University of Illinois Extension
USDA Hardiness Zone - Site Assessment - Stepping Stones to Perennial Garden Design - University of Illinois Extension

USDA Hardiness Zones

USDA Hardiness Zone Map

All horticultural material is classified according to its hardiness. Hardiness refers to a plant's ability to tolerate temperature. The USDA Plant Hardiness Zone map divides the United States into zones according to the average minimum cold temperature. A hibiscus brought to Illinois from Southern Florida will not survive outdoors all year due to its hardiness. Most of Illinois is classified zone 5. Plant material listed for this zone is able to survive the average minimum temperature between -10° and -20°F. Zone 5 has been further divided into 5a and 5b. Zone 5a average minimum temperature is -15 to -20°F, and zone 5b average minimum temperature is -10 to -15°F.

This is air temperature and not wind chill temperature. Wind chill is based on human perception of temperature and doesn't affect plants. The wind associated with wind chill can be damaging however to woody ornamental plants (those that keep a woody portion above the ground year round). Determining what hardiness zone you live in is important when selecting plant material.