Category Archives: Weather & Climate

Assessing Fruit Tree Damage After a Freeze

The relatively surprisingly bad Southeast Texas freezes of January 6th and 7th, 2017, left many of us wondering how our semitropical fruit trees have done. There are really three questions:
(1) The first is relatively obvious: were they killed and if not how bad was the damage?
Questions (2) and (3) are potentially more important since they help us learn.
In the 1980’s, Stewart Nagle, Ph.D. did a careful assessment of citrus damage after the very destructive 1983 and 1989 freezes where temperatures in most places were near the all time records. He went all over the southern half of the state looking at what survived and why. He developed many generalizations about what happened including what did well in freezes and what did poorly. And most of his findings and of course those of many others across Texas and the world are what we use today for good guesses.

As I remember it, he said that for most citrus, their survival depended on several factors:
(1) genetics–kumquats, yuzu, trifoliate, mandarins hardy; pummelos, citron, limes less so.
(2) rootstock–trifoliate and tf crosses: hardiest; sour orange, rough lemon: not so.
(3) active growth– quiescent & healthy: hardy; but active growth or diseased: tender
(4) hours below particular temperatures: less than 32˚F,less than 28˚F, less than 25˚F,less than 22˚F, less than 18˚F, less than 14˚F, less than 10˚F
(5) size of wood killed: leaves only, new growth, twigs, branches less than 1 inch; branches more than 1 inch; trunk; whole tree
To this I would add another factor: exposure
(6) orchard trees exposed to winds out of the north, to winds from the east or west, or overhead are not as sheltered as many house yards where buildings and other trees can provide some warmth protection.

In today’s world with an electronic network across Texas and beyond, it is possible to use this information to learn what caused the damage to your trees. It is also possible to share your conclusions, provided you report the information above for your site to develop a profile of how different fruit trees behaved in various locations and temperatures. This would allow us to understand just how well mangoes and lychees did too.

If you want to do this systematically, you should assess at least twice–at 2-6 weeks and again in about 6 months. Plants on their own roots like some lemons may take even longer to show life.
I have included below an Excel File you can use to do an assessment. freeze-report-template

You could also fill it in in handwriting so there is also a PDF you can download and print.

I have completed my assessment of orchard trees and will post this here so you can see an example when it is available for upload.

Climate Change, Food and Houston’s Future

In February 2015, I presented an invited lecture on the relation of climate change to food production, and its likely affect on Houston’s future.  This is obviously a dicey topic and not easily presented in in 90 minutes.  It was enough to talk about some important issues and questions.

Climate Change, Food, and Houston’s Future

Chill and Low Temperature Zones for Southeast Texas

The pdf table below summarizes 20 years of weather data from National Weather Service Stations in Southeast Texas and some surrounding counties. It is based on winters rather than calendar years (unlike most reported data), reflects the last two decades rather than earlier ones, and attempts to reflect variation as well as what is typical. The chart provides for a specific station the percentage of the years the lowest winter temperature was above the number in the table; it also provides the percentage of the time the winter chill was at or below the range of numbers listed in the table.

Zone Summaries Revised Zone Summary

Randall Chill and Cold Zones for Southeast Texas

This 3 page file has my 2012 Winter Hardiness Zones for annual low temperature 1992-2012 in all National Weather Service Weather Stations with 10 years of data. The winter low temperature zones differ from USDA because the lows I am using are for the winter, not the calendar year. The way NWS reports things, a very cold last week in December and a very cold next week in January are treated as two different annual lows. Since we tend to swing from El Nino to La Nina events, this leads to very misleading data. As well, my tables use the last 20 years, rather than the 1975-2005 that the 2012 USDA model uses.  Since 1975-1990 had some of the coldest days and months ever in the Houston area, this is not very helpful.  As well, I take median average low temperatures at smaller intervals than 5-10 degrees. There is a big difference in fruit tree tolerances for mangoes between 28F and 25F.  Lastly, I list chill zones for the area. These are not quite the same as the low temperatures since they are based on cool temperature duration more than a specific cold night.  The last page is the USDA current zone map which is quite at odds with my data.

Randall Southeast Texas Low Temperature Zones

Winter Chill for Fruit Trees in Southeast Texas 1992-2012

The attached pdf table below summarizes the accumulated chill units at every National Weather Service Weather Station in Southeast Texas between fall 1992 and fall 2012 that reported 10 or more years’ data. What the table makes clear is that all areas experience a very wide range of chilling unit accumulations from one year to the next with the last five years being a textbook case of this. The winter of 2012 was in most places the lowest chill in the last 20 years, while 2009 was one of the coldest.

In the low chill areas, fruit growers should  plant trees requiring very low chill because late frosts are very rare and the plants will fail to thrive if they get too little chill. In areas of medium high and higher chill, the dangers of late freezes are strong, so in some areas it may be best to pick trees with chill toward the center of what is possible.

The attached table lists chill unit accumulation (what once was called chill hours) by percentages of time the winter in the last 20 years had the chill listed in column headers.

Southeast Texas Chill Unit Ranges 1992-2012

Annual Winter Low Temperatures in Southeast Texas 1992-2012

The attached table in PDF  represents a concise summary of annual low winter temperatures (October to April) for all official national weather stations in Southeast Texas for which there is 10 or more years of data since 1992. Most of them are for the full 20 years of data. The table lists weather stations first by Houston area locations (Harris County), followed by other sites in coastal and near-coastal counties bordering these (that is 0 and 1 county from the Gulf).

After this are other “interior” counties in Southeast Texas.  For these, there are 2 or 3 counties that must be passed through to get to the coast. “County numbers to the coast” is a rough proxy for distance from the Gulf, and in rural areas is a very rough approximation of winter temperatures. The warmest areas generally are within a few miles of the Coast, and with some significant warmer exceptions for dense urban areas and Galveston Bay, temps get colder as you go inland. Lastly there is a a table of selected sites north or west of Southeast Texas.  These are included to help interpret information about fruit tree performance in those areas, in order to use such information in Southeast Texas. For example, avocados that do well in Devine, Texas will likely do well in College Station south, because temperatures south of College Station are warmer than Devine, while north of College Station are typically colder, so would need increased protections.

The numbers in the cells represent temperatures F. If for example a cell has a temperature of say 28 in a column labeled 30% (as it does for Intercontinental Bush Airport), this means that 30% of the annual winter lowest temperatures during the last 20 years were at or above 28˚F.  If you are considering planting a fruit tree that needs temperatures above 25˚(like a mango) , you can use this information, plus a comparison between your site and the nearest National Weather Station’s temperatures, to determine how much freeze protection a mango (or any other tropical or semi-tropical) would require.

Southeast Texas Winter Low Temps 1992-2012

Weird Weather

201004 Weird Weather

Climate, Weather, and Landscaping in the Years Ahead

Climate, Weather and Landscaping in the Years Ahead

I gave a short talk on the drought and the increasingly chaotic climate in the Houston and Southeast Texas area.  The talk was in mid-November 2011 at the Organic Horticulture Business Alliance (OHBA) Drought Symposium.

Weblinks for Monitoring Weather and Climate in the Houston Area

As best I can tell, all or most predictions come from the same 10 or so models. Radio/TV/newspapers typically just use government predictions. Commercial media often though exaggerate threats and possibilities possibly for commercial reasons.

General Weather information: Go to ; then click on the map of the Texas Gulf Coast, then click on area map of Houston or wherever. You can enter a zip code there and bookmark it. If it matters a lot whether the prediction is correct, read the detailed discussions at the bottom right of that website. There are hourly forecasts for your zip code for the next 3 days and radar maps that show you how rain is moving through the area.

Longer-term info: A summary of what is expected for the months ahead is updated monthly. Typically, they run 10 computer models and pick the middle one.

What weather happened:

For summary information about historical weather averages and extremes:

For info relevant to landscaping and gardening, go to

For drought:

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