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        PLANT HARDINESS & ENVIRONMENTAL ZONES IN TX

COLD ZONES:  An important horticultural reference is the USDA Plant Hardiness Zone Map shown here. The various zones indicate what the lowest winter temperature will most likely be be based on historical record keeping. It isn't foolproof but serves as a general guide for determining whether a plant will survive the winter in a given area. Other factors like micro-environments also influence survivability. The zone temperature ranges are: ZONE 10: above 30 degrees; ZONE 9: 20 to 30 degrees; ZONE 8: 10 to 20 degrees; ZONE 7: 0 to 10 degrees; ZONE 6: -10 to 0 degrees; ZONE 5: -20 to -10 degrees; etc. A detailed full color plant hardiness map showing individual counties can be ordered from the U. S. Dept. of Agriculture Agricultural Research Service.   The map below is the 2003 version (based on data collected between 1986 and 2002),  to be officially released during this year.  The effects of global warming can be clearly seen when compared with the previous 1990 version (not pictured on this site).

To see the latest USDA Cold Hardiness Map,  click on the link below.  It is a 5 meg. pdf image so expect slow downloading.  

USDA, Agricultural Research Service 2003 Cold Hardiness Zone Map

Did you know that the state of Texas has the largest number of cold hardiness zones within its boundaries than any other state!   It ranges from Zone 6a in the northern panhandle to 9b in the Valley.  Also Texas rainfall west to east ranges from 5" to 40" average per year.  This creates a wide range of plant growing environments.   

Cold Hardinesss Zones in TX: The map below, based on the latest 2003 USDA Cold Hardiness Zone Map


HEAT ZONES: Also keep in mind that plants may also have reverse limits based on heat. For example, certain plants require a certain level of cold to survive or to thrive and can't tolerate warmer winter climates.  Also,  certain plants have special adaptations to deal with the stresses of heat.  Some plants have adapted to thrive in heat.   The American Horticultural Society (H. Marc Cathey) has been preparing a new Zone map according to the number of days per year that temperatures exceed 86 degrees.  Zones range from Zone 1 where temperatures never reach 86 degrees (e.g. Alaska)  to Zone 12 where temperatures exceed 86 degrees over 210 days per year (e.g. Laredo and southern tip of Texas).   Houston and Austin are in Zone 9 (averaging 120-150 days per year above 86 degrees.   The immediate coastline areas (e.g. Galveston) are in Zone 8 (90-120 days above 86 degrees).  This map is now available on the internet to view and also is available for sale by the American Horticultural Society.  The next job  for the AHS is to identify plants that thrive in each of the heat  area zones (similar to the USDA Plant Hardiness Zones).  The Heat Zone study and project has been ongoing for the past 14 years with work still yet to be done.  Why 86 degrees?  That is the point where the growth processes in most plants begin to languish and decline.  Heat doesn't kill plants instantly like cold, but weakens them to the point of eventual death over time.  This important new study will become a new standard for plant survival classification in the years to come and be a valuable tool to gardeners and horticultural professionals. 

 

 

Average Rainfall: Another critical environnmental factor is average annual rainfall in Texas.  This varies considerably between different sections of the State.  Check it out on this TX average rainfall map.

Ecosystems in Texas:  Texas has a broad range of plant ecosystems based on all the environmental factors of rainfall, cold & heat tolerance, soil conditions (percolation, pH, composition, etc).  Although several sources have attemped to map and define these regions, I particularly like this version produced by the TX Wildlife and Parks Division.  This map will help you determine which native plants will thrive in your particular part of TX as well as define the local ecological systems.  Definitions can be found on the internet.

OTHER ENVIRONMENTAL INFORMATION FOR TEXAS GARDENERS

First and Last Freeze Dates:  We've also included a Freeze Date Zone Map of Texas showing the average first and last freeze dates which should help you determine when it is safe to plant or protect  tender crops and ornamental plants.
 
Climate Data for Austin and Central TX (Month by Month):  This analysis of climate in the Austin, TX area was provided by Norman Wagner, a former meteorologist who has analyzed meteorological records over time.


Temperature Conversion - Fahrenheit and Centigrade

 


SPEAKING ABOUT COLD AND PLANT HARDINESS,  THIS PICTURE WAS TAKEN BY A PALM ENTHUSIAST IN DALLAS, TX WHO GROWS MANY VARIETIES OF PALMS THAT  CAN TAKE IT - SNOW AND TEMPS INTO THE MID 20'S.!