Latest Chill & Cold Data
With summer all about us, it seems odd to be reviewing last winter’s cold, but an understanding of winter is important in what we should consider planting. Many fruits, vegetables, and ornamentals either depend on winter chill for success or are damaged by too much cold, so it is important to use plants adapted to local cold. This depends on where you are in Southeast Texas. This is an update of a preliminary report on cold and chill I made two years ago.
How cold it gets on the coldest night of the year affects tropical, semi-tropical, and tender, temperate plants. It is the basis for the most common climate zone ratings. Most of Southeast Texas, for example is classified as zone 9 because typical winter lows are in the twenties. Austin is considered zone 8 because it is in the teens. Zones are further divided in five degree amounts. Houston is described as 9A because low temperatures are typically 25°-29°.
However there are huge differences in temperatures from one neighborhood to the next that may not correspond to the statewide averages for national maps. In any given year, a neighborhood might be zone 10 or 8 depending on the low temperature, and over a ten-year period may not be typically zone 9. So plants that do well in one area may not in another.
This last winter Bergstrom Airport in Austin had a low of 21° (Zone 9b), while Jamaica Beach on Galveston Island registered a low of 39° (Zone 10a) and St. Anne’s Catholic School in River Oaks posted 34°! Hobby Airport recorded a low of 33° while Tomball Airport had 27° and Conroe’s was 25°. Our winter lows were about 20 degrees above all-time records, so the plants didn’t really get a good freeze test.
Plants from the temperate climates are affected by how cold it gets over the whole winter. Temperate climate plants lose their leaves and go dormant in order to resist freezing temperatures. They need to end their dormancy and leaf out when the temperatures are right for new growth, but after killing freezes, so many of them have developed genetic mechanisms for determining how much cool weather has occurred since winter began. When enough chill has occurred, out come the buds. Temperate plants that do well in Milwaukee need much more chill than plants in Galveston.
In northern climates, chill is measured in hours, but in the south is calculated for the warmest areas from average temperatures in January and on temperatures in December and January for areas to our north.
Temperatures are available for most of the airports in Texas for the years 1970 to 2000. Using web data, I have also been able to determine the average temperatures this last winter at many places around our area.
If you compare the 2004 numbers with the 30-year averages, it is obvious that in most places, 2004 had much less chill than has been typical in the past. In fact though, most recent years have been much warmer than the 30-year averages. This may be caused by global warming, or the continuing urbanization of the area, or even multi-year weather cycles. For now the best advice is that you should not try to grow fruits whose chill is higher than the 30-year averages for your area, nor should you attempt to grow fruits with chill more than 100 units below 2004’s numbers.