The Plant Hardiness Zone (PHZ) Map is a standardized geographical tool that provides guidance for gardeners and landscapers. This system, developed by the United States Department of Agriculture (USDA), divides the country into distinct zones based on climate data. The PHZ helps determine which trees, shrubs, and perennial plants are likely to survive the coldest winter temperatures in a specific location. Understanding the hardiness zone is the first step toward selecting plant material capable of thriving year-round.
Identifying the Current Plant Hardiness Zone
The city of Des Moines, Iowa, is currently situated in USDA Plant Hardiness Zone 5b, according to the most recent map update released in 2023. Zone 5b is defined by an average annual extreme minimum temperature that falls between -15°F and -10°F (-26.1°C and -23.3°C). The 2023 map update utilizes thirty years of data (1991 to 2020) and reflects a slight warming trend compared to previous versions. Gardeners should use this 5b classification as the authoritative metric for cold tolerance when purchasing perennial plants.
How the Hardiness Zone System Works
The USDA Plant Hardiness Zone system relies exclusively on the average annual extreme minimum temperature observed over a 30-year period. It is designed to measure a region’s coldest potential winter conditions, which is the most significant factor in determining perennial plant survival. The system does not account for factors such as average yearly temperature, soil type, or the duration of cold snaps.
The entire map is divided into 13 major zones, with each main zone representing a 10-degree Fahrenheit band of temperature. For example, Zone 5 covers a minimum temperature range of -20°F to -10°F. To provide a more granular level of detail, each major zone is further divided into two sub-zones, labeled ‘a’ and ‘b’.
Each of these sub-zones represents a 5-degree Fahrenheit temperature increment. Zone 5a, for instance, has an average annual minimum temperature range of -20°F to -15°F, while Zone 5b covers the slightly milder range of -15°F to -10°F. This division is crucial because a plant rated for 5a may not reliably survive a winter in the colder 4b zone.
Successful Planting Strategies for Des Moines Gardeners
Selecting plants rated for Zone 5 or a lower, colder zone is the most effective planting strategy for Des Moines gardeners to ensure winter survival. Plants rated for Zone 6, while tempting, may not reliably survive the occasional temperature dip to the 5b minimum of -15°F and will require special winter protection. Perennials, shrubs, and trees commonly sold as hardy in the Midwest are generally appropriate, but their specific zone rating must be confirmed before planting.
Timing is another consideration, with the average last spring frost in the Des Moines area typically occurring between April 21 and April 30. This period dictates when tender annuals and vegetables can safely be transplanted outdoors without the threat of a late freeze. Conversely, the average first fall frost usually occurs between October 11 and October 20, marking the general end of the growing season for non-hardy plants.
Gardeners can also leverage microclimates within their own yards to expand their planting options. Urban areas often exhibit a slight warming effect, known as the urban heat island effect, which can make a yard behave more like a warmer sub-zone. Planting sensitive species near a south-facing wall or a large, heat-retaining structure can provide beneficial warmth that may be enough for a borderline Zone 6 plant to survive.
Conversely, areas exposed to harsh northwesterly winter winds or low-lying spots where cold air settles can effectively be colder than the official 5b rating. In these exposed areas, winter protection techniques such as mulching heavily around plant bases, using anti-desiccant sprays, or wrapping vulnerable evergreens in burlap can help prevent winter injury.