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He Li and Donglin Zhang

Kalmia latifolia L. (mountain laurel), a member of Ericaceae (heath family), is a large evergreen flowering shrub native to the eastern United States. It has been considered by many horticulturists and gardeners to be one of the most beautiful

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J.R. Ballington

The genus Kalmia L. is endemic to North America. Kalmia latifolia is the best known species in the genus. It is a rounded evergreen shrub to small tree that ranges from northern Florida to New England. Flower color varies from white to pink, but at lower elevations in the southeastern U.S., pink flowers quickly fade to white. It is a diploid species with 2n = 2x = 24 chromosomes. Kalmia angustifolia var. caroliniana only occurs in the southeastern US. It is a thin upright evergreen shrub to 1.5 m tall. Flower color is either light pink or rosy purple, and the flower pigments appear to be heat stable. It is also diploid. Kalmia latifolia has not crossed readily with any other Kalmia species to date, but a small number of hybrids have been produced. The objective of the present study was to intercross K. angustifolia var. caroliniana with K. latifolia to attempt to develop color stable pink flowered Kalmia hybrids for warm climates. The crosses were made at Cary, N.C., from late April through mid May and included two clones of each species. Only one parental combination was successful and involved a rosy purple form of the former species. With this cross 15 mature seed capsules resulted from 38 pollinations. Numerous seedlings initially germinated, of which about 15% were albinos. Only 38 seedlings survived to transplanting. Thirty seedlings remain relatively vigorous 8 months after potting and are phenotypically intermediate between the parents. Their potential will depend on their ornamental characteristics once they reach maturity.

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Mark H. Brand

To introduce desirable trait genes into Kalmia latifolia, efficient adventitious shoot regeneration methods are needed. Silver Dollar (S$) callus induction and growth in the dark was compared on Woody Plant (WP) medium containing 2,4-dichlorophenoxyacetic acid (2,4-D) (1, 5, 10, 20 μM) or naphthaleneacetic acid (NAA) (1, 10, 20, 40 μM) with and without 5 μM isopentenyladenine (2iP). Both 2,4-D and NAA produced >450 mg of callus from leaf explants in 8 weeks. The addition of 2iP tripled growth for 2,4-D and doubled growth for NAA. Greatest callus growth was obtained on 20-40 μM NAA or 5-20 μM 2,4-D. Shoot regeneration on callus was achieved on WP medium containing 30 μM 2iP or 1 μM thidiazuron (TDZ), but a combination of the two was best, with 68% of dark-grown calli regenerating shoots in 4 weeks. 26% more dark-grown calli regenerated shoots than light-grown calli. The type of auxin (2,4-D or NAA) used to grow the calli did not affect shoot regeneration. For direct shoot regeneration, S$ leaf explants were tested on WP medium containing 5, 15, 30, 45 and 60 μM 2iP. The addition of 1 μM indole-3-butyric acid (IBA) doubled the percentage of leaves that regenerated shoots. 2iP concentrations between 15 and 45 μM supported excellent shoot regeneration, but optimal regeneration (95% of explants, 5.1 shoots/leaf) occurred on 30 μM 2iP+1 μM IBA. Leaf explants of six cultivars were grown on optimal medium with shoot regeneration ranging from 17% to 93% of leaves and 1.8 to 8.2 shoots per leaf, depending on the cultivar.

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John Wachter and Paul E. Cappiello

Terminal stem cuttings of Kalmia latifolia were collected from wild plants (Milford, N.H.) on 12 Nov. and transported on ice to Orono, Maine, for analysis. Samples were processed as follows: 1) stems wrapped in dry cheesecloth; 2) stems wrapped in moist cheesecloth; and 3) stems seeded with crushed ice and wrapped in moist cheesecloth. Prepared samples were subjected to freezing tests to a low temperature of –36C. Following two weeks of incubation at 21C, samples were evaluated for leaf, petiole, stem, and vegetative bud damage. Evaluation of frozen samples revealed: 1) stem tissue remained undamaged to –36C; 2) leaf damage was inconsistent across all handling methods, with no clear LST estimate, and ice seedinggenerally resulted in increased tissue damage; 3) LSTs for vegetative buds and petiole bases were –18C and –15C, respectively, and both yielded definitive and consistent results across all treatments. The results indicate bud and petiole tissue to be the best to use for future studies on LST estimates in Kalmia latifolia.

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Mark H. Brand

The effect of shading during nursery production on the growth, foliage color, and foliar chlorophyll content of container-grown Kalmia latifolia cultivars was investigated. Five cultivars were grown under 40% shade, 60% shade, or full sunlight for a 2-year production cycle. During the first year of production, there were no significant differences in measured growth characteristics for most cultivars in response to light treatment. Shade improved foliar color by decreasing lightness (L*), decreasing chroma, and changing hue angle from a yellow-green to a darker green. Foliar chlorophyll concentration increased under shade. In the second year of the production cycle, the response of foliar color and chlorophyll concentration to shade was similar to that observed in year 1. Plant size, number of branches, leaf area, leaf dry mass, and stem dry mass decreased linearly with increasing shade in year 2. Although shading improves foliar color, it probably should not be employed for container production of Kalmia latifolia in cool, northern production areas due to reduced plant growth during year 2. Shade may be useful in the first year of production to enhance foliar color without reducing shoot growth.

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Anne-Marie Hanson, J. Roger Harris*, and Robert Wright

Mountain laurel (Kalmia latifolia L.) is a common native shrub in the Eastern United States; however, this species can be difficult to establish in landscapes. Two experiments were conducted to test the effects of transplant season and container size on landscape establishment of Kalmia latifolia L. `Olympic Wedding'. In experiment one, 7.6-L (2-gal.) and 19-L (5-gal.) container-grown plants were planted into a simulated landscape (Blacksburg, Va., USDA plant hardiness zone 6A) in early Fall 2000 and in late Spring 2001. 19-L (5-gal.) plants had the lowest leaf xylem potential (more stressed) near the end of the first post-transplant growing season, and leaf dry weight and area were higher for spring transplants than for fall transplants. For spring transplants, 7.6-L (2-gal.) plants had the highest visual ratings, but 19-L (5-gal.) plants had the highest visual ratings for fall transplants three growing seasons after transplanting. 7.6-L (2-gal.) plants had the highest % canopy volume increase after three post-transplant growing seasons. In experiment two, 19-L (5-gal.) plants were transplanted into above-ground root observation chambers (rhizotrons) in early Fall 2000 and late Spring 2001. Roots of fall transplants grew further into the backfill than spring transplants at the end of one post-transplant growing season. Overall, our data suggest that smaller plants will be less stressed the first season after transplanting and will likely stand a better chance for successful establishment in a hot and dry environment. Fall is the preferred time to transplant since capacity for maximum root extension into the backfill will be greater than for spring transplants.

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Mark H. Brand

Container production of recently-developed and popular Kalmia latifolia cultivars has not been fully optimized. A study was conducted using six cultivars grown in full sun, 40% shade or 60% shade. Under 60% shade, plant height was reduced slightly, but shading, at either 40% or 60%, had no significant effect on all other measured growth parameters. Plants were too young to set significant numbers of flower buds, so the study will be continued a second year to quantify the effects of shade on flower bud set. Foliage color was measured using a Minolta CR-200 Chroma Meter. As shading increased, hue angle increased and the chroma and value of the color decreased, indicating that shading produced greener (less yellow), darker and duller foliage colors. Foliar chlorophyll content increased with increasing shading. Higher foliar chlorophyll content correlates with greener leaves in shaded treatments and is likely contributing to the green color. Using moderate levels of shade over container-grown Kalmia could allow growers to produce greener, more marketable plants without sacrificing plant growth.

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Richard K. Kiyomoto

Nursery production of Mountain Laurel (Kalmia latifolia L.) often involves manual disbudding or deadheading flower clusters immediately after flowering, to stimulate the formation of one or several new shoots. Experiments were initiated on populations of K. latifolia `Angel' (41 plants), `Snowdrift' (17 plants), and `Hoffman's Pink' (37 plants) to test the effectiveness of single applications of 0 (water control), 500, 1000, and 2000 ppm ethephon in reducing seed set and stimulating new shoot formation. Ethephon was applied on 9 June 1995 when an average of 52.9%, 53.4%, and 27.3% of the flowers were open in each flowering cluster of `Angel', `Snowdrift', and `Hoffman's Pink', respectively. On 17 to 19 July 1995, data were collected for numbers of green seed capsules per flower cluster and the number of new shoots per plant. One way analysis of variance showed the treatments had highly significant effects on seed capsule numbers per flower cluster and in stimulating the production of new shoots per plant in the three cultivars. The average number of green capsules per flower cluster and new shoots per plant for each cultivar treated with 2000 ppm ethephon were: 2.2 capsules and 57.2 shoots in `Angel', 1.1 capsule and 40 shoots in `Snowdrift', and 6.6 capsules and 39.3 shoots in `Hoffman's Pink'. In contrast, the controls had 20.1 capsules and 2.8 shoots in `Angel', 22.9 capsules and 8 shoots in `Snowdrift', and 27.3 capsules and 2 shoots in `Hoffman's Pink'.

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Amy N. Wright, Robert D. Wright, Jake F. Browder, and Brian E. Jackson

Posttransplant root growth is critical for landscape plant establishment. The Horhizotron provides a way to easily measure root growth in a wide range of rhizosphere conditions. Mountain laurel (Kalmia latifolia L.) plants were removed from their containers and planted in Horhizotrons in a greenhouse in Auburn, Ala., and outdoors in Blacksburg, Va. Each Horhizotron contained four glass quadrants extending away from the root ball, and each quadrant within a Horhizotron was filled with a different substrate (treatment): 1) 100% pine bark (Pinus taeda L., PB), 2) 100% soil, 3) a mixture of 50 PB: 50 soil (by volume), or 4) 100% soil along the bottom of the quadrant to a depth of 10 cm (4 inches) and 100% PB layered 10 cm (4 inches) deep on top of the soil. Root growth along the glass panes of each quadrant was measured biweekly in Auburn and weekly in Blacksburg. Roots were longer in all treatments containing pine bark than in 100% soil. When pine bark was layered on top of soil, roots grew into the pine bark but did not grow into the soil. Results suggest that amending soil backfill with pine bark can increase posttransplant root growth of container-grown mountain laurel.

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Amy N. Wright, Stuart L. Warren, Frank A. Blazich, and Udo Blum

The length of time between transplanting and subsequent new root initiation, root growth rates, and root growth periodicity influences the ability of woody ornamentals to survive transplanting and become established in the landscape. Research was conducted to compare root growth of a difficult-to-transplant species, Kalmia latifolia L. (mountain laurel), to that of an easy-to-transplant species, Ilex crenata Thunb. (Japanese holly), over the course of 1 year. Micropropagated liners of `Sarah' mountain laurel and rooted stem cuttings of `Compacta' holly were potted in 3-L containers. Plants were grown in a greenhouse from May to September, at which time they were moved outside to a gravel pad, where they remained until the following May. Destructive plant harvests were conducted every 2 to 4 weeks for 1 year. At each harvest, leaf area, shoot dry weight (stems and leaves), root length, root area, and root dry weight were determined. Throughout the experiment, shoot dry weight and leaf area were similar for the two species. New root growth of `Compacta' holly and `Sarah' mountain laurel was measurable 15 and 30 days after potting, respectively. Root length and root area of `Sarah' mountain laurel increased during May through December but decreased during January through May. Root length and root area of `Compacta' holly increased linearly throughout the course of the experiment. Final root: shoot ratio of `Sarah' mountain laurel was one-ninth that of `Compacta' holly. Results suggest that poor transplant performance of mountain laurel in the landscape may be related to its slow rate of root growth.