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Plant Hardiness

Plant hardiness is an often misunderstood topic deserving special discussion. It is not as simple as it may appear.

The USDA Hardiness Zone map is based on average winter low temperatures, and doesn't take into account rare extremes. While the map is based on a tremendous amount of data, it isn't perfect. Cold temperatures are only one factor that affect plant hardiness. Cold temperatures for one night are not the same as cold temperatures for a period of weeks, even though the same low temperature is reached in both cases. In many cases, a low temperature of zero degrees may cause cellular damage that will start to heal if the temperature rises rapidly. If the temperatures remain low for several days, cell damage may continue, and result in the death of the plant.

Another phenomenon, seen in the cool areas of the West Coast of the US is the difference in winter hardiness due to a lack of summer heat. In many plants native to warmer climates, summer heat causes increased sugar production, which allows the plants to survive more stress in the winter. In areas without summer heat, a particular plant may only be hardy to 20 degrees F, while in an area with hot summers, the same plant may easily be hardy to 0 degrees F.

Heat hardiness is an issue that has been discussed recently, and while it is critical to those of us in warmer zones, the AHS Heat Zone map is a laughable excuse for a solution. Their heat zones are based only on the number of days above 86 degrees F. The heat map does not integrate data for humidity, or the variance between day and night temperatures which is far more important in determining how far south a plant will grow.

For that reason, we have chosen the 1990 (most recent) USDA Hardiness Map to help you in determining your Zone. The map is clickable to show you more detail for your region.

We are also fans of 'pushing the zone' in our garden to try new and different plants. If you are brave and like to try plants out of zone, we would love to hear your results. Email us to share your experiences.

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