Friday, June 1, 2012

Life of a Cactus Part 7: The Cactus and Freezing Temperatures

All that water stored inside the cactus can be quite a problem certain times of the year.  Freezing temperatures can create ice crystals that burst cells causing tissue damage.  Depending on the amount of tissue that freezes and how well a cactus is adapted to freezing temperatures, freezing can actually kill a cactus.  Most species of cacti are not well adapted to freezing and for that reason most are tropical and subtropical.  Tropical climates never freeze while subtropical climates occasionally freeze.  The extreme southern United States, such as southern Arizona, Florida, and California are all subtropical.  The subtropical Sonoran Desert of Southern Arizona and Sonora, Mexico typically average freezing temperatures a few nights every year or fewer.  Freezing temperatures rarely last more than a few hours and results in the abundance of cacti found in the Sonoran Desert.   Saguaro cacti are the largest cacti that can tolerate freezing, but only as long as freezing temperatures last less than 24 hours.  Areas that receive freezes lasting longer than 24 hours have no saguaro cacti.  Organ Pipe cacti are much more frost sensitive and only survives along the U.S.-Mexico border in Arizona and further south.  Some species such as the Cordon can survive no freezing whatsoever and therefore only survive further south in Mexico.  A handful of prickly pears are able to survive very deep freezes for long durations of time.  Several species, such as Plains Prickly Pear, grow in grasslands of the central United States up to the Canadian border.  One species, Brittle Prickly Pear, grows nearly to the Arctic Circle surviving temperatures as cold as -40 degrees Fahrenheit.  Species able to endure freezing temperatures actually remove water from their cells, essentially dehydrating themselves, so ice crystals will not burst and kill cells.
Brittle Prickly Pear, able to survive temperatures of -40 degrees F by pumping water out of its cells so ice crystals do not burst cells.

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