neighborhood garden for DECEMBER 29,
WHAT IS HEAT
BY GEORGE EDGAR
Most of us are
very familiar with the USDA Hardiness Zones.
Lincoln, Hastings, Grand Island, Kearney, Hebron, and Southeast and South Central
Nebraska are in Hardiness Zone 5 according to the old map. Columbus, most cities in
Northeast Nebraska north of Highway #30, and many
communities north of Highway 92 in Central Nebraska,
are in Hardiness Zone 4. There has been a proposal to change this and
move many areas further North. Zone 5 means that the average minimum
temperatures in the winter are -20 degrees F. to -10 degrees F.
Since the Hardiness Zone Map was published in 1960, American
gardeners have relied on the USDA hardiness map as a standard guide to
plant cold tolerance.
In 1997, The
American Horticultural Society (AHS) released
The revolutionary idea was coordinated by Dr. H. Marc Cathey,
president emeritus of the society.
According to a Horticulture Associate Specialist with the
Louisiana Cooperative Extension Service in an article about Heat Zone
Gardening, “The AHS Plant Heat-Zone Map contains 12 different zones in
the United States
and classifies areas of the country based on the average number of days
per year that the temperature is above 86 degrees F.
Why 86 degrees F.?
This is the temperature where cellular proteins in plants start
Temperature averages for each zone were calculated using climate data
from 1974 to 1995. This is important since some of the hottest years on
record have been in the 1990s.
Zones primarily change due to temperature changes that are
induced by latititude and topography (elevation).
Zone 1 includes areas in the country that do not get above 86
degrees F. in a typical year. This includes only the most remote upper
elevations of the Rocky and Appalachian Mountains.
Zone 12 includes areas where there are 210 days or more annually
of temperatures above 86 degrees F.
This includes portions of the desert southwest, the southern most
tip of Texas and a small section
in south Florida.”
summer and fall the prolonged heat and drought has gardeners very
interested in plants that grow and bloom under these conditions. Many
gardeners are rethinking what they will plant this summer and what
microclimates exist in their garden. They are looking for guidance.
son lives in Albany, NY.
are in Hardiness Zone #5. So both of us have to be careful about cold
temperatures and need to dig up Cannas and Dahlias, and mulch tea Roses,
as the winter temperatures are similar.
With the winter cold both of us can grow Tulips, Daffodils,
Rhubarb and other plants and crops that require so many days of cold
Southeast Nebraska, and South Central Nebraska are in Heat
Zone #7 (60 to 90 days of temperatures above 86 degrees F.) but Albany is only in Heat Zone #4 (14 to 30
days). Therefore, some plants in
grow easier and the blooms last longer because the heat is not as
intense. Southeast, South Central, and Central
Nebraska have both the extreme cold and the extreme hot
temperatures to consider. As the late Harlan
Hamernick, owner of Bluebird Nursery said, “If
the plant will grow in Nebraska, it will grow anywhere”.
The effects of
heat damage are more subtle than those of extreme cold, which will kill
a plant instantly. According to a release by AHS, “Heat damage can first
appear in many parts of the plant: Flower buds may wither, leaves may
droop or become more attractive to insects, chlorophyll may disappear so
that leaves appear white or brown, or roots may cease growing.
Plant death from heat is slow and lingering.
The plant may survive in a stunted or chlorotic state for several
years. When desiccation
reaches a high enough level, the enzymes that control growth are
deactivated and the plant dies.” (1)
The process of
cataloging plants based on a Heat Zone designation is just beginning.
Approximately 500 were initially categorized by the AHS and some
additional ones have recently been completed.
It will take a number of years before all of the plants that have
a Hardiness Zone rating will have a corresponding Heat Zone equivalent.
Hardiness Zone map is a start to educate people about the effect of
prolonged heat on a plant.
The one problem is that the Heat Zone map does not integrate data for
humidity, or the variance between day and night temperatures. Just as
with the Cold Hardiness map, temperatures are only one factor that
affects plant hardiness. In
areas with lots of snow cover, plants may survive deadly winter
temperatures because snow, that is at least 4 inches deep, has an
insulating effect on plants and animals.
understand gardening in the heat, I highly recommend “Flower Gardening
in the Hot Midwest”. This book is written by a Lincoln gardener and
author Linda Hillegass. Her book was published by University of Illinois Press
in 2000, is very good, and of course available at most book stores.
Another book for Southeast, South Central, and Central Nebraska gardeners is “Midwest Gardener’s
Handbook” by Jan Riggenbach. It is published by Earl May Nursery and Garden
Jan lives, gardens, and writes in west
(1) “A New
Landscape Concept-Heat Zone Gardening” by Allen
Gardening”, Time Life Books, 1998 (This book contains pictures and
descriptions of the first 500 plants categorized by Heat Zone.
It talks about the other considerations that affect plants such
as “microclimates”, “sun and shade”, and “the fine art of watering”, and
has a colored “Heat Zone Map”. It is out of print but may be ordered
from used books stores. Also visit the American Horticultural Society’s
website at www.ahs.org.