Frost Probabilities in Texas
Traditionally, peaches have been grown commercially in the high-chilling region (HCR) of Texas (Figure 1). In these areas, freezing temperatures during flowering frequently cause damage to the crop.
At the present time, it is thought severe freezes during bloom occur approximately 20 percent of the years. This study examined temperature records over 32 years at four sites in the HCR, two in the medium-chilling region (MCR), and one in the low-chilling region (LCR) and compared them to mean bloom periods to calculate the relative frequency of damaging cold temperatures during peach bloom, the number of frosts per year and the length of the frost periods that can be expected for each region.
Mean full bloom periods ranged from March 12 to March 17 in the HCR, from March 1 to March 6 in the MCR, and was Feb. 2 in the LCR site (Table 1). Peach flowers are most sensitive from about one week before full bloom until the small fruit stage (Figure 2). During this period, temperatures at or below 28° F for 30 minutes or more will cause commercially important damage to the developing peach crop. Temperatures of 25° F or below will generally cause severe damage to the peach crop.
Freezes at or before the swollen bud stage are generally not an important problem in the major peach-production areas of Texas. Although occasional damage does occur, this can usually be solved by changing to a more cold-hardy cultivar.
Freeze probabilities presented are based on the period from seven days before until 28 days after full bloom and temperatures are arranged into three damage categories. Temperatures of 29° F or greater were considered to cause none to very light damage, those between 28 and 26° F moderate damage, and those equal to or below 25° F severe and widespread damage.
As one moves from the HCR to the LCR the threat of severe crop loss due to bloom freezes decreases dramatically (Figure 3). Conversely, the percentage of years when freezes during bloom are not a factor in production is 47 percent in the HCR, 66 percent in the LCR. In fact, in the LCR, there were no temperatures that were in the severe category.
Table 1. Mean full bloom datesz for location studies in Texas.
|
Location |
Full bloom |
Chill range |
|
Tyler |
Mar. 12 |
750-850 |
|
Stephenville |
Mar. 13 |
750-900 |
|
Bowie |
Mar. 17 |
700-900 |
|
Fredericksburg |
Mar. 15 |
700-850 |
|
College Station |
Mar. 6 |
450-750 |
|
Yoakum |
Mar. 1 |
450-600 |
|
Weslaco |
Feb. 2 |
100-250 |
z
Bloom dates vary by up to 14 days from year to year.
Table 2. Days with 32° F or lower during bloomz
.|
High chill region |
Medium chill Region |
Low chill region |
|
|
Freezes per year
|
3.7 |
2.6 |
1.8 |
|
Avg. Freeze Length
|
1.8 days |
1.6 days |
1.8 days |
|
Years with Freezes
|
81% |
63% |
38% |
z
Bloom includes period of sensitivity from 7 days before until 28 days after full bloom.
Figure 2. Frost sensitive periods of peach flowers.
|
28 |
21 |
14 |
7 |
Full |
7 |
14 |
21 |
28 |
|
|
(days before full bloom) |
Bloom |
(days after full bloom) |
|||||||
|
Not |
Moderately |
|||
|
Frost |
Sensitive |
Sensitive to Temperatures |
||
|
Sensitive |
Below 28° F |
|||
|
No |
Swollen |
Flowers Begin To Open |
All Flowers Open |
Post Bloom Period Still Sensitive to Freezes |
|
Open |
Buds |
|||
|
Flower |
||||
|
Buds |
||||
Freezes in the MODERATE category comprise 33 percent, 18 percent and 12 percent of the freezes in the HCR, MCR and LCR, respectively. These freezes normally cause appreciative damage, but not total loss. Good site selection (elevated with good air drainage) and proper orchard floor management (firm moist soil or shredded cover crop with moist soil) can help minimize but often do not eliminate the damage.
Temperatures in the SEVERE category require additional frost-protection procedures such as orchard heating, wind machines, or sprinkler irrigation (Table 3) to avoid total crop loss. This occurs 20 percent of the years in the HCR and 15 percent of the years in the MCR. Frost-protection procedures are designed to alleviate damage by maintaining temperatures above damaging levels. Weather conditions, however, can become too severe for frost protection to work effectively. Since it is hard to predict how low the temperatures will go any given night, frost-protection procedures normally commence at 32° F. With this in mind, the probabilities and mean values for nights that had temperatures of 32° F or lower were calculated (Table 2, Figure 4). These figures are based on only those years that had temperatures of 32° F or lower to give an accurate picture of a year in which a freeze occurs. This would include 81%, 63%, and 38% of the years in the HCR, MCR and the LCR respectively.
HCR, as expected, had more freezes per year but the average length in consecutive nights is about two for all areas. Consequently, growers in the higher chill areas will need to prepare for an average of four frost-protection nights per freeze year, although to be prepared for an extreme year, preparations for additional nights should be made. This data indicates that the further north and the further the peach-production operation is from the Gulf of Mexico, the greater the chance of having damaging freezes during peach bloom period in Texas. In addition, as you move south, the chance of an advective freeze (windy cold front), which is more difficult to protect against, is less. In more southerly areas of Texas, what usually occurs is that the cold front moves in but does not lower the temperature below freezing. It is only the night after when the temperatures reach critical levels due to radiation-type freezes.
During radiation freezes, in which the night is calm and heat is lost to the sky; trees can be protected with heaters, wind machines or overhead irrigation. But during an advective freeze situation when the wind is strong, very little can be done to protect the trees from damage (Table 3).
Table 3. Types of Freezes. Weather Characteristics and Relationship to Protection Strategies.
|
Freeze Type |
Weather Characteristics |
Relative Effectiveness of Protection System |
||
|
Heating |
Wind Machine |
Sprinkler Irrigation |
||
|
Radiation |
Clear skies Very light or calm winds Temperature inversions usually 4-8° F. Sometimes 8-15° F |
Most Effective. |
Most effective. If weak inversion, supplemental heating recommended. |
Effective if uniform application maintained until ice meltsz. Limb breakage possibley. |
|
Advection |
Skies are clear or cloudy. Moderately strong to strong winds. Temperature inversions 0-4° F |
Least effective. |
Affords no protection since little temperature inversion. |
Extreme caution advisedx. Can cause increased damage due to evaporative coolingz. Decreased effectiveness with increasing wind. Limb breakage possibley. |
|
Advection-Radiation |
Clear or nearly clear sky. Light wind. Temperature inversion 3-5° F |
Moderately effective. Dependent on wind speed and temperature inversion. |
Affords little protection. Supplemental heating recommended. |
Caution advisedx. Decreased effectiveness with increasing wind. Limb breakage possibley. |
z
Sprinklers need to be run continuously until the ice has melted. If it is turned off too soon, flower buds will freeze.y
Prolonged water application will lead to excessive ice build-up and limb breakage.x If continued uniform application of water cannot be maintained evaporative cooling may result in further lowering of temperature and increased damage.
The authors, David H. Byrne, Mickey Flynn and M. Scott Jackson, are Professor, Department of Horticulture, Texas A&M University; Meteorologist-in-Charge and Meteorological Technician, National Weather Service, Southwest Agricultural Weather Service Center, College Station respectively. Originally published in the Texas Horticulturist, January, 1991.