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FROM THE DIGEST

Each monthly issue of the Henry Shaw Cactus Digest includes club updates, columns and articles by members on their favorite aspects of cactus and succulent culture. Follow the link below this item to read select Digest articles -- or join HSCS to receive every article in the print version of the Digest.

Cactus in the Cold

By Joe Merkelbach
 
We tend to think of cacti and succulents as plants that require heat and aridity to thrive. There is enough fact there for it to be generally true, but there are succulents that require lots of water in their growth periods and many cacti that tolerate temperatures below freezing while conserving water.
 
How succulent cacti survive in cold conditions is an interesting topic. Just like people getting used to the increased intensity of the rays of the summer sun to avoid sunburn, cacti have to spend time adapting to conditions. Plants cannot go from one extreme to another for temperature exposure. Rather, they must gradually develop the ability to withstand harsh conditions by long-term experience.
 
Common culture advice is to get plants out into the environment early, as soon as the threat of spring frost has passed, and let them acclimate to particular conditions over a full growing season. Staying out in the fall as temperatures drop allows plants to develop hardiness to survive in the cold.
 
Large ceroid cacti cannot tolerate cold temperatures, at least in North America. Only the saguaro, pipe organ and senita live above the frost line, and all of them are damaged and shaped by exposure to freezing. The arms of saguaros develop where cold has affected the plants. However, opuntias, chollas and various globular cacti thrive while exposed to cold temperatures.
 
There is not a lot of literature about cold weather cactus physiology, but there are suggestions as to the means the plants use. Generally the plants lessen the water content of their cells and build up sugar and starch content so that the intracellular fluid will not freeze.
 
The survival of drought, where water content of the cells becomes very low, is physiologically similar to the ability of cacti to withstand cold. The nearly dry cells cannot rupture due to the expansion of freezing water.
 
So, as those who keep rock gardens and overwintering cacti already know, drainage to prevent standing water is the top priority. The presence of ice on cacti is also harmful, as it can split and contort the plants' skin.
 
There is seldom any icy precipitation in desert areas, so the plants are not adapted to it. A covering of soft snow during dormancy is actually helpful, however, as it lessens the chances of wind damage.
 
Deserts can be cold -- and many actually are -- but they cannot be wet and support plants that must have dry roots.
 
Sources:
http://www.alexandgregory.com/cactus_expert.html
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC159246/
http://www.springerlink.com/content/m0280p123p2w0133/fulltext.pdf

 
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