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.!
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