Sunday, January 29, 2017

The problem with USDA Plant Hardiness Zones

Any system that puts Phoenix, Arizona and Orlando, Florida together has a serious flaw.  But they are both USDA Hardiness Zone 10.
"USDA Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which plants are most likely to thrive at a location. The map is based on the average annual minimum winter temperature, divided into 10-degree F zones."
What it really means:  The USDA Plant Hardiness Zone Map helps determine which plants are unlikely to die of cold weather at a location, not thrive.   There are many other reasons your location will kill unsuitable plants.  The picture below shows part of the problem.
Both city parks are in USDA Hardiness Zone 10

Other Factors

Many other environmental factors, in addition to hardiness zones, contribute to the success or failure of plants. Wind, soil type, soil moisture, humidity, pollution, snow, and winter sunshine can greatly affect the survival of plants. The way plants are placed in the landscape, how they are planted, and their size and health might also influence their survival.

Annual temperature range: Plants grow best within a range of optimum temperatures, both cold and hot. That range may be wide for some varieties and species but narrow for others.  Phoenix was a challenging place to landscape because it was hard to find plants that could survive the 110F summer days and the annual winter frosts.

Duration of exposure to cold: Many plants that can survive a short period of exposure to cold may not tolerate longer periods of cold weather.

Humidity: High relative humidity limits cold damage by reducing moisture loss from leaves, branches, and buds. Cold injury can be more severe if the humidity is low, especially for evergreens.  But high humidity encourages various fungal infections and insect pests.
Roses in Phoenix seldom got any of the "mildews" that plague the Florida coast. Our roses may have been burnt around the edges in June at 110 and 4% humidity but they were blooming gloriously all winter.

Light: To thrive, plants need to be planted where they will receive the proper amount of light. For example, plants that require partial shade that are at the limits of hardiness in your area might be injured by too much sun during the winter because it might cause rapid changes in the plant’s temperature.
Beware of the "full sun" suggestion for plants.  Full sun in Tampa or San Diego and full sun in Phoenix were two different things.  Many plants that need "full sun" elsewhere huddled under the shade of mesquite trees in Phoenix.

Soil moisture: Plants have different requirements for soil moisture, and this might vary seasonally. Plants that might otherwise be hardy in your zone might be injured if soil moisture is too low in late autumn and they enter dormancy while suffering moisture stress.

On the other hand, some plants can survive winter with dry roots but die if they go into winter with wet roots. Yuccas and many cacti come to mind.

The Solution: Local Research

Don't ignore the USDA Hardiness Zones, but don't use them as the only criteria for plant selection. Check books and websites dedicated to gardening in your area, local Master Gardener programs and local water company planting lists.
Local nurseries might not be the best place to ask questions, unless they are specialists in local plants.
 I've seen way too many plants for sale that haven't a chance of surviving in my area, because people ask for them.

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