Tools to help you while monitoring temperatures and protecting your crop

THIS FALL, a couple of cold fronts provided some chill as many growers prepared to apply hydrogen cyanamide. Although chill for November and early December is running below normal hours, accumulation has been greater in many production regions of Florida versus the same time last year.

CLIMATE OUTLOOK

Having an insight into short term (one- to three-month) climate outlook can assist growers in evaluating weather risks they may encounter during the season. It could help them determine probabilities of serious freezes, approximate timing of their harvest window, and other factors. At this time, the National Oceanic and Atmospheric Administration (NOAA) is suggesting we will be in a “weak” La Niña climate phase from December to February, according to the AgroClimate website, agroclimate.org. After February, we are predicted to transition into a Neutral climate phase.

During a La Niña climate phase in Florida, temperatures are typically significantly warmer than normal and precipitation is very low. Warm temperatures during and after bloom could be a welcome change from the colder than normal temperatures experienced during the strong El Niño of early 2016. For blueberries, colder temperatures during and after bloom result in an extended fruit development period and typically later harvests, as was the case for many Florida growers last season.

Although average temperatures tend to be warmer during a La Niña, the chance of freeze cannot be eliminated. Additionally, if the climate phase shifts to Neutral later in February as predicted, the potential for a late freeze could increase slightly.

FREEZE PROTECTION RESOURCES ON FAWN

There are numerous sources for freeze forecast information from government resources such as the National Weather Service or private companies such as Acuweather, Weather Underground, and others. One resource growers should also consider is the UF/IFAS Florida Automated Weather Network (FAWN), which can be found online at fawn.ifas.ufl.edu. FAWN has a network of weather stations throughout the state that record vital weather data every 15 minutes and make it available in a “user friendly” format. Additionally, the FAWN network is linked to real time weather data on the My Florida Farm Weather Network of private weather stations partially funded by the Florida Department of Agriculture and Consumer Services (FDACS) Office of Ag Water Policy and placed on agricultural properties throughout the state.

While real time weather data is extremely useful before, during, and after freeze events, there are several other resources on FAWN to assist growers with freeze protection. The most popular resource for freeze protection information is the Cold Protection Toolkit. The Cold Protection Toolkit is found on the Tools tab of the FAWN home page.

All the resources in the toolkit are designed to assist growers in the freeze protection decision making process. The tools are highlighted below:

Setting a Critical Temperature: The critical temperature is the point at which significant damage can occur in your crop during freezing conditions. Once you set this value, the other tools will take it into consideration for determining recommended actions.
In many crops, critical temperatures can change as the crop stage of growth progresses. There is a link in this section of the Toolkit that provides guidance to growers in determining the critical temperature of many crops.

NWS Forecast — Interactive Map: This resource will allow the user to obtain a “Pinpoint” forecast for a given location. These forecasts are based on archival data from the system of Fruit Frost Stations that were common within most major production regions within the state. This data correlated with main NWS reporting stations throughout the state will generate a forecast for a pinpoint location based on the current forecast for the station.

Minimum Overnight Temperature: Using a tool called the Brunt Equation, an estimate of the low temperature that can be expected for each FAWN Station is calculated. There is also an option for growers to record their ambient and dew point temperatures at sunset to run the brunt Equation for their site.

Most freeze events consist of an advective or windy phase during the first one or two nights, followed by a radiation or calm phase the next one to two nights. Since the characteristics of a cold air mass continue to change during the advective stage, the Brunt Equation may not be as useful in determining an overnight low. During the radiation phase, characteristics of the cold air mass should not change significantly, unless clouds or a shift in wind direction occur during the night.

Forecast Tracker: This is a relatively new resource available in the toolkit and has been very useful for growers monitoring a freeze event. The Forecast Tracker has a chart with one line for the NWS forecast for each FAWN site and another line depicting actual temperature measurements. If growers see the actual temperatures either significantly lower or higher than the forecast, they can make appropriate changes in their freeze protection plan for the event.

Evaporative Cooling Potential: This tool will demonstrate the risk potential for applying water for freeze protection at the various FAWN weather stations. It utilizes the critical temperature you enter for your crop and the ambient and wet bulb temperatures at FAWN stations.

Very dry air associated with cold air masses and wind can result in wet bulb temperatures significantly lower than ambient temperature. If the wet bulb temperature falls to or below the critical temperature for the crop, risks for crop damage could increase should there be a failure in the irrigation system used for freeze protection.

Wet Bulb Based Cutoff Temperature: As with the previous tool, the critical temperature entered for your crop is utilized in the calculation of a safe temperature to turn off an irrigation system being utilized for freeze protection of a crop. On a freeze morning, as the temperature begins to rise, the wet bulb temperature also rises. When the wet bulb temperature is significantly higher than the critical temperature for the crop, it is safe to turn the system off.

CREDITS

photos by SARAH ASCHLIMAN
story by GARY ENGLAND and JEFF WILLIAMSON

Gary England is a regional specialized Extension agent and director of the Hastings Agricultural Extension Center, UF/IFAS. Jeff Williamson is a professor and Extension specialist, Horticultural Sciences Department, UF/IFAS.