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Abstract

Four methods of overwintering container-grown ornamental nursery stock were evaluated. Canopy temperatures of plants unprotected or covered with a thin layer of ice closely followed air temperatures during subfreezing conditions; minimum ambient air temperature was – 16C. Canopy temperatures under white copolymer film fluctuated widely, while canopy and growth medium temperatures of plants continuously irrigated during subfreezing temperatures did not drop below 0C. Root injury of all species and foliar injury of azalea (Rhododendron × ‘Due de Rohan’), euonymus (Euonymus japonica Thunb. ‘Microphylla’ H. Jaeg.), and pittosporum [Pittosporum tobira (Thunb.) Ait.] were least when plants were continuously irrigated or covered with white copolymer film. Foliage of Japanese holly (Ilex crenata Thunb. ‘Compacta’) and dwarf Burford holly (Ilex cornuta Lindl & Paxt. ‘Burfordii Nana’) was not injured with any treatment. Bark splitting of azaleas was most severe under the thinly iced and white copolymer-covered treatments, less severe in the unprotected treatment, and least with continuous irrigation.

Abstract

Rosa hybrida L. ‘Peace’ and ‘Garden Party’ plants covered with mulch in October and November for 2 weeks were less cold-hardy, based on laboratory freezing studies, than unmulched plants. Subsequently, field injury of mulched stems was observed in December and January. Plants covered in mid-December were less cold-hardy than unprotected plants but exhibited less field injury in January than October- and November-mulched plants. Plants covered with Styrofoam cones were not injured as early as perlite-covered plants. There was no significant difference in winter injury when stems were covered with perlite, soil, or leaves. Unprotected plants had less winter stem injury than covered plants during the winter of 1978–79, when the plants were covered with snow. The following winter there was little snow cover, and November- and December-covered plants had significantly less winter stem injury in January than plants with no cover.

The benefit of applying an antitranspirant for protection of cranberry (Vaccinium macrocarpon Ait.) vines exposed to desiccating conditions was evaluated at four different sites, two sites per year, for a period of 1 year each. Overall, plots receiving one fall application of an antitranspirant produced more berries and greater total fruit mass the following year than did nontreated plots. Overall dry leaf mass was not significantly affected. At one site, treated plots had more flowering uprights and more flowers per upright per unit of ground area than the nontreated plots. For cranberry growers who cannot maintain a winter flood, one fall application of pinolene (Vapor Gard) may offer some protection against winter injury. Further research is needed to document long-term yield effects as well as to clarify the role of the antitranspirant in protecting exposed vines and floral buds against adverse winter conditions. Chemical name used: di-1-p-menthene (pinolene).

During the winter of 1991-92. four cultivars of Alstroemeria: `F-180'. `l-5'. `Parigo Pink' and `Parigo Red' were treated with eight different overwintering covers: straw, straw with plastic covering, sawdust, sawdust with plastic covering, hoops with plastic covering, hoops with microfoam covering, microfoam and a control with no cover. All covers had significant effects on the survival of `Parigo Pink' and `Parigo Red'; mulching with straw only gave the best winter protection. There were also significant genotypic differences among the four cultivars: 73% of `Parigo Pink' and `Parigo Red' plants survived after winter, but none of `F-180' or `l-5' survived. In addition, pre-winter evaluation indicated that there were significant genotypic differences among the four cultivars with cold resistance. The cold resistance was highly correlated with winter hardiness. It was concluded that: (1) pre-winter evaluation could be an efficient indicator for winter hardiness selection on Alstroemeria and (2) application of straw provided sufficient winter protection for zone 6 Alstroemeria. Other approaches of mulching need to be further identified in order to protect all Alstroemeria for overwintering in the northeastern United States.

Growers in Northern New Jersey are slowly adopting strawberry plasticulture as an improved production system. One advantage of the system is early fruit production. Early fruit usually brings high prices in the marketplace. With early production, however, there is an increased risk of a late frost damaging flowers and fruit. Removing floating rowcover winter protection earlier than flowering may cause strawberries to bloom later, reducing the risk of frost damage. Supporting the rowcovers above the crop with wire hoops may also provide better winter protection and improve fruit production. In 2 years of field trials, removing floating rowcovers 2 weeks before anticipated bloom reduced early yield and delayed the first harvest by 2 to 3 days. Total marketable yield and average fruit weight were not significantly influenced by early removal. Plants with rowcovers supported with wire hoops did not produce significantly greater total yields or average fruit weights than plants protected by unsupported rowcovers. The wire hoops caused damage to the rowcovers, which may make their use in commercial production impractical.

The capacity of plant materials to resume normal growth after exposure to low temperature is the ultimate criterion of cold hardiness. We therefore determined the low-temperature tolerance of five commercially important herbaceous perennial species. Container-grown blanket flower (Gaillardia ×grandiflora Van Houtte. `Goblin'), false dragonhead [Physoste- gia virginiana (L.) Benth. `Summer Snow'], perennial salvia (Salvia ×superba Stapf. `Stratford Blue'), painted daisy (Tanacetum coccineum Willd. `Robinson's Mix'), and creeping veronica (Veronica repens Loisel.) were subjected to 0, -2, 4, -6, -8, -10, -12, -14, -16, and -18C in a programmable freezer. The percentage of survival of most species was adequate when exposed to -10C. Producers of container-grown perennials are advised to provide winter protection measures that prohibit root medium temperatures from falling below -10C.