AHS Plant Heat Zones – A Great Tool for Gardeners

In my last post to Garden Notes, The Gardening Year 2013, I stated I didn't know whether 2013 was a hotter, cooler or average year, heat-wise. I stated that the weather reporting station at Amarillo International Airport recorded 10 days with temperatures at or exceeding 100° and 12, 13 and 9 days under 90° in June, July and August respectively. The National Weather Service tracts a good deal of weather data, but neither of those two categories are tracked in that manner.

The weather service keeps tracks of the average days above 90°, and the averages are 14, 21 and 17 for June, July and August. We can't do an actual comparison of numbers, since days where maximum temperature reached 90° would not be included in either. So I went back to the data.

AHS Heat Zones

Rather than go back and include the number of days where 90°+ was the maximum temperature, I have focused my attention on a stat of interest to gardeners, the

Days 86 Degrees and Over
Heat Zone 7 60-90 Days
Heat Zone 8 90-120 Days
Heat Zone 9 120-150 Days

AHS Plant Heat Zones. We are all aware of the USDA Plant Cold Hardiness Zones, but heat affects a plant's survivability too. The heat zone map was introduced by the American Horticultural Society in the late 90''s-early 2000's, (the AHS Plant Heat Zone Map is copyrighted 1997). The map divides the United States into regions based on the number of "heat days"-- temperatures over 86 degrees – it experiences. Eighty six degrees F is “the point at which plants begin suffering physiological damage from heat”.

“The effects of heat damage are more subtle than those of extreme cold, which will kill a plant instantly. Heat damage can first appear in many different 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. That is the point at which plants begin suffering physiological damage from heat.” (AHS Heat Zones.)

The American Horticultural society goes on to say that there are many other factors that can affect, both positively and negatively, the ability of a plant to survive heat, chiefly the extent a gardener provides for the general needs of any individual plant. So all is not lost when it gets hot. For that reason, this is why we don't have a wholesale die-off of heat sensitive plants during the heat of summer months. However, gardeners do experience heat related die off among their plants from time to time, depending on the severity and duration.

Maximum daily temperature data from the National Climatic Data Center was analyzed from 1974-1995 for most areas of the country in compiling the heat zones. The zones range from Zone 1 (less than one heat day) to Zone 12 (more than 210 heat days). Amarillo and most of the Texas Panhandle falls within Heat Zone 8, 90-120 days in which the maximum temperatures reaches 86° or higher.

I could find no reference in climate data that summarized this critical point of 86° for the years since 1995. Therefore, I studied monthly records from 1996 to 2013 to see if Amarillo was still in AHS Plant Heat Zone 8 region. To simplify matters, I included 1994 and 1995 to work with two decades of data, then compared the last 10 years to the previous 10 years to see if our area has been cooler, warmer or on average.

Heat Days

My initial interest was in learning how many days reached and exceeded 86° and what was the average number of days over these two 10 year periods. I wasn't able to find our specific data for the average number of days used in compiling the heat zone map back in the 1990's, however, one could access this data from the national Climatic Data Center and calculate that average for 1974-1995 by going through the records month by month, year by year, as I did for 1994-2013. One point that became immediately clear as I scanned the months and years of temperature data was the great temperature fluctuation between years, between months, and weeks.

For Heat Zone 8, the number of heat days falls between 90-120 days. The 20 year average from 1994-2013 was 102 heat days. Six years recorded heat day totals

Avg Days 1994-2013
Month Heat Days
March 1
April 3
May 11
June 20
July 27
Aug 24
Sept 13
Oct 3

below the 90 day threshold, (88, 86, 88, 85, 83, and 83 days), classified as Heat Zone 7. Our last cooler year was in 2009, recording 88 heat days. Three years recorded heat day totals above 120 days (134, 133 and 130 days). The year 2012 actually recorded the most heat days, 134, with 2011 at 133 days and 1998 at 130 days – solidly within Heat Zone 9 (120-150 days). If you can remember how uncomfortable the summers of 2011 and 2012 were, you'll have a good idea of what our climate will be like if this warming trend continues (just think Wichita Falls). 2013 recorded 116 heat days; we experienced 14 more heat days than our 20 year average. But it did seem cooler to me, recording 18 and 17 fewer heat days than 2012 and 2011.

The most recent 10 year period from 2004-2013, the average is 106 days, and for 1994-2003, the average number of heat days was 99. So, we averaged 7 more heat days a year this past 10 years than the previous 10 years. However, Amarillo and surrounding area is still solidly within the middle ground of Heat Zone 8.

Important to note, there were no heat days recorded during the months of January, February, November and December for this 20 year period. March averaged one heat day, April and October averaged 3, May-11; September-13; June-20; August-24 and July experiences the most heat days with an average of 27.

Still Lots of Variability in Heat

Just like peppers, there is a lot of variability in heat from one month to the next. Were the heat days just in the upper eighties, the nineties or topping 100 degrees? 86° degrees is hot and begins the damage in heat sensitive plants, but not that bad, really (when one is used to it). In scanning the daily and monthly temperatures, some months seemed on a seesaw up and down. Only 5 months recorded heat days every single day of the month during the 20 year period, 3 of the months in July, 2 months in August.

Once the temperature reaches the century mark, I don't know what 100° heat does to plants, but my cells seem to melt. During 1994-2013, the Amarillo weather

Avg Days 1994-2013
Month Century Days
May .65
June 3
July 3.3
Aug 2.45
Sept .55

station recorded 197 days in which the temperature reached and/or exceeded 100°, for an average of 10 century days a year. Nine of the twenty years experienced 10 or more days, and 11 years experienced less than 10 century days.

The most recorded was 50 days in 2011, followed by 24 days in 2012 and 23 century days in 1998 – the same years the Amarillo area experienced a Heat Zone 9 climate. Some years just destroy the average, 2011 is an example of that – recording just over 25% of the century days for this 20 year period.

So, 2013 was exactly average by reaching the century mark on 10 days, however, taking our 2011's fifty century days and recalculating for 19 years, the average is slightly under 8 century days a year. With that in mind, the Amarillo area experienced more than average century days.

On the cooler side, two of the six years that fell within Heat Zone 7, also experienced no century days. However, two of those cooler years also recorded 10 days each in which it reached the century mark.

Annual Mean Maximum/Minimum Temperature

Which leads to a final indicator of warmer/cooler/or average thought, the annual mean maximum and mean minimum temperatures. The average of the annual mean maximum and minimum temperature for 19 of the past 20 year period is 71.81° max and 44.36° min (This stat was not yet available for 2013, so the most recent 20 years is actually 19 years – 2012-1994 and for 10 years is really 9 years – 2012-2004). The two 10 year periods break down as follows:

  • 72.21° max/44.77° min for 2004-2012
  • 71.46° max/44.0° min for 1994-2003

Both the mean minimum and the maximum temperatures averaged 3/4 of a degree warmer 2004-2012 over the course of those 9 years (day in and day out) than the previous 10 years. Even more telling, the mean max/mean min for 2012 was 77.7°/48.6°; and for 2011 – 74.5°/44.6°. 1998, the year with the third most heat days of 130, the annual mean max/min was 72.9°/45.1°.

Comparing 2012 with our 19 year average mean max/min temperatures, 2012 averaged nearly 6 degrees warmer daily maximum temperature, and four and a quarter degrees higher minimum temperatures throughout the year, considerably higher than the 2011 mean max/min temps; even though 2011 experienced more than twice as many 100° days including the highest temperature recorded at Amarillo at 111° and the highest monthly mean maximum temperature of 99.7° in August 2011 (for this 19 year period)!

The mean maximum/minimum temperatures were from NOAA's National Climatic Data Center. However, The National Weather Service Weather Forecast Office in Amarillo shows the average maximum temperature to be 76° and the average minimum at 46.8 for 2012. The averages given by the two entities for 2011 are the same. I've used NOAA's National Climatic Data Center data as they covered all the years under this study; the National Weather Service Weather Forecast Office in Amarillo only covered some of the years.

The average annual temperature, as reported by the National Weather Service Weather Forecast Office for 2012 was 61.4°, and 59.6° for 2011. 2012 was the second warmest year on record!


But looking at the numbers and comparing them for the cool, average and hot years (corresponding to AHS Plant Heat Zones 7, 8, and 9), it is easy to see that some years are distinctly cooler, average and warmer. Often, we can only determine this towards the end of the year. Some years can start out cool, average or hot, but end up differently, overall. In the past 20 years, there were six Heat Zone 7 years, 11 average or Heat Zone 8 years, and three Heat Zone 9 years, too little to draw definite comparisons, other than they are cooler (Heat Zone 7) or warmer (Heat Zone 9) than our average (Heat Zone 8).


For gardeners, what conclusions can we draw from all this weather data? Looking at the numbers after the season has ended doesn't necessarily help a gardener while it is happening. There is a significant tool gardeners can use in caring for their plants, and few conclusions I can make.

The most important indicator readily available in real time on a day to day basis, is whether the temperature reaches or exceeds 86°, and how often during the growing season. Gardening need not be done on a trial and error basis. We have the information available to us to make better choices.

  • Use the AHS Plant Heat Zones and match the plant to the climate. The area within the Texas Panhandle is nearly completely AHS Plant Heat Zone 8.
  • Knowing which plants are cool season or warm season plants is important for water conservation.
  • Some cool season plants will bolt or die no matter what you do with prolonged duration of heat days, for instance, cool season vegetables. Cool season plants are better suited to September through May. Conserve water and let them go by June.
  • Warm season vegetables will do better May-September, as long as they're provided with well amended soil, that is mulched and watered to the needs of the plants.
  • When our area experiences greater than average heat days, gardeners and homeowners should water more. Likewise, when experiencing less than average heat days, less water is needed.
  • If the majority of your landscape is composed of warm season, C4 plants, you'll be able to get by on less water than cool season C3 plants. You'll have far more success and less plant loss too.
  • Warm season plants can be planted in the fall – it's warm enough. No need to wait until May of the next year. Of course, there are exceptions to all things; know your plants. For instance, seeding out buffalo or bermuda grass just prior to dormancy in October is not a good idea.
  • The AHS Plant Heat Zones support two of the organic gardening principles:
    • Use the best species for the microclimate and
    • Plant in the preferred season
  • Additionally, if one follows the recommendations made by the American Horticultural Society about the Heat Zones, you'll be following the Seven Basic Principles of Gardening.

20 Year Weather Data


Year Days 86°+ Days 100°+ Mean Temp Max/Min Low Temp Yearly Precip Highest Monthly Mean Max Temp
2013 116 10 * 3 15.2 *
2012 134 24 77.7/48.6 8 11.78 96 July
2011 133 50 75.4/44.6 -6 7.01 99.7 August
2010 112 3 71.2/44.6 2 26.54 92.0 June
2009 88 5 71.5/43.3 4 21.15 92.0 June
2008 94 6 72.1/43.3 6 22.42 92.4 June
2007 94 3 70.3/44.9 7 22.5 93.0 August
2006 97 2 72.1/44.3 4 21.88 91.8 June
2005 104 1 70.6/44.5 -3 15 91.7 June
2004 86 1 69.9/44.8 -1 27.31 87.7 July
2003 88 10 72.2/44.1 4 13.42 94.3 July
2002 109 0 70.7/43.9 5 18.25 91.8 July
2001 109 14 72.3/45.7 11 18.69 98.4 July
2000 119 16 72.6/44.6 8 18.39 96.9 August
1999 85 0 71.1/44.3 12 26.57 90.9 August
1998 130 23 72.9/45.1 0 17.15 97.5 July
1997 83 0 68.2/42.9 2 24.95 91.3 July
1996 83 10 71.7/42.9 -3 20.51 95.3 June
1995 90 4 71.0/43.2 6 18.33 91.0 Jul&Aug
1994 95 15 71.9/43.3 1 16.54 95.3 June
Totals 2049 197   70 383.59  
20 Year Avg 102.45 9.95   3.5 19.18  
19 Year Avg     71.81/44.36     93.63
10 Year Avg 2004-2013 105.8 10.5   2.4 19.08  
9 Year Avg 2004-2012     72.21/44.7     92.9
10 Year Avg 1994-2003 99.1 9.2 71.46/44.0 4.6 19.28 94.27
Heat Zone 7 85.5 4.3 70.77/43.72 3 22.32 91.9
Heat Zone 8 103.5 6.7 71.48/44.23 4.5 19.43 93.43
Heat Zone 9 132 32 75.0/46.1 0.67 11.98 97.73
*Not Available            



All climate data is from NOAA Climatic Data Center or National Weather Service Forecast Office, Amarillo as follows:

To download and print out a free weather calendar, go to http://www.srh.noaa.gov/rtimages/ama/2014_Calendar.pdf.

AHS Plant Heat Zone Map

For a printable copy, go to http://www.upsidedownpatiogarden.com/when-to-plant.html, click on the map, copy and paste to your Office program and print.

Rain gages and other official CoCoRaHS weather monitoring equipment is available at http://www.weatheryourway.com/. CoCoRaHS is an acronym for the Community Collaborative Rain, Hail and Snow Network.

I still haven't found a source that reports daily ET rates for our area (evaporation and transpiration). However, ET Gages are available at http://www.weatheryourway.com/cocorahs/etgage.htm for $218.00 plus shipping. Please read the pdf file for operating instructions before ordering to see it this is right for you.

Angie Hanna, January 11, 2014