Our discussions on tender and hardy plants have led us to another category of plants called half-hardy. As with all plants, initially, these plants are categorized by the United States Department of Agriculture (USDA) as hardy in certain zones. But often, these plants perform outside of the set parameters, which causes them to be identified as half-hardy.
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To illustrate, a plant labeled as tender at a specific temperature may survive colder weather and frost, like more cold-hardy plants. The challenge is once the plant outperforms the guidelines, there isn’t a definite temperature range to pinpoint when the plant is actually tender. This means the USDA plant hardiness guidelines will not accurately determine whether half-hardy plants will survive or die in a climate.
Several factors allow half-hardy plants to survive in colder winter weather. These factors include their root system, environment, and the chemical make-up of the plant. To have an extended life, these plants must have well-established roots, which have acclimated to environmental conditions. Factors like light, soil type, moisture level, humidity, and other natural elements affect whether a plant survives. Thus, where possible, managing environmental conditions and protecting plants with coverings during bad weather will help. Also, the internal design of the plant, like its chemical make-up of sugars, proteins, etc., determines whether the plant will withstand colder conditions and frost.
It is important to note that even though these plants may survive colder temperatures and frost, a severe frost or hard freeze is likely to kill them. Because of this limitation, half-hardy plants are mostly considered annuals even when traditionally categorized as perennials. The take-away is to know the USDA hardiness zone for plants. You can find which plants are considered tender or hardy in your area by reviewing the USDA plant hardiness zone map. Once you know the plant’s tolerance for cold, understanding the factors that allow the plant to survive outside of those guidelines is key.