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Tomasz Anisko and Orville M. Lindstrom

The effect of water stress imposed at three dates in late summer and early fall on cold hardiness was examined in Rhododendron L. `Coral Bell', `Hinodegiri', and `Red Ruffle'. The persistence of the water stress-induced cold hardiness was also examined following plant recovery from the stress. Container-grown plants were exposed to three weeks of reduced water supply starting 8 Aug., 29 Aug., or 19 Sept., while control plants were well watered. Cold hardiness of leaves, lower, middle, and upper stems was evaluated with laboratory freeze tests. Reduced water supply independent of time initiated increased cold hardiness by 1 to 4C in the majority of the tested plant parts in the three cultivars. Cold hardiness of all plant parts tested strongly depended on the current water status of the plants as indicated by the stem water potential. In most cases, 3 weeks after rewatering, the cold hardiness of previously water stressed plants did not differ from that of nonstressed plants.

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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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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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Orville M. Lindstrom

The cold hardiness of seven deciduous hardwoods, red maple (Acer rubrum L.), white oak, (Quercus alba L.), green ash (Fraxinus pennsylvanica Marsh.), sweetgum (Liguidambar stryaciflua L.), sugar maple (Acer saccharum Marsh.), river birch (Betula nigra L.) and black cherry (Prunus serotina Ehrh.) were evaluated weekly during the fall, winter and spring for three consecutive years. All trees evaluated were established (20-40 years old) and locatd on the Georgia Station Griffin, GA. Each species developed a maximum cold hardiness of at least -30 C by mid-January or early February each season. Response to temperature fluctuations varied with species. Red maple, for example, lost less cold hardiness due to warm mid-winter temperatures than the other species tested, while white oak tended to respond more quickly to the temperature fluctuations. Data will be presented comparing the response of cold hardiness to mid-winter temperature fluctuations for each species for the three year period.

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Robert L. Wample and Andy Bary

Cold-hardiness evaluations and soluble and insoluble-nonstructural carbohydrate analysis of dormant Vitis vinifera L. cv. Cabernet Sauvignon buds and cane tissue indicate a positive relationship between soluble carbohydrates and primary bud cold hardiness. Seasonal variations in soluble and insoluble carbohydrates appear to be related to changes in air temperatures and the dormancy status of the tissues. No differences were found in bud cold hardiness and only limited differences in carbohydrate levels of buds or stem tissues collected over 3 years from early harvest, normal harvest, or unharvested vines. These findings contrast with the widely held opinion that delayed harvest or failure to remove fruit results in reduced cold hardiness as a consequence of low storage carbohydrate content of the plants.

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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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Norman E. Pellett, Nancy Rowan, and John Aleong

Florets of eight provenances representing three native North American azalea species [Rhododendron calendulaceum (Michx.) Torr., R. prinophyllum (Small) Millais, and R. viscosum (L.) Torr.] being grown in Burlington, Vt., were compared during three seasons for cold hardiness by laboratory freezing during cold acclimation. There was a large variability in the number of florets killed within an inflorescence in response to freezing temperatures. Cold hardiness of florets of the three species ranked, from most to least hardy, were R. viscosum, R. prinophyllum, and R. calendulaceum. Some differences were noted in cold hardiness of florets of provenances, but these were not necessarily related to latitude or elevation of origin. Cold hardiness of most provenances showed a significant linear relationship with the daily mean temperature of the 3 days preceding freezing tests. Ambient temperatures just before subfreezing test temperatures may affect winter injury more than provenance differences for these species.

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D.F. Warnock, W.M. Randle, and O.M. Lindstrom Jr.

Proper acclimation of onion (Allium cepa L.) seedlings can enhance winter freeze survival; therefore, the effects of photoperiod-temperature combinations, photoperiod, and plant age on the cold hardiness of short-day onions were investigated. Following acclimation at various photoperiod-temperature regimes, different-aged plants were frozen to various subzero temperatures in an ethylene glycol bath and evaluated for cold hardiness. Older plants were more cold hardy than younger plants. An 11-hour photoperiod-decreasing temperature (20/15 to 10/5C day/night) treatment improved plant cold hardiness over other photoperiod-temperature regimes. Various photoperiods (8-, 11-, 14-, and 24-hour) applied during a 14-day, 3C acclimation treatment before freezing had little effect on plant cold hardiness. However, day 7 foliar and day 14 root evaluations indicated that 81-day-old plants given an 8- or 11-hour photoperiod during the 3C acclimation treatment were less cold hardy than older plants (91 or 112 days) given the same acclimation photoperiod.

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Norman E. Pellett and Mary A. Holt

Abstract

Five evergreen rhododendron cultivars were compared for flower bud cold hardiness in laboratory freezing studies on 6 dates. ‘Roseum Elegans’, ‘Catawbiense Boursalt’ and ‘Boule de Neige’ were more cold hardy on most sampling dates than ‘America’ and ‘Lee’s Dark Purple’. The relative cold hardiness of these 5 cultivars was consistent on several winter dates over several years. Injured florets were black and readily separated from uninjured white florets upon dissection of the inflorescence bud after freezing.

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Orville M. Lindstrom

Leyland cypress [×Cupressocyparis leylandii (A.B. Jacks. and Dallim.) Dallim. and A.B. Jacks.] plants were transplanted into the field monthly from Aug. 1989 through Mar. 1990, and laboratory cold-hardiness estimates of these transplants were obtained monthly for two winter seasons. Cold hardiness estimates obtained in Dec. 1989 and Jan. 1990 revealed that the Nov. and Dec. 1989 transplants were 6C less cold-hardy than those transplanted into the field earlier in the year. There was little difference in cold hardiness due to transplant date during Feb., Mar., and Apr. 1990. In the second year of the study, on the same transplants, cold hardiness varied among transplanting dates. In Dec. 1990 and Jan. 1991, those transplanted in Jan.-Mar. 1990 were up to 9C less cold-hardy than those transplanted earlier in the season. However, in Mar. and Apr. 1991, those transplanted in Jan.-Mar. 1990 were equally or more cold-hardy than those transplanted earlier in the season. Transplanting Leyland cypress into the field in August to November appears to be the best time to ensure development of cold hardiness in early winter, whereas January to March planting appears to promote greater cold hardiness in the spring months.