This is the time of year when we receive and are asked questions on rhododendron die-back and proper soil preparation for success in growing these beautiful shrubs.
In response to a question on die-back: Die-back of the leaves and end (terminal) portions of the branches is caused by different tyes of fungi. The fungi is spread by splashing water, rain, infected soil and gardening tools. If there is a small wound or drying twig or leaf, the fungi will enter the plant through these areas and cut off the important flow of nutrients and water. Hence, the tips wilt and die-back. When the weather becomes hotter, the plants under stress will die more rapidly.
Prune out and destroy all infected and wilted branches several inches below the visibly diseased tissue (canker). Be sure and sterilize your pruning shears in a mild solution of bleach and water before each cut.
Try spraying with a basic copper fungicide after blooming, and then repeat two more times at intervals of 14 days.
Rhododendrons: Growing and Planting
Another soil-inhabiting fungi (called phytophthora) is capable of killing the entire plant. It starts at the roots, destroys them, and works its way up the stem. When the stem is girdled, the entire plant wilts and dies. This wilt and root rot is caused primarily by heavy, poorly drained soils. No chemicals will help this problem, but you can try drying the plant out and discouraging the fungus.
Rhodies thrive on constant moisture but only in well-drained soil. If your soil is heavy, you must improve it with organic matter. Plant rhodies in raised beds or even plant them above the existing soil grade in order to make sure this devastating soil-inhabiting fungi won’t be a problem to your sensitive plants.
I recommend planting or even mounding plants on top of the existing ground level. They use 100 percent organic material, consisting of a mixed size fir bark (75 percent) combined with -3/8-inch minus crushed lava rock (25 percent). Then, each year an additional 3 to 4 inches of humus is added as a mulch around the base of the plants.
Fertilizer such as the timed-release Sierra 17-6-10 Plus Minors or Osmocote for planting mix is used with great success.
Extra time and energy spent on soil preparation pays off!