Selecting Plants for Local Weather using Zones

Before selecting gardening plants, consider the plant hardiness zone, heat zone and basic plant requirements. Basic plant requirements are sun, water and nutrients.

Plant Hardiness Zone Maps

Plants have the ability to adapt to a variety of growing conditions, but most gardeners like to select plants that will thrive locally. The USDA plant hardiness zone map was developed by the United States Department of Agriculture to show where specific plants can grow optimally. This was designed using average winter temperatures. Cold hardiness refers to a plant’s ability to tolerate the average annual minimum temperature in that region. An interactive USDA zone map is available, just enter the zipcode.

The lower the hardiness number, the more cold tolerant the plant. Zone 1 includes Fairbanks Alaska; zone 7 covers Oklahoma City, zone 10 is Miami.

The plant zones are divided into subzones to cover microclimates. These are identified as 3a, 7b, etc. Major metropolitan areas are usually a subzone. The high percentage of impervious surfaces, large buildings and vehicles raises the temperature slightly. Other factors are large bodies of water, areas that routinely see increased moisture, lower or higher temperatures, different elevations, more.

This map is applicable to the United States. Information on global gardening zones is available for other countries.

Factors to Consider When Using the Plant Hardiness Zone Map

• Environmental Stressors. In this modern world, many factors can cause stress on plants. Acid rain, air and water pollution, urban areas with night security lights are among the factors that can cause plant stress.
• Artificial Environments. Landscaping techniques and urban environments often result in placing plants in artificial environments, meaning they would not grow in these locations in their natural home. These could include raised beds, freeway landscaping, large building landscaping, and other places where plants are not in the natural soils in the area. Hardiness and heat zone maps are not enough resources to make informed plant decisions in these situations.

Additional Factors to Consider When Selecting Plants

• Matching frost dates and days to plant maturity. Use information on the plant label to determine if a plant is appropriate for the location. Tomatoes may go from seeds to ripe tomatoes in 53 to 90 days. Check with the National Weather Service or Cooperative Extension Service for average last and first frost dates to expect. Select plants that fit within those average dates. In northern climates, many root and cool season crops are planted well ahead of the predicted last frost date. Cool season crops such as lettuce do well in cooler soil temperatures. Sensitive plants such as tomatoes or petunias are susceptible to frost damage in early spring as well as the fall.
• Day length. Day length is a critical factor for plant growth. In early spring, soil temperatures may allow plants to begin growing, but optimum growth is not reached until more hours of daylight are available.
• Heat. Many plants are damaged by prolonged periods of excessive heat. In most areas, this can be compensated somewhat by providing shade in the late afternoon. Paying attention to notations for sun, part-shade and shade will help select these plants for appropriate locations.
• Water requirements. Plants native to an area have adapted to the normal rainfall of that climate. Introducing non-native plants requires and understanding of the plants’ water requirements. Most annual flowers are non-native and thus usually require more water and have higher maintenance requirements. Rain gardens and rain barrels can help take full advantage of rainfall.

Heat Zones and Heat Effect on Plants

Some plants that grow in colder temperatures do not thrive in warmer zones due to heat tolerance. Heat and cold both have strong impacts on plants. Extreme cold will kill plants instantly – think of frost hitting a plant on the first cold night of fall. Heat is equally damaging, but slower and more subtle. Symptoms may include flower buds withering, leaves drooping, leaves turning brown or white due to chlorophyll leaching, roots may stop taking up nutrients. Plants may survive for some time, but will not thrive or produce and may appear stunted.

A heat zone map was developed by the American Horticultural Society to help gardeners select plants that do well in warm climates. There are 12 zones with guidelines for the average number of days each year an area typically experiences temperatures over 86 degrees Fahrenheit (30 Celsius).

The zones range from Zone 1 (less than one heat day) to Zone 12 (more than 210 heat days). For example: central Minnesota averages 14 – 30 days above 86 degrees F and is zone 4, while central Texas averages 120 – 150 days and is zone 9.

Utilize Plant Zone and Plant Growth Information

Gardening is a fun hobby for many. To maximize enjoyment and the investment, research plant growth information so plants that will grow successfully in the area will be selected.