Hardiness zone
/ˈhɑɹdinəs zoʊn/
Definition
Hardiness zone refers to a geographic rating system used in gardening to describe the average annual extreme minimum winter temperature of an area. It helps indicate whether a perennial plant can usually survive outdoors there year-round. In the US, zones are numbered by temperature range and may be split into a and b sections. It does not describe summer heat, rainfall, soil, or overall climate suitability.
Practical Relevance
- Cold survival planning: A hardiness zone helps predict whether a perennial plant can survive winter outdoors in a given area. It is most useful for trees, shrubs, bulbs, and other plants expected to live for more than one growing season.
- Limits of the rating: This tool focuses on winter cold, not summer heat, humidity, rainfall, soil drainage, or wind exposure. A plant may match the local hardiness zone and still struggle if those other conditions are unsuitable.
- Microclimate effects: Local conditions can shift real-world performance within the same region. South-facing walls, urban heat, slopes, and sheltered courtyards often feel warmer, while low spots and exposed sites can be colder.
- Choosing safer margins: Gardeners often get better results by selecting plants rated slightly colder than the local hardiness zone. That buffer can reduce damage during unusually severe winters or sudden temperature swings.
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