Tips for Beginners: Some Thoughts on Growing Rhododendrons in Difficult Climates
Joe B. Parks
Dover, New Hampshire
In trying to grow rhododendrons, soil and mulch are the two most important factors. Of the two, mulch is probably the more important. I have seen wild plants in Newfoundland growing in full sun (and in flower) on limestone rock with a fraction of an inch of soil between them and the limestone but with a rotting conifer mulch over their roots (plus, of course, an adequate supply of water). Such plants are, of course, survivors of no telling how many hundreds (or thousands) of seedlings that attempted to grow under such harsh circumstances. So, while the same thing might not be possible in a garden, it does tell us that, given the minimums necessary for survival, these plants will perform in horrible situations.
Of course, the first question is "What are the minimums?" My experience is that these are 1) an acidic, highly organic soil, 2) a good mulch, and 3) adequate moisture (continuously damp but not wet). Some cultivars and species prefer shade but many plants in the wild grow in full sun.
Rhododendron roots require a high oxygen level (that's probably why they are shallow rooted) and an organic soil that supports mycorrhiza (the fungus that invades their roots and promotes their growth). The soil also must be properly acidic (between pH 5 and pH 6 is ideal) so that nutrients will be water-soluble and thus available to the roots.
If the soil becomes too highly acidic (lower than about pH 5) some nutrients start becoming overly abundant and thus can actually act as poisons; others become less water soluble and thus in short supply.
In highly acidic soil, there is a further problem. Aluminum, which prevents cell division at the root tips (thus preventing root growth), starts converting to a water-soluble form at about pH 5.2 and rapidly becomes more and more soluble as the soil becomes more and more acidic.
Soil with too low acidity (above about pH 6.2) causes iron, a necessity for photosynthesis, to change into an insoluble form and thus, being unavailable, the plants become chlorotic. Thus, the suggestion is that soil acidity should be kept within a pH 5 to 6 range.
If the pH is too high (above pH 6), use sulphur to lower it - never, never ever use aluminum sulfate as it can, by preventing root cell division, kill your plants. Add sulphur over a period of several weeks, not all at once as too much can burn the roots.
If the soil is too acid (below pH 5), use ground limestone to raise it (never use lime; it works too fast and may "burn" or kill the roots.
Importance of Calcium
It is very important to note that all plants (including rhododendrons) require calcium. Calcium is involved in water movement within the plant. Soils that are properly acidic for rhododendrons are typically low in calcium. I have often wondered if a calcium shortage adds to plant stress in dry periods. And since calcium leaches out of the soil, it needs to be regularly replaced. To supply calcium to soil that is properly acidic, i.e., between pH 5 and pH 6, use gypsum (calcium sulfate) as it does not materially affect the pH. I mix it with fertilizer for ease of application but it can be applied separately if you prefer. I use about a tablespoonful per large plant every year.
Add Organic Materials
The soil provided by builders, which sometimes includes construction trash, is usually unsuitable for rhododendrons. Regardless of the soil situation, you can't go wrong by increasing the amount of organic materials in your planting soil - for clay or sandy soils it is an absolute must. A high level of organics in the soil helps offset nutrition and acidity problems since the organic materials tend to both act as a buffer and, due to breakdown by bacteria and fungi, serve as a nutrient source.
I do not hesitate to recommend that you make your planting soil 50 percent organic - if it is mostly sand or clay, that is the absolute minimum. Peat moss alone is not too good, but mixed with ground bark (3 or 4 parts of bark to 1 of peat) or other coarse organic material it should improve your soil mix greatly. The object is to provide a good nutrient base for bacteria and fungi, maintain an even level of moisture and assure that the roots receive plenty of air.
Bark is better at promoting drainage than is peat, thus the larger amount recommended in the mixture. Fresh grass clippings are most undesirable and should not be used. Instead, they should be mixed with soil and allowed to compost for at least a year; leaves, pine needles, corncobs and most organic materials are also best if partly - but not completely - composted before use. Pine needles are so loose that they can be used directly as mulch, but be aware that my tests show that their exclusive use will substantially increase the acidity of your soil. Partially composted material supports an active bacterial and fungi population that is so important for rhododendron success. Note the word "partially"; fully composted material alone, though useful, does not provide a sufficient nutrient base for soil organisms.
The soil must also be moist but well drained - no standing water for more than a few hours. And this is another reason to minimize use of peat moss; though a good moisture holder, in wet situations it will cause the soil to remain overly wet, thus reducing the availability of oxygen to the roots.
In planting your rhododendrons, be sure to keep the root ball as intact as possible to insure that you don't lose the mycorrhizal fungi that are already growing with the roots. The "native" soil provided so nicely by the developer may well be devoid of these necessary organisms. However, potted rhododendrons should have three or four vertical, inch-deep cuts from top to bottom of the root mass. Otherwise, they will be slow in growing out into the surrounding soil.
Lastly, make sure there is adequate mulch (2 to 4 inches depending on how loose it is) around your plants. From my observations in the wild, a good mulch may be even more important than proper soil - plants I have seen growing in the limestone areas of West Virginia and Newfoundland all have a good mulch of rotting organic material on the surface. As the various soil organisms break down the organic material, nutrients are released to the plant's roots in a usable form (usually as a water-soluble chelate). Mulch also provides a good moisture reservoir and buffer against heat and drying winds. As mentioned above, tight materials such as grass clippings, maple leaves and peat moss, for example, are a no-no because by retaining water they reduce the oxygen supply and may thus kill the roots. I use 2-year-old wood chips for mulching but only because I have an inexhaustible supply. Ground bark, oak leaves, pine needles or any material that doesn't pack tightly but breaks down readily will work. However, don't pile mulch up against the trunks of your plants. By keeping them too moist it can promote Phytophthora invasion.
Plant location is also important and particularly so in stressful climates. If at all possible, I would provide shade - summer and winter in regions where there are no native rhododendrons. Coniferous trees are probably best but lacking those, deep rooted deciduous trees such as oaks are useful, or build a lathe house or plant on the north or east side of the house. Winter sun is particularly stressful because it tends to bring the plants out of dormancy and, if the ground is frozen, to cause desiccation; thus the west and south sides of a house are undesirable. And since some cultivars such as 'Roseum Elegans' and its kin are particularly sensitive to the desiccation problem, I would prefer to avoid them in a dry cold climate.
You probably know all of this and have already tried everything possible, but I hope that, just maybe, somewhere in these comments there may be a bit of info that will help you in solving some of the problems of growing these lovely plants in a stressful climate.
It's no accident that there are no wild rhododendron or azalea species growing in the mid-continent. But that doesn't mean they can't be grown there. Certainly, Leonard Miller has solved the problem in stressful northeast Oklahoma. You just have to find techniques that get around the problems. And therein lies the pleasure: the search for and the finding of hidden solutions to problems.
Joe Parks is a member of the Massachusetts Chapter.