JARS v52n3 - Companion Plants: Mountain Laurel, A Plant with Many Faces

Companion Plants: Mountain Laurel, A Plant with Many Faces
Richard A. Jaynes
Broken Arrow Nursery
Hamden, Connecticut

New selections and cultivars of mountain laurel, Kalmia latifolia , continue to demonstrate that there is more to this species than previously thought. Much has been learned of the culture and breeding, but much more remains to be done. This article describes a few new cultivars, summarizes recent developments, and suggests projects needing attention. More detail is contained in the book Kalmia latifolia, Mountain Laurel and Related Species , Timber press, 1997.

The light pink to near white flowers of the native mountain laurel are attractive and impressive enough for the species to have been named the state flower of Connecticut and Pennsylvania. Mountain laurel has even been claimed to be the most attractive evergreen shrub of North America. Most native mountain laurel has light pink flowers; however, named selections range from pure white to rich pink and red budded, as well as cinnamon burgundy within the corolla. Unique newer selections deal largely with pigment distribution, flower form, variation in leaf size and shape, and plant habit.

New Selections

'Peppermint' has bright red stripes that radiate from the center of the open corolla to the lip. The unique 'Shooting Star' cultivar originated from a single plant found by Hollis and Marjorie Rogers in 1972 at Hanging Rock State park, Danbury, N.C. The lobes of this plant are deeply cut and may appear petaled. Through a series of controlled crosses I have developed other selections, all having the same unique corolla shape: 'Comet' is white flowered and similar to 'Shooting Star' but was selected for a denser growth habit; 'Galaxy' has the "banded" burgundy pigmentation within the corolla; and 'Meteor', to be introduced in the year 2000, is light lavender rose when grown in light shade instead of full sun.

One of the most recent and unusual discoveries was a double flowered mountain laurel found growing in a yard in New Zealand - on the opposite side of the world from its native range. Named 'Madeline', it will become available in the United States when material received in sterile culture is old enough to flower and we can demonstrate that the plants produce the same clear pink, double flowers of the original.

The miniature (semi-dwarf) or small-leafed laurels (forma myrtifolia ), although rarely found in the wild, have been known for over 100 years. They are distinguished by smaller foliage and a slower rate of growth than the species. Because the distinguishing features of the myrtifolia form are controlled by a single recessive gene, it has been possible to breed new small-leafed cultivars with the same range of flower colors available in the species. The original miniatures were light pink to white, like the first one I named 'Elf'. The newer ones include the rich pink flowered 'Tiddlywinds' and 'Tinkerbell', the bold banded 'Minuet', and the red budded 'Little Linda'. These vary somewhat from each other in leaf shape, foliage color, and growth habit, but all are clearly "miniature" mountain laurels.

These semi-dwarfs are destined to become more popular because they are so useful in the landscape where larger growing selections would be overpowering. Nurserymen love them because they are rapid growing when young but have more controlled growth after they flower and are planted in the landscape.

The recently named 'Willowood' (from Woodlanders Nursery, Aiken, S.C.) has narrow leaves like the earlier named 'Willowcrest'. However, the leaves of 'Willowood' are smaller and the plant habit is more diminutive than its predecessor.

Genetics and Breeding

The inheritance of several distinct traits in foliage, growth, and flowers is now understood. Therefore it is possible to select plants for various combinations of desired traits, as the miniature and Shooting-Star series demonstrate. A new miniature selection having a Shooting-Star-type flower is under test, and the combination of traits for red buds and bands has produced the cultivars 'Kaleidoscope', 'Keepsake', and others. As additional traits are discovered, like the double flowered 'Madeline', the opportunities for new combinations increases. And, of course, there is always the desire to extend and improve existing cultivar types with plants having increased pest resistance, glossier dark green foliage, denser habit, etc. It's been said for all plants that the perfect cultivar has never been developed. Improvements are always possible.

It might seem that all the exciting natural variants of mountain have been discovered, but new finds strongly suggest otherwise. Several variegated forms of the mountain laurel have recently come to my attention and surely one of them will be worthy of naming and introducing. Also just discovered is a procumbent mountain laurel growing in Wahalla, N.C. It is l6 feet (1.8m) across, generally less than a foot tall and has clean foliage. It is a remarkably attractive plant compared to the floppy, irregular procumbent plants I have seen up to now and will probably be introduced as 'Creepy'. Another source for dense, compact laurels may also have been discovered with the first known broom. As he has with other unique selections, Clarence Towe, of Walhalla, N.C, brought this find to my attention.

In other words, more kinds of attractive mountain laurel are still in woods and along roadsides to be found and developed. To make speedier progress, we need more people using these unique plants and growing seedlings from controlled crosses or even just from selected seed parents. Maybe a few rhododendron breeders and growers can be convinced to put part of their efforts into Kalmia !

Growing Laurel and Future Research

Anyone familiar with growing Kalmia knows it can be finicky. Mind you, the plant is virtually a weed in parts of its native habitat covering millions of acres in the eastern United States. For soils where the drainage and aeration are limited, Richard Bir and associates at the University of North Carolina have clearly shown that incorporating large amounts of coarse organic material, such as aged pine bark, is of great benefit.

Nurserymen who grow Kalmia in containers often have a problem getting their young vigorous plants to set flower buds. Two plant growth regulators have proven effective in stimulating flower bud set when sprayed on plants. The use of these materials (Bonzi and SuMagic, or others) may become common for the large nurseries.

A problem for field growers of large plants is not the lack of flowers, but too many of them before the plants reach the desired landscape size of 30 inches (75cm) or more. The many flowers, of course, result in a prolific set of seed capsules that preempts vegetative growth and effectively retards growth. Recent research by Dr. Richard Kyomoto at the Connecticut Agricultural Experiment Station in New Haven demonstrated that the growth regulator ethephon (Floral) applied at about 1000 ppm as flowers begin to open prevents seed capsule development. The effect is similar to mechanical removal of flowers (deadheading) at the end of the bloom period; new vegetative shoots are stimulated to grow. Unfortunately, this material is not a likely solution for deadheading plants in the landscape, because it has to be applied when the flowers are most attractive and they collapse soon after spraying. Perhaps there are other growth regulators that will prevent seed set but not bring about premature loss of the flowers. Such a material would be useful on rhododendrons as well.

Dr. Peter Del Tredicci at the Arnold Arboretum has raised an interesting question about whether micro-propagated (tissue cultured) laurels can endure extreme stress as well a seedling laurels. Like Rhododendron maximum , seed grown mountain laurel is characterized by burls; swellings at the base of the plant that harbor many dormant buds. These buds are stimulated to grow after the plant is injured by cutting, fire, or drought. Micropropagated plants, on the other hand, produce no burls and thus might be less tolerant of extreme stress. This is a question that begs for some good comparison studies.

There is still much to be learned about breeding Kalmia , as well. Controlled crosses with other related genera, especially Louiseleuria and Leiophyllum , just might be successful. Furthermore, existing hybrids of Kalmia latifolia x K. hirsuta have not been fully exploited to extend the adaptability of these good garden plants to warm regions such as the southeastern United States. And crosses of the Cuban Kalmia ericoides have never been attempted with any of the other six Kalmia species.

Development of seed-sterile cultivars would negate the need to deadhead mechanically or chemically. Examples of such a sterile plant in a related genus is the Pieris hybrid 'Brouwers Beauty'.

We are gradually learning how the many new mountain laurel cultivars respond to different growing conditions, but this is a slow process. University and arboreta test plantings in Maine, Connecticut, and North Carolina are a beginning. But screening for heat and cold tolerance, disease and insect resistance, and higher pH and heavier soils tolerance is never fast or easy. More people growing more seedlings under a wide range of conditions would surely expand our selection of cultivars and determine where they could best be grown. The rich diversity of Kalmia latifolia is impressive and more than previously imagined. I encourage anyone with an interest to become involved with mountain laurel and its close relatives.

Richard Jaynes, a member of the Connecticut Chapter, authored the book Kalmia latifolia, Mountain Laurel and Related Species.