Journal ARS Article
Vol. 59: No. 2: Year 2005
Tips for Beginners: Thoughts on Growing Rhododendrons in Difficult Locations
Joe B. Parks
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 survivors of no telling how many hundreds 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.
Rhododendron roots require a high oxygen level (that's probably why they are shallow rooted) and an organic soil that supports mycorrhizae (the fungus that invades their roots and promotes their growth). The soil also must be properly acidic (between pH 4.5 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 4.5) 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 4.5 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 4.5), use ground limestone to raise it (never use lime; it works too fast and may "burn" or kill the roots.
Importance of Calcium
Add Organic Materials
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 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, corn cobs and most organic materials are also best if partly - but not completely - composted before use. Partially composted material supports an active bacterial and fungi population that is so important for rhododendron success. Fully composted material alone, though useful, does not provide a sufficient nutrient base for soil organisms. 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.
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.
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.
Note: This article has been edited from the original.