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Orville M. Lindstrom and Michael A. Dirr

Cold hardiness levels of six cultivars of Chinese elm (Ulmus parvifolia Jacq.), `Select 380', `Orange Ribbon 740', `Emerald Isle', `Emerald Vase', `Drake', and `King's Choice', were determined over eight sample dates from 31 Aug. 1988 to 16 May 1989 and for `Emerald Vase' and `Drake', over three dates from 14 Feb. 1988 to 25 Apr. 1988. All cultivars tested achieved a maximum cold hardiness in December and January of – 21 to – 24C, except `King's Choice', which survived exposure to at least – 30C. `Emerald Isle' and `Emerald Vase' acclimated earlier (both – 9C on 31 Aug.) and reacclimated later (– 6 and – 9C, respectively, on 16 May) than other cultivars tested. `Emerald Vase' and `Drake' exhibited similar cold hardiness levels over the two years tested.

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C.L. Haynes, O. M. Lindstrom, and M. A. Dirr

The effects of timing of pruning in relation to cold hardiness of X Cupressocyparis leylandii (A. B. Jacks. and Dallim.) Dallim. and A. B. Jacks. `Haggerston Grey' and Lagerstroemia L. `Natchez' were evaluated on 6 test dates from August 1989 to March 1990. Pruning treatments decreased the cold hardiness of both taxa compared to unpruned controls on 5 test dates. Cold tolerance of `Haggerston Grey' decreased for 4 to 5 months following the August and October pruning compared to the unpruned controls. `Haggerston Grey's cold tolerance were reduced by 6C in February. October and December pruning of `Natchez' reduced cold hardiness by 4C in January. However, cold hardiness of January and February pruning treatments was similar to unpruned controls. In general, the data indicated that plants of `Haggerston Grey' pruned in October through February were less cold hardy than plants pruned in August. Ideally, `Natchez' crape myrtle should be pruned in late winter.

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S.M. Scheiber, C.D. Robacker, and M.A. Dirr

The genus Abelia contains ≈30 species, but A. × grandiflora, its cultivars, and A. `Edward Goucher' are the primary taxa grown. The nursery industry has stated that Abelia R. Br. taxa are important economically, and new selections or cultivars with increased cold hardiness, richer pink-rose flower colors, unique foliage colors, and compact habits are desired. Breeding and selection work in the genus is very limited due in part to limited access to germplasm. Pollen storage enables breeders to cross taxa with incongruent flowering cycles, save time and resources by eliminating the need to grow vast amounts of plant material, and incorporate otherwise unavailable germplasm into a breeding program. An experiment was conducted to determine the optimum levels of temperature and humidity for the long-term storage of A. chinensis and A. × grandiflora `Golden Glow' pollen. Temperature and humidity levels were analyzed by incubating undesiccated pollen of a given taxon at four humidity levels (0%, 50%, 80%, and 100%) for 72 h at 5 °C. Following incubation, the pollen was stored in glass vials at each of the following temperatures: 5, -20, and -70 °C. All combinations of temperature and humidity were tested. Pollen viability was assessed after 60 days by in vivo germination tests on styles. Abelia chinensis pollen germinated following storage at all temperature and humidity levels. Pollen of A. × grandiflora `Golden Glow' pollen germinated following all treatments except storage at -20 °C.

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C.L. Haynes, O.M. Lindstrom, and M.A. Dirr

Decreasing photoperiods and decreasing temperatures induce cold acclimation and the accumulation of soluble sugars in many plants. Two cultivars of southern magnolia differing in cold hardiness and acclimation patterns, were monitored to determine photoperiod × temperature interaction on cold hardiness and soluble sugar content. Cold hardiness increased with low temperatures and short photoperiods. Total soluble sugars, sucrose, and raffinose consistently increased in the leaves and stems of both cultivars in response primarily to low temperature. `Little Gem' was less responsive to photoperiod than `Claudia Wannamaker'

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C.L. Haynes, O.M. Lindstrom, and M.A. Dirr

Cooling treatments of 2, 4, and 6C/hour or warming at 25, 4, or 0C influenced the cold hardiness estimates of x Cupressocyparis leylandii (A.B. Jacks. and Dallim.) Dallim. and A.B. Jacks. (Leyland cypress), Lagerstroemia indica L. (crape myrtle), and Photinia ×fraseri Dress `Birmingham' (redtip photinia) at four times during the year. New growth from all taxa, especially spring growth, was injured or killed at higher temperatures by the fastest cooling rate and/or by warming at 25C. Cold hardiness of Leyland cypress was unaffected by the cooling and warming treatments. Crape myrtle had a significantly higher lowest survival temperature (LST) when warmed at 25C than at 4 or 0C. Photinia leaves and stems cooled at 6C/hour or warmed at 25C generally resulted in a higher LST than those cooled more slowly or warmed at lower temperatures. Cooling rates of 14C/hour and warming at 0 to 4C should be used in freeze tests with Leyland cypress and crape myrtle. For leaves and stems of photinia, 2C/hour cooling and warming at 0 to 4C should be used.

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C.L. Hayns, O.M. Lindstrom Jr., and M.A. Dirr

The effects of late summer, fall, and winter pruning on the cold hardiness of × Cupressocyparis leylandii (A.B. Jacks. and Dallim.) Dallim. and A.B. Jacks. `Hag gerston Gray' (Leyland cypress) and Lagerstroemia L. `Natchez' (crape myrtle) were determined. Pruning in late summer through early winter significantly reduced the cold hardiness of both taxa. The maximum difference in cold hardiness between pruned trees and controls for × Cupressocyparis leylandii `Haggerston Gray' in October, December, January, and February was 3, 3, 2, and 6C, respectively. The maximum difference in cold hardiness between pruned plants and controls for Lagerstroemia `Natchez' in December, January, and February was 3, 4, and 2C, respectively. Early spring pruning of Leyland cypress and late winter or early spring pruning of crape myrtle are suggested from these data.

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C.L. Haynes, O. M. Lindstrom, and M. A. Dirr

The effects of timing of pruning in relation to cold hardiness of X Cupressocyparis leylandii (A. B. Jacks. and Dallim.) Dallim. and A. B. Jacks. `Haggerston Grey' and Lagerstroemia L. `Natchez' were evaluated on 6 test dates from August 1989 to March 1990. Pruning treatments decreased the cold hardiness of both taxa compared to unpruned controls on 5 test dates. Cold tolerance of `Haggerston Grey' decreased for 4 to 5 months following the August and October pruning compared to the unpruned controls. `Haggerston Grey's cold tolerance were reduced by 6C in February. October and December pruning of `Natchez' reduced cold hardiness by 4C in January. However, cold hardiness of January and February pruning treatments was similar to unpruned controls. In general, the data indicated that plants of `Haggerston Grey' pruned in October through February were less cold hardy than plants pruned in August. Ideally, `Natchez' crape myrtle should be pruned in late winter.

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S.M. Scheiber, Carol D. Robacker, and M.A. Dirr

Flowering evergreen shrubs that are compact and resistant to pests are in great demand in the nursery and landscape industries. The genus Abelia contains 30 species that vary in many traits including flower color, growth habit, and hardiness. Abelia × grandiflora (Andre) Rehd. and its cultivars are the most widely grown Abelia taxa and are characterized by pest resistance, an abundance of pinkish white flowers, long flowering period, and glossy evergreen foliage. Interspecific hybridization among Abelia × grandiflora, its cultivars, and other species in the genus Abelia offer the potential for new cultivars; however, seed germination within the genus has been described as slow and inconsistent. Experiments were conducted to test procedures to increase germination percentages and rates. Each Abelia seed is enclosed in a leathery achene. The effect of achene removal was examined in combination with cold, moist stratification for 60 days at 4 °C, immersion in 100 ppm gibberellic acid for 24 h, and no treatment. Treatments were replicated five times with 15 seeds per replication. Seeds were sown on sphagnum peat, and grown under mist in the greenhouse. Weekly germination counts were recorded for 8 weeks. Seeds with attached achenes germinated at a significantly higher percentage than those without achenes. Cold, moist stratification and gibberellic acid treatments were not significantly different than the control. No significant differences were found within the achene treatments for relative rate of emergence, but significant differences were found for the time until 90% of final emergence was reached.

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Orville M. Lindstrom, Tomasz Anisko, and Michael A. Dirr

Although differential thermal analysis has been routinely used to evaluate cold hardiness, the relationship of deep supercooling ability and plant survival are not well understood. In this study, we compared the seasonal profiles of changes in low-temperature exotherm (LTE) occurrence with visually determined cold hardiness of Acer rubrum L. `Armstrong', Fraxinus americana L. `Autumn Purple' and Zelkova serrata (Thunh.) Mak. `Village Green' growing in three locations representing plant cold hardiness zones 8b, 7b, and 5a. Between December and February, LTEs in Acer rubrum `Armstrong' and Fraxinus americana `Autumn Purple' occurred at temperatures around 10 to 25C lower than the lowest survival temperatures. The mean difference between LTEs and lowest survival temperature was not significant for Zelkova serrata `Village Green' from January to April and for Acer rubrum `Armstrong' and Fraxinus americana `Autumn Purple' in March. Data indicated that LTEs could be used as an estimate of lowest survival temperature in Zelkova serrata `Green Village' but not in Acer rubrum `Armstrong' and Fraxinus americana `Autumn Purple'. This study demonstrated that LTEs may not reliably estimate cold hardiness in all species that deep supercool. Factors other than freeze avoidance ability of xylem may limit stem survival at temperatures above the LTE.

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C.L. Haynes, M.A. Dirr, and R. Severson

The cold hardiness of Magnolia grandiflora `Claudia Wannamaker' and `Little Gem' was determined under 8, 12 and 16 hour daylengths. Temperature was maintained at 25C day and 20C night. In addition, specific and total carbohydrates of both cultivars were analyzed. Cold hardiness and carbohydrate content were tested at the beginning (0 week), middle (5 week), and end (9 week) of the study. As expected, both southern magnolia cultivars were more cold hardy after 9 weeks at 8 hour daylengths with -9C cold hardiness estimates, as compared to 12 and 16 hour daylengths. The 12 and 16 hour daylengths resulted in similar cold hardiness estimates of -6C after 9 weeks. Additional cold hardiness and carbohydrate information will be presented.