1.1 PLANT HARDINESS ZONES

Before selecting plant material, it is important to know our region's basic climatic parameters. The USDA Plant Hardiness Zone Map for the entire United States helps classify the winter hardiness of plant material so that we may select species of plants that will thrive in our unique environment. Oklahoma City falls within Zone 7a, meaning that plants classified 1 to 7a should tolerate our average annual low temperatures. Figure 1 illustrates the average annual minimum temperatures for the Midwest United States. Plants that are classified 7b to 10 are not likely to be cold hardy, and may act as annuals in Oklahoma City's environment.

1.2 AVERAGE TEMPERATURES

Not only is it important to know the cold hardiness of plants, but also how well they fare in the extreme temperature ranges that Oklahoma exhibits. The average temperature in the Oklahoma City area is between 56-58 degrees Fahrenheit. However, the extremes may soar over 1008 and fall well below 08. Although most plants will tolerate short periods of extreme temperatures, prolonged periods of heat or cold may affect the viability of plants on the edges of their zone classification. Also to be noted are the dates of the first and last hard freeze. This can be especially helpful when planning activities such as pruning, planting annuals and perennials and sowing grass seed.

1.3 PRECIPITATION RATE

Along with average temperatures, we must consider the area's precipitation rate to make appropriate plant selections. The normal precipitation rate for the Oklahoma City area is between 32" and 36" annually. However, Oklahoma has been known to experience periods of drought, and often a lack of snow and rain in the winter will result in a winter drought. 1

1.4 SOIL CHARACTERISTICS

Providing anchorage, access to minerals, nutrients, and moisture, soil is the basis of plant health. Factors such as soil texture, structure and depth can affect a plant's ability to access water, nutrients and oxygen. It is important to have a well-balanced soil type that will provide the right mixture of air, water and nutrients for optimal plant health. Soil particles range in size from large rocks and gravel to sand, silt and clay. In combination with one another, these particles form the soil type, which can be classified using a Soil Textural Triangle. (See Figure 5)
The arrangement of the particles in the soil creates pores where air and water can travel. These pockets also

provide spaces for the migration of roots. Compaction of the soil causes this delicate soil structure to collapse, restricting air and water circulation and healthy root development. Native Oklahoma soils are rich in clay, which is a heavy, easily compacted soil type. Techniques, such as amending soils to help create a better balance of the soil structure, will provide plants better access to water and minerals. Soil tests can provide us with critical information on mineral deficiencies, and offer suggestions for amending the soil. Refer to Section 7: Resources and References for locations for soil testing.

Within the soil, chemical and electrical bonds hold soil particles together, sometimes affecting a plant's ability to absorb certain minerals and nutrients. Soils with high alkalinity (a pH greater than 7.0), like ours in central Oklahoma, cause certain nutrients to bind, making them unavailable for roots to absorb. Phosphorous, Manganese, Iron and other micronutrients are particularly affected by the pH of the soil.2 Supplementing these minerals with chemical fertilizers is not the only remedy. Amending the soil with organic matter, along with specifying plants that will tolerate alkaline soils, may help reduce the need to fertilize.

Also influencing the health and longevity of plants is the depth of the soil in which they are planted. More water and nutrients become available when the soil has sufficient depth, and roots will grow deeper to reach them. Deeprooted plants have a better chance of surviving drought, have better access to varying minerals and nutrients, and can resist strong wind forces due to better anchorage in the ground. Layers of rock, clay or other impermeable materials can create a barrier through which roots cannot penetrate. This results in flat, often shallow root formation, and does not allow proper drainage. Poor drainage can be just as detrimental to a plant's health as a lack of water. In this case, preparing the bed or planting area prior to the installation of plant material may require some excavation and backfilling with an appropriate blend of soil and organic material.

In general, soils have a delicate balance of air, water, minerals, nutrients, and living organisms. In the right combination, these components can be the perfect medium for your healthy plants. To maintain the integrity of your soil, protect it from compaction and chemical contamination.