The United States Department of Agriculture (USDA) Plant Hardiness Zone (PHZ) map classifies regions based on their average annual minimum winter temperature to determine which perennial plants are most likely to survive winter. While the map is widely used, gardening in the Denver metropolitan area requires additional consideration due to its unique high-altitude, semi-arid climate. Understanding Denver’s PHZ designation is the first step, but cold tolerance is only one part of the successful gardening equation.
Denver’s Official Hardiness Zone Designation
The Denver metropolitan area is split between USDA Plant Hardiness Zones 5b and 6a. This dual designation reflects localized variations, or microclimates, across the city and surrounding suburbs. The zone designation is determined by calculating the average lowest temperature recorded during the winter season over a specific period of time. Many parts of the urban core, particularly those benefiting from the “urban heat island” effect, fall into the warmer 6a category. This split is a recent development, as the Denver area was historically categorized as colder, generally falling into Zone 5b or even 5a. The updated designation reflects more accurate data collection and a gradual warming trend in the region’s winter minimum temperatures.
Decoding the Hardiness Zone System
The USDA PHZ system is a structured temperature scale that divides North America into 13 distinct zones. Each zone represents a \(10^\circ\)F range of the average lowest winter temperature. Each zone is further divided into two subzones, labeled ‘a’ and ‘b’, which mark \(5^\circ\)F increments. In the Denver area, Zone 5b is defined by an average annual minimum temperature range of \(-15^\circ\)F to \(-10^\circ\)F. The slightly warmer Zone 6a is characterized by a range of \(-10^\circ\)F to \(-5^\circ\)F. This \(5^\circ\)F difference between 5b and 6a is significant, as it determines the survival of certain marginally hardy perennial plants.
Beyond Cold Tolerance Heat and Altitude Factors
While the PHZ map guides cold tolerance, Denver’s distinctive climate requires gardeners to consider factors beyond minimum winter temperatures. The American Horticultural Society (AHS) Heat Zone map addresses the opposite extreme, measuring the average number of days per year that the temperature exceeds \(86^\circ\)F. Denver typically experiences 15 to 30 days over \(86^\circ\)F, indicating substantial summer heat stress for many plants. Denver’s high elevation (one mile above sea level) contributes to low humidity, intense solar radiation, and rapid temperature fluctuations. The thinner atmosphere allows for higher levels of ultraviolet (UV) light, which can cause leaf burn on sensitive plants. Even plants rated for Zone 6a may struggle if they are not adapted to arid conditions and intense sun exposure.