QBARS - v29n2 The Perversity of Milled Sphagnum

The Perversity of Milled Sphagnum
Judson Hardy, Silver Spring, Md

It is well known that if fertilizer is withheld, milled sphagnum moss used as a germinating medium can keep small rhododendron seedlings in a state of healthy non-growth for at least two years. The addition of nutrients under proper conditions of light, heat, etc., will cause the checked seedlings to resume normal growth.
Tests conducted by the author during 1973-4 demonstrated that this material, mixed with perlite, can also serve as an excellent medium for growing-on containerized yearling deciduous azaleas without the use of fertilizer. Unfortunately, this was exactly the reverse of hoped-for results!
In the fall of 1973, faced with a surplus of vigorous young deciduous azaleas in outdoor beds the thought occurred that the growth-checking properties of milled sphagnum possibly might be used as a means of "storing" surplus yearling plants within a compact area by replanting them closely together in containers, using unfertilized milled sphagnum and perlite as a growth-restraining but viability-retaining medium. A year or so later, I thought, when a crop had failed, or more bedding space or containers were available the checked plants could be brought back into a normal growth cycle by replanting in a suitable medium in containers or bedding rows with the addition of fertilizer. The alternative would be the always traumatic and heart-rendering act of consigning the surplus plants to the compost pile (since neighbors and relatives are already overstocked with my surpluses of previous years).
To test this idea, the following experiment was conducted:
On November 18, 1973, two strong plants each of R. vaseyi , arborescens and prunifolium were lifted from their outdoor beds and soil washed from the roots under a strong spray. All six plants were replanted in a 2-gallon plastic container, using a medium of four parts fresh milled sphagnum and one part of coarse perlite. Since the container was 8½" wide at the top and of the same depth, only the bottom 6" was filled with the medium. The plants were spaced about 1½" in from the container wall and about 3" apart.
Using a cheap aluminum cake pan 12" long by 5" wide and 4" deep, four Ilam hybrid azalea seedlings were planted in the same medium. In this case the medium was mounded about 1" above the sides to provide a little extra depth. The plants were spaced in a row about 2½" apart. They too, had the soil washed from their root systems.
After thorough watering the containers were placed in a lath and plastic covered pit for the winter. During occasional warm periods they were watered as needed.

With the return of spring, the containers were placed under high deciduous shade on a black plastic ground cover, along with several dozen sibling plants from the same seedling batches, which had spent the winter in specially prepared beds under lath and plastic protection. The container mix used for these latter plants consisted of equal parts of Canadian peat, well-rotted leaf mold and fine sand, to which gypsum, super phosphate and cotton seed meal had been added. These plants also were given a 23-19-17 foliar spray three times during the growing season. The test plants received no fertilizer, although it is possible that on one occasion, they were slightly exposed to drift from foliage nutrients being sprayed on the other containers.
It soon became apparent that the azalea species test plants had no intention of cooperating in the experiment, since they grew right along with their fertilized neighbors. The Ilams under the test grew well but not quite as strongly as their fertilized brethren. Color of foliage and susceptibility or resistance to insects and disease were the same in all plants, fertilized and unfertilized.
On September 5, 1974, the test plants were removed from their containers, every inch of which was filled with a thick mass of roots interspersed with perlite. No evidence of intact sphagnum was discernible.

The roots of each test plant were pulled apart with little difficulty and apparently little damage. After photography, each plant was repotted individually in my regular container mix, with added gypsum and super phosphate, and labeled for further observation.
The following table shows the average size of test and comparison plants at the beginning and end of the experiment.

Avg. Height
Nov. 1973
Avg. Height
Sept. 1974
Avg. Height
Nov. 1973
Avg. Height
Sept. 1974
R. vaseyi 6" 15" 6" 13"
R. arborescens 6" 15" 6" 13"
R. prunifolium 6" 15" 6" 13"
Ilam hybrid 3" 8" 3" 10"

The only conclusions I am able to extract from these results are the following:

  1. Although washed free of most of their soil the roots of the test plants managed to bring with them bacteria and other microorganisms which, the liberal oxygen supply provided by the perlite, caused the sphagnum to break down quickly into the chemicals needed for growth. This is in contrast to the virtually sterile state of fresh milled sphagnum moss used for germination, which thus permits the checked growth phenomenon observed when nutrients are withheld.
  2. Except for the exorbitant cost, milled sphagnum and perlite would be an excellent bedding or container medium for growing-on liners.
  3. Probably the same results would have been obtained had I used ordinary commercial Canadian sphagnum peat and perlite.
  4. First-year surface overcrowding of plants in prepared beds should not hinder growth under adequate fertilization if the medium is loose and friable to a depth of at least 6", permitting root growth downward when restricted laterally.