neighborhood garden for DECEMBER 29, 2012






          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 Plant Heat Zone Map.  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 experiencing damage.  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.” (1)

          This past 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.

          Our youngest son lives in Albany, NY. Both Lincoln and Albany 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 temperature.

          However, Lincoln, 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 Albany 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.

          The Heat 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. 

          To better 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 Centers. Jan lives, gardens, and writes in west central Iowa.


          (1) “A New Landscape Concept-Heat Zone Gardening” by Allen Owings)

          (2) “Heat-Zone 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

Copyright 2012