Plant Hardiness Zones


The USDA hardiness zone is a mechanism developed to describe the geographic range of adaptation of plant species based on climatic conditions, specifically minimum average temperatures. Since the potential for a plant to survive in a specific location is frequently limited by its cold tolerance this is a very useful concept for commercial horticulture and landscaping. The USDA cold hardiness system uses 11 zones numbered 1-11. A lower number indicates a colder minimum average temperature.  Each zone is further divided into 2 sub-zones labeled a and b.

USDA Hardiness zone table and map

The National Arboretum has created an interactive hyperlinked map of the USDA hardiness zones. Click on the map above to explore this resource. What is the hardiness zone for State College?

The hardiness zone does not include information on maximum summer temperatures which can also greatly affect plant survival in a region. To address this the American Horticultural Society (AHS) has developed a plant heat zone map. This map is used like the USDA cold hardiness. This map is copyrighted by AHS but you can look at a copy at their website. What is the AHS heat zone for State College? Click on the link below to find out.

AHS Heat Zone Map. 

Climate and precipitation records are available from the National Climatic Data Center. This site is particularly useful to examine long term temperature and precipitation patterns for a location and can be useful in selecting plants for a region you are not used to. Explore the NCDC website. Find the recording station closest to your home and look at the climate record for the last year. NOAA the National Oceanic and Atmospheric Administration maintains a website that contains a great deal of information on climate trends and the impact climate changes are having on the US. Explore the NOAA website.

Natonal Climatic Data Center


Exercise: Complete this exercise in the Week 6 module of the class Canvas website

Long and short term changes in the climate may influence plant survival in a region. The National Arbor Day Foundation has developed a map showing changes in Hardiness zones from 1990 - 2015.  

Click on the Arbor Day Foundation website link and explore the map of changes in USDA hardiness zones.

How has our hardiness zone changed in State College? 

If the trends continue what will our zone be in 2025?

List 3 plants we have studied that are commonly used as annuals in State College that would be perennial if our hardiness zone increased to 7?

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