Tips for Beginners: Fertilizing Rhododendrons
J. Powell Huie
Westport Point, Massachusetts
Reprinted, with revisions, from the Rosebay, published by the ARS Massachusetts Chapter
Every year in early spring and on a particularly cold day, I feel compelled to help my rhododendrons recuperate from the previous winter by giving them a generous serving of fertilizer. But after some years of experience I have concluded that in many cases I am not doing them a favor and, in some, causing considerable harm.
In general, the following practice seems to give the best results: Each spring after setting out in a lathhouse newly acquired 1- or 2-year-old plants, the majority of which come from Oregon and Washington states, are watered-in and mulched with oak leaves picked up by a lawn mower. I then scatter around each a small amount of low nitrogen fertilizer, such as Holly-tone®, a 4-6-4 mixture, recommended for acid loving plants 1 . One must resist the temptation to be overly generous, which can be deadly, particularly for the dwarf species and hybrids. One teaspoon or less around around the dwarfs will suffice. Even with this application, new leaf growth will not appear until the following spring. Keep in mind that most rhododendrons reach maturity only after up to ten years or more of normal growth, equally as true of slow-growing dwarfs as well as the larger varieties, and should not be forced to do it in less time. All that is required for normal growth of a young plant is a suitable growing medium, enough mulch to keep their roots cool in summer, enough moisture so the roots do not dry out, and the small amount of fertilizer recommended above. Thereafter, each spring again mulch and lightly fertilize. Above all be patient.
When the plants are old enough to be moved to a landscaped area, they are set out in a mixture of topsoil, bark and any available compost, and possibly a very small amount of peat moss. (The compost I use is a mixture of leaves and grass clippings put in a pile the previous year. I never bother to turn it.) A handful of superphosphate is then scattered around the plant and watered in. 2 This will carry some of the phosphate down among the roots to encourage more rapid root growth. An alternative is to mix the superphosphate into the soil prior to planting. Then, after mulching, scatter around the plant a small amount of fertilizer such as that used in the lathhouse. For the next four or five years in the late winter or spring, again add mulch and lightly fertilize. By this time the previous year's mulch has begun to break down, and unless there is a positive sign of nitrogen, phosphate or potash deficiency, all that they then need is an annual addition of mulch, usually provided naturally from the leaves of shade trees or the plant itself.
An elepidote rhododendron having 5 or 6 inches (12.5 or 15 cm) of growth each year does not need additional nitrogen as long as the leaves are a healthy green in color. Too much nitrogen will cause lanky growth and ruin the appearance of the plant, particularly true of those located in heavy shade. Also, there is a misguided belief that in order to have a profusion of flowers on mature rhododendrons, one should make an annual application of phosphate. I followed this practice for a number of years but after discontinuing it saw little difference in the number or quality of the trusses. Phosphate is relatively insoluble, and some of that applied with the original fertilizer will remain in the soil for a number of years. Given a well-drained soil, enough decaying mulch above the roots, and sufficient moisture, an older rhododendron may be just as happy without additional help.
Holly-tone® analysis: 4.0% nitrogen, 6.0% phosphate, 4.0% potash, 3.0% calcium, 0.5% magnesium, 5.0% sulfer, 0.02% boron, 0.1% chlorine, 0.0005% cobalt, 0.05% copper, 1.0% iron, 0.05% manganese, 0.0005% molybdenum, 0.1% sodium, 0.05% zinc.
2 Phosphate is essential for bud formation, but excessive amounts do not increase bud formation. A rule of thumb for fertilization is 1 cup in 100 square feet (10' x 10'), a very light fertilization.
J. Powell Huie is a member of the Massachusetts Chapter.