   Plant Selection - July 22, 1998 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Plant selection is the process of matching site conditions to a plant species. This sounds simple but Yavapai County Cooperative Extension regularly receives calls from clients asking why their tree is dying. In some cases, our reply should be "why is this tree living?" Seriously, our harsh site conditions and arid environment are a certainty and any plant that we plant here should be well adapted. Some of us like to grow native plants in our landscapes. We know that natives are well-adapted but they are sometimes difficult to find in nurseries. Exotic plants are also a viable option and today's market offers an ever expanding list of choices. Whatever your fancies, there are many plants that can be successfully grown in the Verde Valley. This article should clarify the plant selection process. The first decision involves deciding how much growing space is available: both above and below ground. Height restrictions, width restrictions, and available root space should all be considered. Do not plant trees and shrubs closer to a building than one half the mature width. Powerlines and roof overhangs should also be considered. The Sunset Western Garden Book lists the mature size of plants suitable to our area. Roots of trees and shrubs extend 1.5 to four times the width of the canopy. Roots of a 20 foot wide tree could spread from 30 to 80 feet from the trunk. This rule can be forgiving. However, it is best to provide adequate root space and avoid underground features such as septic systems and underground utilities. Once growing space has been determined, the plant should be well adapted to the local environment. Climate zones and cold hardiness maps are available through the Cooperative Extension office. These should be considered a starting point, but localized microclimates may further determine the success or failure of outplantings. Consider how hot it is when you walk across a parking lot in July. The heat is being reradiated from the asphalt. Along streams and rivers it is much cooler. The abundance of water creates an oasis which can support a different array of species. Also, cold air drains from high to low elevation so these low lying areas will be cooler in winter. North and east exposures are cooler than south and west exposures. These guidelines will help you determine whether your planting site is especially hot or cold for your general climate zone. A soil depth of 18 to 24 inches is sufficient for most trees and shrubs. Consider adding soil if you have less than six to eight inches. Urban soils can be compacted due to construction, vehicle or foot traffic. This limits water penetration, air exchange, and root growth. Flood irrigation can also compact fine textured soils. Plants adapted to desert conditions usually do poorly in compacted soils. The relative amounts of sand, silt and clay (soil texture) on the site will determine how much water and air that a soil can provide to plant roots. Sandy soils are well drained, well aerated, and resist compaction. Clay soils do not drain well, are poorly aerated, and easily compacted. Knowing your soil texture is useful when selecting plants and can usually be determined by a Cooperative Extension Master Gardener. Soil drainage can also be reduced by shallow bedrock, caliche, or clay layers. Other factors that can influence plant selection are water quality, insects, diseases, birds and mammals. Salinity of irrigation water can be a serious problem in some areas of the Verde Valley. Large amounts of dissolved salts decrease the plant's ability to absorb water through the roots thereby stressing the plant. Known plant pests and diseases should also be considered when selecting plant species. The University of Arizona Cooperative Extension Publication: Drought Tolerant Trees for the Verde Valley lists plants by size and offers several choices for each size class. For more information about tree selection, planting, care, and other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113. Or E-mail us: mgardner@kachina.net. Be sure to include your address and phone number. |