Rooting It Out
Mark Konrad, MD
This article stems from a fortuitous short cut I developed while root pruning young seedlings before transplanting. Instead of using scissors or clippers to trim roots, I use a sharp pocket knife and a 2" x 4" block of wood to do the chore quickly and expeditiously. The method is similar to the butcher cutting away excess fat.
This simple experience led me to think about the cultural demands related to root function. Because the root system is out of sight, perhaps we do not reflect enough on this most important plant aspect.
Root pruning applies to young plants with a root imbalance, as well as to older plants. For example, cutting back extended root tips, facilitates transplanting. Proper preparation of the root mass allows for better contact with the soil thereby eliminating undue shock.
Container grown plants offer a special challenge. To break up the root ball of root-bound plants, remove as much soil as possible. This is not always easy. Use a knife to make peripheral vertical quadrant cuts if the plant is badly root-bound. This will interrupt continuous root circling.
Even with all this preparation and care, transplants require special watering attention to make sure enough moisture is present until the plants are fully established. If this is not done, the plants will be at risk since poor watering practices will cause the root ball to harden and become impervious to outside moisture.
Rhododendrons have shallow fibrous root systems and require porous soil with a high humus content. Rhododendrons are generally considered to need an abundant supply of oxygen to their roots. Responding to this need we should be careful not plant too deeply.
Rhododendrons like moisture, but not excessive water on their roots. A mulch helps to preserve the availability of moisture for all seasons and to keep the soil cool during the summer. The amount of mulch depends on the heaviness of the soil. A thin sandy soil requires more mulch than a heavier soil.
Mulching allows for natural plant nutrition. The formation of humus from the decomposition of mulch allows nutrition to be supplied to the plant naturally. Even though nitrogen is needed for the above process, my experience indicates that the lack of this element does not seem to be a major problem under natural conditions. From this evidence, I conclude that rhododendrons do not have an abundant need for supplemental nitrogen. Oak leaves and pine needles rate highly with me for mulching.
Use fertilizer sparingly. Random use of fertilizer on established rhododendrons adds an unpredictable variable since any late growth stimulation could make the plants more vulnerable to cold injury. If for some reason commercial fertilizer is used, I suggest applying it in late winter or early spring.
Proper soil acidity is a necessity with the ideal pH ranging between 4.0 and 5.5. Soil testing is very informative and helpful.
The considerations above are interactive and interrelated. Rhododendrons grow easily if the ideal requirements are met. When plants anguish one or more of the above cultural ingredients could be absent, a plant doing poorly may reflect improper cultural conditions rather than a genetic weakness or a climatic unsuitability. Not only may the root physiology be interrupted but poor conditions can serve as a fertile media for the introduction of soil pathogens such as rhododendron wilt ((Phytophthora cinnamomi) along with an increased risk of its rapid spread. A weakened plant is naturally more vulnerable to environmental stress.
In summary, the old truism, "Getting to the root of the matter," still applies, that is where it all begins. When in doubt, try being a student of Mother Nature, she has been an instructor for a long, long time.
Dr. Konrad, Great Lakes Chapter member, often shares his gardening experience with ARS Journal readers. Some recent articles include: "A Marriage Made in Heaven," Vol.43:l, Winter 1989, and "Rhododendron 'Ecstasy' and Her Sisters," Vol.43:3, Summer 1989.