By Stephen D. Robinson, Commercial Horticulture Agent
In the Commercialized Green Industry, we are aware of the weather events that can include heavy rain, high winds, and prolonged periods of unseasonable hot weather. Significant weather events can create very difficult or damaging conditions for both native and exotic plants in West Central Florida over a short or long-term, period of time.
Conversely, forecast warnings that include the adverse conditions of both freeze and frost are also of great concern when tender cold sensitive plants are used in West Central Florida. Freeze and frost warnings or advisories provide predictions that there is the potential for damage to sensitive plants during a freeze event. When a frost and freeze advisory or warning is issued by the National Weather Service it is important to understand the implications of macro and micro weather phenomenon. It is this document’s intent to inform the Green Industry as to the risk of both frost and freeze events.
A Freeze Warning will be issued when temperatures are forecast to be below 32°F within the next 24-hours by the National Weather Service.
A Hard Freeze Warning is issued when temperatures are expected to be below 28°F for at least three hours as determined by the National Weather Service.
A Frost Warning is not always provided by the National Weather Service since frost events are generally tied to or are inclusive of a freeze warning. This short article is cautionary as to the unpredictability of damaging frost.
Depending on wind, humidity, and dew point variables, patchy frost can form with air temperatures as warm as of 38-42°F. Areas of frost can occur at temperatures of 33-37°F. Widespread frost can occur with temperatures below 32°F.
Non-cold tolerant plants are injured or killed by a freezing event resulting from either inter-cellular (occurring between cell) or intra-cellular (occurring within a cell) freezing. As free water in the plant freezes it expands by 9% in volume. The expansion of frozen water known as “ice” ruptures the cell wall structures of the frozen plant’s interior vascular system and non-vascular tissues. The rupturing the cell wall from both interior and exterior forces opens the pathway to a sudden drop in turgor pressure or turgidity. As a result, plant desiccation quickly follows, and the freeze damaged plant can no longer take in moisture from the soil. Within a few hours the frozen plant has no effective translocation processes to rehydrate itself or manufacture glucose or sucrose for food production. Death can be complete in under six hours.
Frost damage is a pervasive atmospheric phenomenon that does not always require the air temperature to drop to less than 32°F unlike freeze damage. How does killing frost occur when the ambient air temperatures are as warm as 38-42°F?
In later publications, I will explain the atmospheric processes that create widespread frost and freezing temperatures while temperatures on the observed thermometer might read 34°F. I will also clarify how frost can occur when ambient air temperatures are as high as 42°F. Finally, I will also define micro-climates how they can greatly increase or decrease local air temperatures. We often call this meteorological phenomenon a cold pocket or frost proof area. If you would like to ask any questions on frost and freezing phenomenon, please e-mail me at robinsons1@ufl.edu
Stephen D. Robinson
Commercial Agent
Pinellas County Extension Service
12520 Ulmerton Road Largo, Florida. 33774
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