Understanding Zones

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The USDA plant hardiness zones from 1990. The USDA is in the process of revising the map.

Gardeners can be
conservative or adventurous, traditional or daring. The conservative or
traditional gardeners choose plants they know will grow well in their gardens.
They depend on these plants for a successful garden year after year. The
adventurous or daring gardeners want to push the limits and grow plants that
are not commonly found in their areas. For these gardeners, success (and
failure) is part of the fun of gardening. Both types of gardeners can benefit
from understanding zone maps and hardiness ratings when choosing perennials
that fulfill their garden aspirations and annuals that add seasonal color.

Hardiness zones
or ratings are listed in gardening magazines, reference books, plant catalogs
and on seed packets. A hardiness rating is based on a map developed by the U.S.
Department of Agriculture. First published in 1960, the map divides North America into 11 hardiness zones. Each zone is
determined by a 10-degree Fahrenheit difference in the average minimum
temperature. Zone 1 is the coldest and Zone 11 has the warmest winter

A plant listed
as hardy in Zone 4 indicates it should survive winter temperatures as low as 30
degrees below zero F, which is the average minimum winter temperature according
to the USDA map. A Zone 9 plant is hardy only to 20 degrees F. Some references
provide a range of zones in which the plant will grow. A plant listed as hardy
in Zones 4-9 means it will grow in all of those zones. However, there are many
factors that affect a plant’s ability to grow in a particular climate including
exposure, altitude, moisture, soil type and even snow cover. These conditions
create variations between and within zones.

While it’s
unlikely that a plant listed as hardy in Zone 7 would survive in Zone 4, you
may be able to grow plants recommended for warmer zones. Your garden may have a
microclimate that’s not typical of the actual zone where you live. Variations
in temperature, moisture, soil and light can be used to place plants where they
have the best chance of growing. Trees and buildings provide shade and
protection from strong winds, while south-facing fences and walls absorb heat
to create a warmer microclimate.

You may give
plants special attention to help them survive out of their recommended zone.
For example, gardeners living in Zone 5 might be able to grow a Zone 6 or even
a Zone 7 plant if their gardens are in protected locations or they provide
extra care such as growing plants in a warm south-facing garden and the plants
are covered with mulch or snow during the winter. Another option is to move
plants into an unheated garage or another protected location that doesn’t get
as cold as the outdoor temperature. Some gardeners may even move tender plants
indoors during the winter months.

While cold
temperatures can damage plants, extreme periods of heat can injure plants while
they are actively growing during the summer time. In 1997, the American
Horticultural Society developed a Heat Zone map that divided the United States
into 12 heat zones. Each zone indicates the average number of days that are
greater than 86 degrees F, the temperature at which plants start to suffer
damage from high temperatures. Zone 1 has the lowest number of heat days and Zone
12 has the highest number. While not used as often as the hardiness zone, it is
still a valuable tool when choosing plants for your garden. The most
comprehensive reference with heat zone designations is the Great Plant Guide
published by the American Horticultural Society. This book lists both hardiness
and heat zone information for about 3,000 plants.

In addition to
reference books, your local garden center or county extension office may help
you identify your zone information. To find your hardiness zone by using your
zip code, check the National Gardening Association’s website under
the Reference section. To find your heat zone check at the AmericanHorticultural Society’s website under the
Gardening Q&A section, Heat Zone Finder.

No matter what
type of gardener you are, your choice in plants may seem almost endless.
Knowing about hardiness and heat zones can help you make the right selections
for your garden.

For more
gardening information, visit the National Garden Bureau’s website.