Caring For and Planting Rhododendrons
Print this page
In order to understand the culture of rhododendrons it is helpful to visit mountainous areas where they flourish. If you take a fistful of soil at their roots you will find that it is not dirt, but dead and dying plant debris. The fallen leaves, twigs, and flower parts decay into what is called humus. Humus is very porous and with slight water retaining characteristics provides perfect drainage. Observing a plant growing happily in a crevice of a rocky cliff is further evidence of the perfect drainage requirement of rhododendrons. The roots may be found growing many feet away searching for moisture and nutrients in the thin layer of humus.
If you remain in the mountains very long you will realize that there is more rainfall than at lower elevations. In fact, very often you will find that you are in a rain cloud. Although it may not be raining, the grass and leaves are wet with dew. Thus you witness the two basic requirements of rhododendrons: (1) abundant water and (2) soil with good drainage. By providing these requirements, we may enjoy in our gardens at lower elevations the exotic species and hybrids of rhododendrons.
Of lesser importance is the plant’s location in your garden. Some rhododendrons will flourish in full sun and some in partial shade. A happy medium is probably the best, with high filtered light from surrounding tall trees. By removing the lower branches of the trees the rhododendrons can enjoy the full early morning and late afternoon sun. Some protection from drying winter winds may be necessary either from a building or a plant windbreak. Since the drying winds come most often from the southwest, planting rhododendrons on a northeast slope or the northeast corner of your house would be ideal.
To provide good drainage, elevated beds are necessary on clay soils. Of course, these beds will quickly dry out and the water needs must not be neglected. On banks or slopes holes may be dug for the planting as long as the bottom of the hole is trenched out. The trench may be filled with rocks and humus so that water will never stand in the bottom of the hole. Before planting it is wise to fill the hole with water and observe complete drainage through the trench.
When making a single planting on level ground in clay soil, the elevated bed should be about 5' in diameter. The plant should be placed on top of the ground without digging a hole. If a hole is dug it should be very shallow so that water will drain out. The humus may then be placed about the rootball to barely cover the top of the ball. Each year more humus should be added to the bed in the form of a top dressing of composted leaves, bark, grass clippings, peat moss, etc. This will help keep shallow roots cool and moist. The mulch should be composted (well-decayed) and not this year’s crop of leaves or clippings, because they tend to shed water from the bed and take up nitrogen from the soil in the process of decaying.
Most commercial growers today propagate rhododendrons in containers with a soil mix of 2 parts composted pine bark, 1 part peat moss, and 1 part sand. The mixture varies with some using only composted pine bark. There is much discussion of rhododendrons being “acid loving” and of the need to keep the pH of the growing medium between 4.5-5.5. Actually, if the proper growing mix is used the decaying organic matter will naturally lower the soil’s pH. The home-owner/gardener should be aware of this and when applying alkaline lime to the lawn avoid getting it too close to the rhododendrons.
Very often the containers purchased from a nursery are “root-bound.” The roots grow out to the sides of the container and then circumferentially around the inside of the container and back into the rootball. If planted in this manner it will die in two to three years because the roots cannot escape the ball. When the dead plant is lifted from the bed one finds the rootball to be exactly as it was when planted two to three years before with no roots extending out beyond the ball. The encasing peripheral roots had actually choked the plant to death. This tragic circumstance occurs all too often and is a prime reason for failure with container grown plants. We cannot stress too vigorously the importance of the proper care of the rootball prior to planting. It is folly to purchase an expensive plant that will surely die without proper care.
When the plant is root-bound the peripheral roots should be torn or cut away before planting. The entire ball should be loosened by gently pressing and pulling and breaking its rigid contour. This loosening effect may be aided by directing a forceful stream of water from a garden hose. Another recommendation is to use a sharp knife to slice the sides of the ball from top to bottom. This will help open up the ball and allow the inner roots to escape.
One question that gardeners frequently ask is: “When is the best time to plant rhododendrons?” A container grown plant (that is not rootbound) can really be planted safely anytime. However, since you may not know if the plant is rootbound until you lift it from its container, it would be safer to make plantings in the early spring or fall. April, May, September, and October are the best months for planting. The worst months are June, July, and August, when the plants are in active top growth. Fall plantings are best because the winter rains maintain ground moisture better than in summer. Early fall plantings allow time for the roots to get out of the rootball and into the moist plant bed before ground freeze occurs.
Another frequent question is: “When should I fertilize and what should I use?” New plantings should be fertilized immediately after blooming. Early spring application will stimulate new growth that will obscure the blossoms. Late summer (after July 15th) fertilization should be avoided to prevent late new growth that may be damaged by fall freezing. Established plantings of two or more years should be fertilized at least once every other year. A general recommendation is to use a fertilizer ratio of 2:1:1 or 2 parts nitrogen, 1 part each of phosphorous and potassium. The best and most accurate way to fertilize is first to have the soil tested by your state soil-testing laboratory. This can be facilitated by your agricultural extension office or garden center.
If properly planted and given an extra quota of water, a Rhododendron will outlive the one who plants it by many years.