QBARS - v33n1 Kalmia latifolia, Selections and their Propagation

Kalmia latifolia Selections and their Propagation
By Alfred J. Fordham, Hopkinton, Mass.
Consultant, Weston Nurseries
Reprinted from the Rosebay, Massachusetts Chapter

While at the Arnold Arboretum I worked on the propagation of Kalmia latifolia and have prepared a table showing the outcome of that effort. It gives data concerning 29 experiences, most of which show a high degree of success.
Through the years many cultivars of Kalmia latifolia (Mountain Laurel) have been selected as natural variants either in the wild or from nursery rows. Oddly enough, K. latifolia 'Rubra', one of the first cultivars of this native American plant to appear in the records of the Arnold Arboretum, came from the English nursery firm of Veitch and Son in 1886. From native sources, the Arboretum received such Kalmia latifolia cultivars as 'Obtusata' (1886), 'Polypetala' (1870), and 'Myrtifolia' (1885).
Despite the fact that good garden forms were first described about a century ago, few were carried in nursery lists. This can be explained by the fact that they were previously considered difficult to propagate from cuttings and percentage of success was small when they were grafted.
In the early 1940's, Edmund Mezitt of Weston Nurseries became interested in mountain laurel selections, particularly those with deep red buds, pink flowers, etc. Plants with similar characteristics were planted side by side to insure cross pollination. The resultant seeds were sown and plants were grown on further selection. This program led to some spectacular clones. I am now associated with Weston Nurseries as a consultant and one of my involvements is an effort to get these into commerce. By using the procedures described below the project is now underway.

POLYETHYLENE CHAMBERS

Much of the mountain laurel propagation herein described was carried out in polyethylene chambers which have some distinct advantages. Nutrients do not leach from the cuttings as can happen when cuttings which root slowly are placed under mist. The chambers are carefree and can be left for long periods of time without attention. There is little chance of loss through human or mechanical failure. Many taxa normally considered difficult can be rooted in high percentages.
The chambers were constructed on benches with side walls about 5" high. They were first lined with 2 mil polyethylene film. Bottom heat was provided by heating cables, so about 1 inch of medium was placed in the bench and cables installed at that level. To disperse the heat more evenly, 1/8 inch galvanized hardware cloth was placed in contact with the cable. The bench was then filled with medium consisting of equal parts horticultural grade Perlite and sphagnum peat moss. Welded joint wire of 2 by 4" mesh was shaped to form supporting frames which held the 2 mil plastic covering about 8 or 10 inches above the rooting medium. Bottom heat was maintained at about 75° F.

CUTTING PREPARATION

Mountain laurel cuttings are made from current year's growth and can be taken as soon as the growth ripens. The stems are cut to a uniform length, (any leaves that would be below the rooting medium are removed) and they are wounded so that they will produce well distributed root systems. This is accomplished by slicing two slivers of rind downward for a distance of 1 to 1½ inches on opposite sides at the base of the cutting. This procedure removes physical barriers to root emergence and exposes more surface to the action of root inducing substances. Wounding both sides of the cutting is important to prevent a lopsided root ball. Mountain laurel roots slowly, taking from 4 to 6 months.

ROOT INDUCING MATERIALS

Some of the most effective treatments in our experiences were 5-second dips using IBA plus NAA at various strengths or treatment with 2,4,5, TP powder formulations. Because of clonal variation, there were instances where IBA plus NAA was superior, while in other cases, 2,4,5, TP proved better. In the course of our current work at Weston Nurseries, a number of formulations are being tested. When sufficient propagating wood is available, we use as many as eight treatments. The treatments proving most effective, however, are 2,4,5, TP at 1,000 parts per million or IBA plus NAA at 2500 parts per million each.
We are grateful to the Research Department, Agricultural Chemicals Division, Amchem Products, Inc., Ambler, Pa. for providing the experimental materials being used. The "quick dip" preparations are diluted from concentrated solutions while the 2,4,5 TP formulations are in talc.

PROPAGATION FROM SEED

The five-chambered, globe-shaped seed capsules of mountain laurel ripen in autumn and later split to release their seeds. Seed examined in December shows some, but not all, capsules open at their tops. Nature's design prevents spilling of the minute seeds, yet allows high winds to whip them out and carry them away from the mother plant. A dispersal adaptation such as this leads to wide latitude in collection time and one can expect to find some seeds remaining in the capsules throughout the winter.
Most very small seeds do not have benefit from pretreatment by cold; mountain laurel, however, is an exception. Germination of 20 to 50% can be obtained without stratification but it is increased and unified if its seeds are subjected to a cold treatment. This can be accomplished by sowing the seeds in pots or other containers and placing them out-of-doors for the winter in a sheltered location such as a cold frame. An alternate method of stratification would be to put the containers of sown seed in a polyethylene plastic bag which is bound at the mouth with a rubber band to make it vapor-proof. The cold requirement is then satisfied by putting the bag in a household refrigerator for three months.

MOUNTAIN LAUREL SELECTIONS

In the Boston area, mountain laurel blooms about June 1st and its course of flowering is longer than that of most woody plants. The span between bud opening and flower drop covers about 3 weeks. Flowers of red budded forms and those with banded corollas tend to flower later and to persist for greater periods. At Weston Nurseries, there are several blocks of mountain laurel from which selections are made. Those thought to have merit are moved to particular areas where they can undergo further observation. In order that evaluations be uniform, only one person is involved in making judgments.
During the last few years R. Wayne Mezitt has undertaken this endeavor.

IMPORTANCE OF CHARACTERISTICS OTHER THAN FLOWERS

In seedling populations of mountain laurel, one finds plants with varying characteristics. Some with special horticultural merit display bright red, yellow or orange stems on which many plants are contrasted against dark green leaves. One particularly striking specimen at Weston Nurseries is characterized by compact growth habit, red stems and dark green leaves with red mid-ribs. Some of these beautiful features are prominent throughout the year and therefore should receive consideration when selections are being made. The plants described here were grown in full sun. Were they grown in shade or partial shade their coloration might not be so intense.

IBA = Indole butyric acid
NAA = Naphtaleneacetic acid
2,4,5 TP = trichlorophenoxypropionic acid

LITERATURE CITED
Fordham, Alfred J., Mountain Laurel and Its Propagation Plants & Gardens, 27 (2); 44-47
Fordham, Alfred J., Woody Plants Hard to Root, International Plant Propagators Society Combined Proceedings 16: 190-193