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.
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.
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.
PROPAGATION OF KALMIA LATIFOLIA BY CUTTINGS Taxa Time
Remarks 'Rubra' 3 Mar 66 10 IBA - 1%
100 Excellent Plants of same clonal line forced dip in greenhouse - wood very firm - under polyethylene 'Rubra' 3 Mar 66 10 IBA - 2%
90 Excellent Plants of same clonal line forced in greenhouse - wood very firm - under polyethylene 'Rubra' 3 Mar 66 10 IBA - NAA 500 ppm ea.
100 Excellent Plants of same clonal line forced in greenhouse - wood very firm - under polyethylene 'Rubra' 3 Mar 66 10 IBA - NAA
1,000 ppm ea
100 Excellent Plants of same clonal line forced in greenhouse - wood very firm - under polyethylene 'Rubra' 3 Mar 66 10 245 TP 5,000
90 Excellent Plants of same clonal line forced in greenhouse - wood very firm - under polyethylene 'Rubra' 30 Mar 67 10 245 TP 1,000
90 Excellent From 1-year old rooted cuttings in greenhouse - wood starting to firm - under polyethylene 'Rubra' 30 Mar 67 10 245 TP 1,000
30 Fair to Good Wood as above - placed under mist; leached badly 'Rubra' 30 Mar 67 10 IBA - NAA
1,000 ppm ea
90 Excellent Wood as above - placed under polyethylene 'Rubra' 30 Mar 67 10 IBA- NAA
1,000 ppm ea
50 Fair Wood as above-placed under mist; leached badly 'Rubra' 4 Aug 66 6 245 TP 5,000
83 Excellent Cuttings from 8-year old plant in
nursery row - under polyethylene
'Rubra' 23 Jan 67 22 IBA- NAA
5,000 - ppm ea. 5-second dip
91 Excellent Cuttings from 9-year old dormant plants in cold storage unit - all same clone - under polyethylene 'Rubra' 23 Jan 67 22 245 TP 1,000
86 Excellent Cuttings from 9-year old dormant plants in cold storage unit - all same clone - under polyethylene 'Rubra' 20 Dec 67 93 245 TP 1,000
91 Excellent Cuttings from 22-month old rooted cuttings in nursery row under polyethylene Selection 27 Aug 70 15 245 TP 1,000
73 Excellent From large plant, Beverly, Massachusetts darker than normal foliage - under polyethylene Selection 27 Aug 70 15 IBA - NAA
2,500 ppm ea.
100 Excellent From large plant, Beverly,
Massachusetts - darker than
normal foliage - under
'Rubra' 4 Aug 66 6 0.8% IBA in - w/ Thiram Powder 50 2 Good
From 7-year old plant nursery - under polyethylene 'Rubra' 4 Aug 66 6 none - control for above 66 Excellent; As above Compact form 29 Nov 68 6 245 TP 5,000
100 Excellent From plant in nursery - under polyethylene 'Rubra' 20 Dec 67 30 245 TP 1,000
87 Excellent; From older plant in nursery - under polyethylene 'Rubra' 20 Dec 67 30 IBA - NAA
5,000 ppm ea.
84 Excellent As above Selection 17 Dec 74 12 0.8% IBA -
w/ Thiram powder
100 Excellent From older plant in nursery under polyethylene 'Fuscata' 28 Jun 67 11 245 TP 1,000
82 Excellent Under mist - later removed to under plastic because of leaching 'Fuscata' 28 Jun 67 11 245 TP 5,000 ppm ea. powder 100 Excellent Under mist - later removed to under plastic because of leaching 'Fuscata' 28 Jun 67 11 IBA -- NAA
2,500 ppm ea.
100; Excellent Under mist later removed to
under plastic because of leaching
'Fusca ta' 28 Jun 67 11 IBA - NAA 82 Excellent Under mist later removed to
under plastic because of leaching
'Polypetala' 20 Dec 67 12 none 50 Excellent Taken from very old plant received at Arnold Arboretum 1885 - under polyethylene 'Polypetala' 20 Dec 67 12 IBA - NAA
2,500 ppm ea.
80 Excellent As above 'Polypetala' 20 Dec 67 12 245 TP 1,000
66 Excellent As above 'Silver Dollar' 17 Dec 74 8 8% IBA -
w/ Thiram 15%
100 Excellent From 2-foot grafted plant - under polyethylene
IBA = Indole butyric acid
NAA = Naphtaleneacetic acid
2,4,5 TP = trichlorophenoxypropionic acid
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