Plant Responses to Early Spring Fever

The seasonal events in the life cycle of plants are regulated by a number of environmental cues including temperature and day length.  How do plants survive during colder temperatures? Woody plants start preparing for winter by shunting the water in their twigs, branches and trunks to their core and root system.  This movement of water is triggered by certain chemical compounds.  As a result, the plant cells contain higher concentrations of sugars, salts and organic compounds which lower the freezing point of cells and tissues enabling them to survive lower temperatures.

What happens to plants when temperatures spike ahead of the last average frost date?

After the coldest days of winter and as the days get longer and the temperatures rise, plants need to leaf out as early as possible in spring to take full advantage of the growing season before the heat of the summer arrives.  This involves moving water into developing leaves and flowers, reducing the concentration of sugars, salts and organic compounds in their tissues and the resulting loss of protection from cooler weather.  For any species, leafing out too early is a risk because late frosts can damage or kill young leaves and flowers.

According to the National Centers for Environmental Information, in the greater Phoenix area, the average risk of frost is from November eighth through March 27 with guaranteed dates of frost from November 27 through February 25. In downtown Phoenix, the average risk of frost is from December 11 through January 26. In Tucson, at the University of Arizona campus, the average risk of frost is from December 18 through January 19.  Greater Tucson averages are November 28 through February 16.

If unusually high temperatures cause plants to leaf out or produce flower buds that are then killed by late low temperatures or frost, nothing should be done!  Removal of the dead or damaged plant parts may stimulate additional growth (pruning is a growth stimulating activity) or expose additional sensitive parts to the cold weather.  Plants are able to compartmentalize damaged areas, preventing further damage.   Wait until the threat of a late frost is past and after new growth has started again before any pruning occurs.