Hardiness Zones Explained

April 6, 2023
Sue Adams

What is a planting zone?

A planting zone, or hardiness zone, is a categorization that the USDA uses to divide sections of the country based on weather pattern date. Plant types have their own hardiness zones, often listed on the products sold. Planting zones are a guideline to help gardeners choose plants. The USDA’s website has a map showing zones for the country as well as individual states.

Have planting zones changed?

Yes! Global warming has had an impact, so the USDA updated their information from 1990.

Factors that can change the zone you’re in

Because of micro climates, your zone might not act as it should. Physical structures such as walls, houses or other buildings may impact your zone. Structures made of brick, cement or stucco will hold and radiate heat. A large body of water such as a lake or river tends to moderate the temperatures of inland areas nearby. Topography plays a role as well. Warm air is lighter than cold air, so valleys can have more frost problems than hills. The southern side of a slope warms up faster in the spring. Clever gardeners impact their planting area by mulching, using cold frames, planting on balconies and rooftops.

What about frost?

Ah, it’s a beautiful day, and you can’t wait to start planting! New trees and shrubs, most herbaceous, perennials, pansy, snapdragon, lettuce, spinach, peas, chard, collards, beets, carrots and radishes can tolerate a frost. Semi-tender plants that will tolerate a light frost include broccoli, cauliflower, petunia, lobelia, geranium, dusty miller, osteospermum (cape daisy), nemesia, verbena, potatoes, corn and parsley.

Whoops! I planted and there’s a frost warning!

Cover the plants with a light weight, breathable and insulating fabric – blankets, bed sheets, towels. Or cover with straw, newspapers, leaves or other organic matter. You can also make your own cloche or hot cap by cutting off the bottom of a gallon milk jug and removing the top. You can cover your lettuce with the hot caps, and when the lettuce is ready to harvest, move the hot caps to tomato plants – instant little greenhouses!

Meaning of Hardiness, or Planting, Zones

Hardiness Zones

Here is information from the USDA:

USDA Local Hardiness Zones

Sources: Old Farmers Almanac, Home & Gardens, Fine Gardening and Mark Adams