Growing zones, most commonly known as hardiness zones, are geographically defined areas which specify the types of plants that can survive and thrive within them. Some growing zone maps are determined by winter temperatures. But there are many factors to consider beyond the cold when deciding what to plant.
Factors that Affect Plant Growth
Heat – Increased temperatures can negatively affect seed germination and plant growth while causing other plants to grow faster. Extreme temperatures can create heat stress causing permanent damage to plant development.
Level of Humidity – Too much humidity interferes with a plant’s ability to evaporate water or draw nutrients from the soil while increasing the likelihood of pests and diseases.
Vernalization – This is the particular duration of low temperatures. Some flowering plants need a certain amount of cold ‘hibernation’ in order to jump start the flowering process.
Moisture – Too much water can rot the roots of a plant impeding the plant’s ability to draw oxygen from the soil. Not enough water reduces the plant’s capacity for pulling nutrients from the soil.
Frost – Without accompanying snow cover, frost descends deep in to the soil. Extended periods of unrelenting frost can destroy plan-needed nutrients and organisms to the depth of the frost
Snow Cover – Constant snow cover restricts the depth of the frost layer, allowing plants and other organisms to survive below the surface. It also prevents wind and water erosion.
Length of daylight – Photoperiodism is the reaction and development of a plant to the length of light and darkness it experiences. It is important to understand your plant and crop requirements. Long-Day plants (LD) require over twelve hours of sunlight, Short-Day plants (SD) require less than twelve hours of sunlight and Day-Neutral plants (DN) do not rely on specific daylight amounts, instead developing in stages, by age, or by vernalization.
pH in soil – The measure of acidity affects available nutrients in soil. As nutrient absorption is necessary for plants to thrive, an improper pH balance, either too acidic or too alkaline, can detrimentally affect growth and production.
Microclimates – A smaller climate zone within a climate zone based on atmospheric differences. Detailed description below.
Wind – Slight winds can have a positive plant benefit by strengthening root systems. Persistent strong winds can cause breakage and uprooting of plants. Wind chill, effectively creating a colder overall temperature, can also be detrimental to plants, stunting their ability to produce.
Gardener’s Edge Zone Map:
-60° to -50° F
-51° to -44° F
-45° to -30° F
-31° to -20° F
-21° to -10° F
-11° to 0° F
1° to 10° F
10° to 20° F
21° to 30° F
31° to 40° F
41° to 50° F
51° to 65° F
Based on information from the USDA – this map takes the average annual minimum winter temperatures from 1976-2005 and divides the country into thirteen 10-degree color-coded Fahrenheit zones
Zone Map Shortcomings: It is essential to understand the coldest temperatures within any given zone as many plants simply cannot survive frost or lengthy durations of cold. As our world climate changes, however, it is equally important to identify extreme heat conditions since these will also affect the gardens and crops we can grow. Researching summer heat levels, winter snow cover, microclimates and additional growing factors many of which are listed earlier in our guide is the gardening due diligence that is becoming increasingly important.
Microclimate: Within any zone there can be conditions that cause a temperature rise or drop that is substantially different from the climate immediately surrounding that area. This microclimate, may be warmer or colder or wetter than the rest of the zone where it resides thus allowing or disallowing a plant common to the larger zone, to survive. Microclimate areas may be quite small like a portion of a garden bed or several square miles large. Recognizing and identifying microclimates within your growing area can determine the location of your plantings.
The continental United States is comprised of Zones 3-10. Hawaii and Puerto Rico stretch from Zones 9-13, and Alaska begins at Zone 1 and reaches Zone 7.
List of Zones by Major Cities:
Based on the Gardener’s Edge map, zones for a selection of US cities