Mercy Olmstead, Ph.D., Stone Fruit Extension Specialist

One of the biggest concerns for any fruit grower is the threat of a frost or a freeze killing flower blossoms or fruit.  On the evening of February 11^th and morning of the 12^th, 2012, low temperatures caused by two different frost/freeze events heavily damaged peaches in North Central Florida, including two research sites in Citra and Hastings, FL.

There are two main types of cold events that can cause damage to flower buds or newly set fruit, radiation frosts and advection freezes.   Radiation frosts occur on clear, calm nights with low humidity.  Heat from plants and the soil gained during the day escapes into the upper atmosphere, allowing cold air to settle near the ground.   In these cases, site selection is extremely important in areas where a slope exists, as cold-tender or low-chill varieties should not be planted where cold air typically settles (Figure 1). 

Figure 1. Proper orchard site selection can help to avoid cold air pockets at the bottom of slopes and to ensure cold air drainage. 

Several methods can be used to “warm” the air near the ground in radiative freezes, including warming air near the surface (using orchard heaters), using wind machines to mix the air and bring warm air near the surface, and using ice formation to encase the buds or fruit (Evans, 2008). Ice formation on plant tissues keeps tissues at 32°F. In addition, water emits a small amount of heat as the ice layers form.  This “latent heat of fusion” releases energy as the water moves from a liquid to solid state.  If the temperature of the ice layer on plant tissues can be maintained at 31-32°F throughout the freeze period, then the tissue encased in ice can be kept at 32°F, preventing damage.  However, if the water application rate is halted (due to a frozen sprinkler), or the volume required is insufficient due to the presence of wind (Figure 2), the tissue could drop to the wet bulb temperature, causing more severe damage than if no water was ever applied.  The wet bulb temperature is measured using a sling psychrometer with a piece of wet material (typically a sock) attached to the end of the bulb.  This temperature can also be calculated with a number of assumptions taken into account using FAWN online: http://fawn.ifas.ufl.edu/tools/irrigation_cutoff/about.php. 

Critical temperatures for peach buds and flowers can be found in a Washington State University Publication available online: http://treefruit.yakima.wsu.edu/weatherbuds/budhardiness/BudCharts/Peach%20Bud%20Chart%20EB0914.pdf. 

Figure 2.  Volume of water required to use ice formation as an effective method of frost protection (Gerber and Martsoff, 1965). 

Advection freezes can often be more severe because cold air is accompanied by windy conditions, rendering the aforementioned techniques ineffective.  These windy freezes are often part of a large cold air mass accompanied by steady wind speeds of at least 5 mph.  Unlike radiation frost events, thermal inversions (warm air rising, cold air sinking) do not form, despite minor radiation loss from plants and the soil. 

On the night of February 11^th, 2012 and into the morning of the 12^th, our research orchard at Citra experienced an advection freeze event.  We had sustained winds of at least 10 mph throughout the night (Figure 3).  Based on the data available at the time, irrigation to begin the ice formation process began at approximately 3 AM.  The wind speed had begun to drop, and there was sufficient water pressure and volume to start the ice formation process.

Figure 3.  Minimum temperature (°F) at approximately 2 feet above soil level (blue line) and maximum wind speed (mph) (red line) on February 11-12^th, 2012 (FAWN data).

The following night, February 12^th, was a clear, fairly calm night with lower relative humidity  that resulted in a radiation freeze event(Figure 4).  On this night, the water was turned on earlier than February 11^th  with calm winds and dropping temperatures.  However, two successive nights of ice loads damaged some of the trees in the orchard, especially the ones that had not yet been pruned (‘UFSun’; Figure 5).  The ‘UFSun’ suffered severe damage; some trees were damaged on 3 out of the 4 scaffolds.  Although these trees will be kept through the season for observation, if sufficient buds do not form to replace lost scaffolds, they will be pulled from the orchard and replanted. 

Figure 4.  Minimum temperature (°F) at approximately 2 feet above soil level (blue line) and maximum wind speed (mph) (red line) on February 12-13^th, 2012 (FAWN data).

Figure 5.  Ice loads on pruned ‘Flordaprince’ trees (left), and on unpruned ‘UFSun’ trees (right) at the Plant Science Research and Education Unit in Citra, FL, February 2012. 

Figure 6.  Peach flowers encased in ice.

Figure 7.  Freeze damage on peach flowers. 

Good ice formation occurred on the 2^nd night (Figure 6), but damage was already visible from the 1^st night of the advective freeze (Figure 7).  Three days later, fruit were sampled from the orchard to determine the degree of fruit damage from the two nights of freezes.  Fruit that were still alive were green, with a white embryo and pit still in the process of forming, while dead fruit had brown mush, indicating a dead embryo (Figure 8). 

Figure 8.  Sectioning fruit to detect alive (left) and dead embryos (right).

So, what did we learn from this frost/freeze event?  We definitely will finish pruning all of our orchards by Christmas this year to avoid tree damage from successive ice loads.  With the warm winter, we had nickel-sized fruit and some canopy already on some of the low-chill varieties like ‘TropicBeauty’ and ‘UFSun’, which further weighed down tree branches. 

It is very helpful to have a weather station or temperature recorder that you can access via the Internet, if your orchard site is not outside your back door.  As technologies advance and become less expensive in the future, many commercial operations would benefit from investing in on-site temperature monitoring.  The UF blueberry program has recorded at least 1-2 degree differences in our fields from the Citra FAWN station, which is 2 miles to the west of our field plots. 

This year, FAWN rolled out a text message “freeze alert” system that has a $50.00 fee per season (http://fawn.ifas.ufl.edu/freeze_alert/).  Using this system, you can set a number of parameters, such as the freezing threshold for different stages of bud of flower development, and then the system will send a text message or alert as an indicator of when to begin frost/freeze protection activities. 

Finally, this advective freeze event was the first that I have experienced since coming here to Florida 2 ½ years ago.  Although January-February 2009 was a tough year, successive ice loads from several radiation freezes were the downfall of many of the scaffolds in our research orchard. As water use gets more highly regulated, alternative frost protection methods will need to be investigated, and are being researched among faculty here at University of Florida for several specialty horticulture crops. 

Here is to hoping that next year is a better year and we have a good cropload!  Cheers.

References

Evans, R.  2008.  The ABCs of frost management.  Available online:
<http://www.ars.usda.gov/SP2UserFiles/person/21563/Frost%20Protection%20in%20Orchards%20and%20Vineyards-2008.pdf>

Florida Automated Weather Network.  2012.  Data from Citra Station in Alachua County, FL.  Available online: <http://fawn.ifas.ufl.edu/>

Gerber, J. F. and J. D. Martsolf. 1965. Protecting citrus from cold damage. Univ. Fla Ext. Cir. 287.