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Selecting and caring for plants in Utah’s high mountain valleys is a challenge. Much of the stock sold along the Wasatch front will not survives the colder temperatures and heavy snow loads. Others do not do as well because the summers are cool and the growing seasons are much shorter. With this in mind, it is important to understand some of the >needs of gardeners in these valleys. > >Plant hardiness is an often misunderstood topic. In most cases it refers >to cold hardiness but it can be used in different contexts. Hardiness is >genetic. Some plants are hardier than others, even why some cultivars >are hardier than others of the same plant. These plants have been bred >or selected as being hardier clones. Sometimes this happens naturally >as, plants acclimate or change over time to their climate. This is why a >plant species growing in a southern location may not be as hardy as the >same species growing in a northern location. > >Different parts of the plants are also different in their temperature >response. Roots, stems, leaf buds, and flower buds are hardy to >different temperatures. Herbaceous perennials die to the ground in >winter but their roots survive and produce new shoots the following >spring. It is also why some woody plants have leaves but no flowers. The >flower >Buds are less hardy than the leaf buds and are killed by cold. If >flowers bloom at a certain height, but not above, it is because the >snow cover protected the flower buds during the extreme cold. > >Hardiness is a function of location in a different sense as well. A >particular, plant such as >verbena may overwinter in lower warmer valleys but die during the winter >in colder locations.. Keep this in mind when getting information from >other areas that call a plant annual or perennial. > >Hardiness zones are geographic zones shown on maps that share the same >range of average annual minimum winter temperatures. Few references list >hardiness zones for heat--in other words, how high a temperature can a >particular plant endure. >Maples, lilacs, and many of our herbaceous perennials cannot take the >heat of hot climates, or need more cold than they get there to bloom >properly. > >To begin, check to see what hardiness map you are using. There are two >in gardening >publications: one from the USDA based on data from about 1930 through >1960 and seen prior to 1990; and a revised USDA map seen from 1990, >based on more recent data reflecting a period of cold extremes since the >mid-1970s. Only the location of zones on the map has changed on the new >USDA version. > >The zones are averages, which means some years may be colder. Even >though a plant may be listed as hardy in a particular zone, an unusually >cold year may come along and kill it. If a plant is listed as hardy in a >zone or two colder, it is likely hardy. On the other hand, a plant >listed for one or two zones warmer may also grow in a site under certain >conditions. > >These conditions, which determine whether a plant will survive in a >particular site, together are known as the microclimate. They include >soil type, exposure to sun and wind, and other factors, such as slope >and proximity to buildings. After the zone in which a site is located, >or macroclimate, is determined, these microclimate factors should be >considered. > >Mulches moderate soil temperatures, keeping them from getting as cold >and possibly injuring roots. They are especially useful on exposed sites >where protective >snow cover may blow off. Sites exposed to winter winds, usually from the >north and west, can cause evergreens to dry out, resulting in winter >injury, such as leaf burn > >Keep the hardiness zones and their limitations in mind when choosing a >plant. Then keep the microclimate factors in mind when placing it for >planting. Although this will not guarantee hardiness it reduce loss of >plants from winter injury. Larry A. Sagers >Regional Horticulturist >Utah State University Extension >Thanksgiving Point Office >Plant of the week