JARS v39n3 - From the Container into the Ground

From the Container into the Ground
Dr. Philip Waldman

Reprinted from New York Chapter Newsletter

Rhododendrons and azaleas have been growing happily for thousands of years generally under woodland conditions. Modern nurseries now grow these plants under full sun, in containers utilizing soilless mixes with heavy applications of water and fertilizer. In the transition from the nurseryman's container to the homeowner's garden, special after planting care is absolutely necessary until these plants become established in their new surroundings.
The key ingredient in a container grown plant's growth and survival is simply water, not some miracle treatment. A containerized plant's water requirements are much greater than an established plant's, rather similar to a house plant. The practice of placing a newly planted rhododendron or azalea among established plantings and irrigating all plants equally, will result in under-watering the new transplant. Water does not move from the finer soil particles to the coarser container mix. If a plant growing in a container dries out, the root ball will tend to contract away from the walls of the pot, making rewetting very difficult. This identical situation will also occur with an un-established plant in the ground, as the roots will shrink away from the surrounding soil when dry. Careful individual irrigation is necessary to prevent this situation.
Rhododendrons and azaleas generally should be planted in an area where they receive some shade during the hottest part of the day. Although they are regarded as shade-loving plants, they will not thrive in dense shade. These plants do respond to greater light with more flowers and a better shape, but their moisture requirements are also increased greatly. A favorable planting site also offers some protection from the winter sun and wind.
In pre-inflation days, it was said to plant a 50 cent plant in a five dollar hole, although a 50 dollar hole may be more appropriate today, the idea remains the same. It is imperative to dig a hole at least three times the diameter of the root ball. Even after many years of growth, the roots will have difficulty breaking out of the original planting hole into the surrounding undisturbed soil, as the two surface interfaces have a detrimental effect on root growth and water movement. It is a better practice to prepare an entire bed area at the same time by incorporating organic matter and rototilling the area to obtain a uniform soil consistency.
For hundreds of years the common practice when planting rhododendrons and azaleas was to add peat moss to the planting hole. This is known as amending the backfill. However adding amended backfill such as peat moss, bark, etc. to a planting hole has not been proven to be beneficial and does not appear to warrant the additional work and expense. (1, 2, 3, 4, 5)
Since the feeder roots of rhododendrons and azaleas in the ground generally lie in the upper 3-4 inches of soil, it does not seem to be logical to add peat moss 1 foot down from the surface to improve water retention. The use of amended backfill in poorly drained soils will aggravate the aeration problems, creating a "bathtub" effect, especially during periods of heavy rainfall or overwatering. Very few rhododendrons will survive in poorly drained soils. The digging of extra deep holes, or raising beds to obtain proper drainage are often necessary. The practice of placing a shovelful of gravel beneath the root ball does not significantly help the problem.
The fertility of the soil is important to sustain the growth of these plants. Composted materials can be added to backfill, to increase fertility, providing they add nutrients to the soil. However with the advent of modern slow release fertilizers, it is now more efficient to feed rhododendrons throughout the growing season. Small doses of continuous fertilizer are better than the practice of one massive dose in the Spring. This insures the plant of high nitrogen levels in its leaves to sustain growth, and to better withstand the rigors of winter. Modern nurseries also fertilize rhododendrons in November, after the first frost to maintain high nitrogen levels and to minimize leaf drop and the translocation of nitrogen from older leaves to the newer ones. It is also important to measure the pH of the soil to insure the proper acidity of approximately 5.0 - 5.5 for rhododendrons and azaleas. Lime will raise the pH if necessary, whereas sulphur will lower it, in order to reach the correct level.
Most container grown plants are either pot-bound or approaching that condition. It is necessary to cut or loosen some of the roots to disrupt the circular root growth and force the plant to extend roots into the surrounding soil. Two popular methods for achieving this goal are slashing the roots with 4-8 vertical cuts 1" deep, the length of the root ball or butterflying the roots by making a deep cut across the bottom half of the root ball and subsequently spreading the lower half of the root-ball when planting. This practice also brings more feeder roots towards the surface.
The plant should be planted with ⅛ of its root ball above the natural grade, as the lower roots will have a better chance of survival. After planting, a saucer should be built around edges of the old root ball and immediately watered in with a hose to eliminate air spaces in the soil. Backfill with more soil if excessive settling occurs.
The use of mulch for newly planted rhododendrons is highly recommended. A proper mulch insulates the root system of a plant, moderating temperature changes. It is important to maintain a loose airy material which does not mat down over a period of time. These materials can consist of bark, wood chips and other organic material which does not decompose too quickly. Peat moss makes an unsatisfactory mulch, since it will tend to repel water when dry, and to remain too soggy when wet. Mulch should not be greater than 3 inches deep, as greater depth leads to the smothering and death of lower roots. As the rhododendron roots try to grow into the surface of a deep mulch, the plant is also more susceptible to winter and wind damage, and drought problems. The mulch should be kept several inches away from the stem and bark. Failure to do so can lead to bark disease and damage from insects and animals.
In conclusion, select an appropriate planting site and bring a hose to it. Place the plant in the ground utilizing proper preparation and techniques and water it. After planting care requires old fashioned common sense and water.

Literature Cited
1.  Ingram, D.L, R.J. Black and C.R. Johnson, 1980. Effect of backfill composition and fertilization on container grown plants in the landscape. SNA Res. Conf. Ann Rept. 25:68-71
2.  Ingram, D.L, and H. Van De Werken. 1978. Effects on container media and backfill composition on the establishment of container grown plants in the landscape. HortScience 13:583-584.
3.  Munday, V. and O.E. Smith. 1979. Landscape establishment of containerized azaleas grown in three different media. Ornam. South 1(7): 12-14
4.  Whitcomb, C.E. et al. 1976. Soil amendments and plant growth; a summary. Okla. Agr. Expt. Sta. Res. Rept. P-741
5.  Corley, W.L. 1984. Re-evaluating the value of amending planting holes with organic material. (Amer. Nur. 151 (6); 113-116)