to 3 inches of heat moss and 2 to 3 inches of rotted cow manure are rottilled in and worked with a sub-soiler to a depth of nearly 18 inches. After they are lined out, the plants are cultivated and hoed every week or so, as time permits. In March or April they are fertilized with a complete acid fertilizer.
        I also know of a nursery in New Jersey where excellent rhododendrons are grown in the field without a mulch. Only large trees grow here and there to break the direct rays of the sun. The soil is a clay-like loam and the area has a high water table varying from possibly 2 to 5 feet depending on the season. I am convinced this high water table and heavy loam play an extremely critical role in the ability of Rhododendrons to thrive without a mulch. (The water table is the level below which the soil is naturally saturated with water. At one time there was a belief that water rose great heights through the soil by capillarity but experiments to determine the rise of water through a column of soil have shown that the rise does not generally exceed a few feet. The maximum ascent can not exceed 8 feet. Although the rise is slower in clay-like soils it climbs to its greatest heights in them.) This heavy loam is further enhanced in its capacity for water retention by the application of peat humus, and before planting partially rotted cow or horse manure is spread over the area and incorporated. Clay colloids, as we know, have tremendous power to absorb water and the huge surface area of clay particles (as high as 25,000 square feet per pound of clay as compared to 250 square feet per pound of sand) makes clay loam ideal for ericaceous plants for which cool roots and copious moisture are vital. It should be stressed here that a clay loam is excellent but not clay itself which is too dense for rhododendron roots.
        At this nursery, the rhododendrons are grown under lath the first year, with a light Pine needle mulch. They are transplanted to a woodlot the following spring, spaced 18 to 24 inches on center and mulched with either shingle tow (the ends from red or white Cedar shingles) or wood chips. They remain undisturbed until they are 18 to 21 inch plants at which time they are transplanted, in the early spring, to fields without a mulch. They are then cultivated along the rows and weeded by hand between the plants. Nature supplies a chickweed mulch for the winter. The plants are fertilized in the spring with a complete commercial plant food. A mulch here, too, if practicable, would eliminate the small amount of leaf burn that might occur on some plants during extremely severe winters or summers, but by and large the plants are excellent. I have been told that somewhat the same procedure is successfully used to grow rhododendrons in Massachusetts.
        These few paragraphs on the growing of rhododendrons without a mulch seemed to fit in with a discussion on the value of various mulches but they must not be misconstrued to infer that I advocate that method of cultivation, for I do not. Again let me re-emphasize the condition is strictly local.
        The wise use of commercial fertilizer is as important a part of the program for growing vigorous rhododendrons as is the use of insecticides and fungicides. It is not against the use of these fertilizers that the complaint should be made but against the foolish and reckless abuse of them. This abuse, unintentional as it may be to the average home owner is responsible for the ultimate death of hundreds of rhododendrons every year. Perhaps there is reason to excuse or at least understand these costly mistakes made by a well meaning public who are either uninformed or misinformed. We can hope that through the influence of garden clubs and just such a society as ours, that these errors can be brought to a minimum.
        More significant and deplorable to me is the deliberate over-stimulation with fertilizer by nurserymen in their crazy rush to "beat the market." These men, who have a responsibility to the public, are doing rhododendrons a shocking disservice. Their flagrant disregard for the future of the rhododendron after it leaves their hands cannot be excused. Whatever their motive, whether it be money or to satisfy an insatiable thirst for acclaim, shallow acclaim at its greatest depth, the practice is shameful. Fortunately bad news travels fast and in a short time these selfish abuses form their own tightening noose. And it should be so. Luckily many plants so treated often die in the nursery before being sold to the public. But there are some that get out. I have seen plants so over-stimulated by fertilizers that at first they died at an alarming rate but later tapered down to an occasional death now and then, for as much as three years afterward. I know of one nurseryman who imported grafts from Holland in the spring and planned to fertilize them weekly during the entire summer to get nice bushy plants by autumn. It was his intention to unload them before winter but his plans were cut short in midsummer by their death. On the less drastic side I have seen small hybrids 1 to 2 years old so over-fertilized that they showed injury until they were plants 24 to 30 inches tall. Even when this size they had no floral buds or only an incidental one and the general appearance of the plants was poor. Adjacent to them were plants of the same age that were grown according to the rules of proper culture, and although they were a fraction smaller in height and width, they were sturdy, well budded and of fine appearance.
        In the wild it is quite easy to under stand the interminable cycle of nature, that of life and death which had its beginning with the advent of life itself and which will hold true until nothing stirs in this world. Nature is self-renewing. In the forests we see the death of trees or shrubs which were crowded out by their neighbors. Lower limbs lost in the struggle for light drop to the ground. Ice and lightning further smash down trees or portions of them. Disease and insects take their toll. Finally even the noble giants in their turn succumb and return through decay to the soil from which they sprang. The underbrush, annuals and perennials and even animals, give back eventually that which they borrowed. And so it goes in nature, time without end.
        Look at rhododendrons in the wild. I have crossed rhododendron forests far beneath their crown of leaves, crawling and climbing through the almost impenetrable tangle of twisting branches, a dead and dying thicket of limbs and twigs fallen victim to natural accident and decline. No order here in the bitter struggle for survival. Occasionally the canopy above is broken by the crashing fall of a monarch of the forest or perhaps it has closed over one again where once one fell to earth half a century or more before. In nature rhododendrons have their beginning as tiny seedlings and as the years pass, their roots, not confined to a limited area, run rampant wherever food is to be found, perhaps dozens of feet in one long meandering, searching direction. Only in this way by continuous extension through the soil can a continuous absorption of mineral salts take place.
        In our gardens the outlook is not so favorable. Instead of the rich forest floor we have a thin veneer of mixed top soil and subsoil compressed beneath the weight of the giant earth movers brought in by the building contractor. There is practically nothing to begin with, and at best the most conscientious gardener can provide for only a few years' root growth. Rhododendron roots are notoriously fastidious in their need for a light friable medium. They do not venture far into a hardpan soil and consequently, what nourishment the plant is to receive must come from a confined area in most gardens. Leaves are a fine form of organic matter and future humus but they are in themselves poor in food value. So if the plants are to continue in a healthy state they must be fed and fed a complete plant food. Soil scientists have long since refuted the ancient belief that nitrogen, phosphorus and potash alone supply what plants require. They demand much more. Today the essential elements are classified as "main," "intermediate," and "minor." The main elements are nitrogen, phosphorus, and potassium. The intermediate elements are calcium, magnesium and sulphur. The minor elements are iron, manganese, boron, zinc, copper, and molybdenum. To complete the list, carbon, hydrogen and oxygen are derived from the soil water or atmosphere. Regardless of how small the requirement, these minerals are indispensable to the function of plants. Some act alone, in combination with others or as catalysts or carriers. For example, pure chlorophyll when reduced to ash leaves a residue only of magnesium and yet without iron, as a catalyst we would have no chlorophyll.
        Perhaps the soil condition in woodlot plantings of rhododendrons is less drastic than is the general rule for backyard and foundation plantings. Even so in these cases we have no cycle of nature and whatever trees and shrubs are left standing are competing for the minerals in the soil and unquestionably force their roots into the feeding area of the rhododendrons. It may be that under extremely advantageous circumstances rhododendrons in a woodland will thrive without fertilizing but such examples are few and far overbalanced by those that need assistance.
        The exact concentration of aluminum ions in the soil which is lethal to rhododendrons has been established by scientific experiments but I do not have the figures at hand. Aluminum ions build up in the soil when aluminum sulphate is used as an acidifier and the cumulative effect of repeated applications of this chemical prove fatal when the critical amount is reached.
        Except in very dilute concentrations aluminum is distinctly toxic to plants. In culture solutions evidence of toxicity to corn has become apparent in concentrations as low as 1 part per million. This of course is not true of Rhododendrons but it is an example of how small is the essential amount of aluminum. We must try to eliminate aluminum sulphate as an acidifier in rhododendron culture. Ordinary commercial sulphur is much safer.
        In conclusion, the intelligent use of commercial fertilizers can spell the difference between healthy, thriving plants or those which are stunted and languishing. Perhaps we ought to regard these fertilizers as we would a doctor's medicine; a small amount as prescribed by the physician can be of inestimable value, but an overdose can be fatal.


Volume 10, Number 1
January 1956

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