Care of Rhododendrons
B. F. Lancaster
After growing rhododendrons along with many other ornamental shrubs for many years I have arrived at the following conclusions, providing proper care is taken in choosing a suitable location for the different varieties or species and the few simple planting rules, recommended for these shrubs, are followed at planting time, rhododendrons will thrive and bring more pleasure with less actual time and effort than any other shrub I have had experience with.
In reference to planting location, consider the extreme variation in size, habit of growth, foliage texture, color and time of bloom and the different conditions under which they grow in their native habitats. One can easily find a type of rhododendron adapted to almost any situation in the modern landscape.
A few basic rules, easily remembered and accomplished should be sufficient for any one to successfully grow and enjoy these plants where ever climatic conditions are at all favorable.
In reference to climatic conditions I would like to digress briefly from my subject to report on a test planting in a locality that has not been considered very favorable for growing rhododendrons. Several years ago I gave my brother (E. F. Lancaster) a collection of rhododendrons and azaleas, with planting and cultural directions, which he planted at his home in Walla Walla, Wash. The list includes the following; hybrids. R. 'Catawbiense Alba', R. 'Blandyanum', R. 'Cynthia', R. 'Doncaster', R. 'Everestianum', R. 'Kate Waterer', R. 'Lady Clementine Mitford', R. 'Mrs. R. S. Holford', species R. augustinii, R. lutescens, R. racemosum, R. indicum var. rosaflora, 2 or 3 R. mucronatum species and about a dozen varieties of deciduous and evergreen azaleas. All of these plants have been well cared for and seem to thrive and bloom as well there as in the Portland area. The only injury occurred during last winter's ('48-'49) below zero weather. There was a little leaf burn on two or three plants in very exposed positions, but no bud injury has ever occurred there. One or two of the more tender evergreen azaleas succumbed during that severe winter which was very extreme all over the northwest. We intend to continue these tests using varieties not quite as hardy in the near future.
Rhododendrons produce many of their hair like roots very close to the soil surface, therefore resenting any active cultivation in that area such as spading or hoeing. Any weeds appearing may be pulled while small with very little injury. A two or three inch mulch of peat moss or leaf mold obtainable under conifers or oak trees will be found beneficial to their health. This mulch, applied preferably in the fall, serves the fourfold purpose of providing winter protection, retaining moisture during hot dry weather, adding humus to the soil as it decays and keeping weeds down. Alder or oak sawdust also makes a splendid mulch if a little high nitrogen content fertilizer is added to compensate for the loss of nitrogen in the soil occurring during the breaking down process of such materials.
If a proper mulching program is carried out year after year very little fertilizing of any kind is required to keep these plants in excellent growing condition. A mixture of up to one half of lime-free well rotted barnyard fertilizer may be added to this annual fall mulch if a little added vigor seems required. The same results may be obtained by using one of the commercial fertilizers prepared especially for acid loving plants and applied on top of the mulch, sparingly, not over a. cup full to a three foot plant. This application may be made from early spring on but not later than the middle of June. Fertilizing at a later date is very apt to promote too much late growth that may not be hardened in enough to survive an occasional early frost.
When a plant appears sickly with slightly yellow leaves, a handful or two of Epsom salts on top of the mulch, watered in well will many times give it the needed boost, in fact most of us have found an occasional application of Epsom salts a very good soil conditioner where rhododendrons and azaleas are grown.
Successful rhododendron culture demands a moist condition, not soggy or waterlogged, in their root areas at all times. Their roots also demand a certain amount of aeration. This is made possible by using loose material, such as peat moss, leaf mold, etc., in the soil when planting. This demand for aeration was brought very forcibly to our attention after the flooding of large areas along the Columbia river two years ago. Rhododendrons and evergreen azaleas with their roots entirely immersed in water for only a few days failed to survive. Many deciduous azaleas under the same conditions came through with only slight injury and just a few lost. Perhaps the fact that they experience a fuller dormant period than the evergreen sorts and have not become fully active at that time would account for the difference in this mortality. However we have noticed that many of these have deeper root systems than the evergreen sorts and possibly do not demand as much aeration. This last conclusion is borne out by the fact that other surface rooted shrubs succumbed when submerged while the deeper or tap rooted varieties came through unscathed.
Maintenance of this moist condition around the plants should be supplemented in hot dry or windy weather by over head watering of the foliage, preferably in late afternoon or evening to compensate for the excessive loss of moisture through their leaves under drying conditions. This precaution will greatly lessen the chance of sun or wind burn, which may occur on the older growth as well as the new leaves at such times.
However faithfully you adhere to this dry weather overhead watering never let it substitute for the thorough watering of the root system once or twice a week through the open hose or soil soaker in the dry season.
When cooler fall weather approaches many rhododendron growers recommend and practice a tapering off or with-holding of water from the plants to discourage any late growth and harden or ripen the wood for the rigors of winter. This practice of with-holding water at that time can prove a decided disadvantage to them if carried too far. We have many fall seasons with extended dry spells. Allowing them to dry out at this time is very apt to cause a depletion of reserve plant food along with what little moisture they have in their systems, leaving them in a weakened condition for the winter season. I have observed some very sick looking plants during the months of September and even October and have made sure that lack of moisture was the cause. In the writer's opinion the advice of "not over watering in late summer and fall" could well be substituted for the old with holding practice. The loss of a few late terminal shoots that fail to harden by a chance early frost will likely improve the plant's shape by increasing the number of shoots near that same terminal the following season.
With the approach of freezing weather especially in areas where where winter temperatures hover around zero and below or in foundation plantings with overhanging eaves, it is especially recommended that these plants go into freezing period thoroughly moist. Freezing being essentially a drying process, necessitates the presence of enough moisture if the ground were to freeze solid. A mulch around the trunks and stems will prove beneficial to those few varieties which suffer from bark splitting at the ground level and among the lower branches.
Removal of all old blooms a short time after blooming is highly important as much of the plants vitality is used in producing seed if seed pods are allowed to develop. The stem of the old flower truss becomes quite brittle and snaps off easily with thumb and forefinger. Removal may also be accomplished with sharp shears. Care must be exercised in either case so as not to injure the new growth buds or shoots at the base of the flower stem.
A few varieties may require disbudding if maximum flower development is desired. The need for this is apparent whenever the flower buds are too tightly packed on a compact growing plant to allow development of well rounded trusses.
Usually very little pruning is required by these plants after they are established in a proper location. However they may be cut back rather drastically if the necessity arises without any permanent injury. Certain varieties of plants with rather rangy growth habits may be induced to branch and grow more symmetrical with a little judicious pruning at the proper time, and best accomplished just after bloom time or before the new growth is under way. Always cut back to a terminal growth just above the rosette of leaves where dormant buds appear at the junction of leaf stem and branch. This type of pruning may be carried back through several years growth if the plants have become overgrown for their surroundings or if you wish to develop a more compact plant. The importance of pruning while the plants are still small can hardly be over estimated. This is best done by pinching out the terminal bud growth when it has attained an inch or two of growth. This operation forces the side buds into growth and a few weeks later you will find several shoots at that point where only one would have appeared if this had not been done. Always make sure that these little side buds are present before pinching back, as a few varieties will be found that produce practically no buds at this point. This terminal bud pinching may well be carried on among the branches as the plant develops to blooming size, from there on the removal of old bloom trusses will substitute for the most of this bud pinching program.
A little different type of pruning may be done on both deciduous and evergreen azaleas which produce dormant buds along the entire length of their stems. Both types may be pruned severely if necessary to a desired shape or reduce the ranginess of some varieties. Many of the evergreen kinds will stand sharing and bloom more profusely under this treatment while others develop very evenly and compact, and require very little pruning other than the removal of an occasional dead or overgrown branch. All pruning is best done immediately after blooming or before the new growth is very far advanced.
Finally I would impress the fact that a little observation and judicious pruning or pinching back will greatly assist the development of your plants to fit them to their surroundings and fulfill your expectations.