J.E. Shields jshields@indy.net
Tue, 28 Dec 2004 11:53:02 PST
Dear Ken, John, and all,

Ken is generally right, we don't know what makes some plants hardy to cold 
weather and others tender.  At least in many cases.  It isn't true in all 

The physiology of a bulb or any plant changes when it switches from active 
growth to a resting phase.  Bulbs that have to survive cold weather are 
thought to convert much of their stored starch to sugars (glucose, 
fructose, sucrose) with the onset of cooler temperatures or shorter 
days.  Starch is insoluble and does not act as an anti-freeze.  Soluble 
sugars like glucose, fructose, sucrose, and perhaps glycerol are effective 
anti-freeze agents.  It makes good sense.  Plants that can prepare for 
winter in an orderly way, can survive the cold.  Plants caught unprepared 
by unseasonable freezes are often killed.

Tender plants can not always prepare for cold weather, even when the 
progression to winter weather is normal, as we all know.  The fascinating 
ones are those that "ought" not be able to survive (so far as we know) but 
that then do, at least in some microclimates.

There are reports describing some of the changes that have been found in 
some types of bulbs when they prepare for cold weather.  See  "The 
Physiology of Flower Bulbs," by August De Hertogh and Marcel Le Nard, 
Elsevier Science Publishers, Amsterdam (1993), for instance.

When plants go back into active growth, the sugars are either used up or 
reconverted to starch.  A perfectly hardy plant like a dormant daylily can 
be killed when in active growth by a couple days of sub-freezing 
temperatures.  One winter, I had numerous trays of young daylily seedlings 
growing in the greenhouse in January.  We had a power outage, the 
temperatures went well below freezing inside the greenhouse for several 
hours, and every single daylily seedling died.

Plant hormones that lead a plant to dormancy or to prepare itself to 
survive freezing temperatures include abscisic acid and ethylene.  Plant 
hormones that waken plants from dormancy to active growth include the 
gibberellins and auxins.  Abscisic acid and gibberellins are natural 
antagonists in regulating the physiology of plants.  Abscisic acid prepares 
the plant for stress; gibberellins prepare the plant to grow.

Jim Shields
in central Indiana, where any sensible plants are far below ground and fast 
asleep right now.

Jim Shields             USDA Zone 5             Shields Gardens, Ltd.
P.O. Box 92              WWW:    http://www.shieldsgardens.com/
Westfield, Indiana 46074, USA
Tel. ++1-317-867-3344     or      toll-free 1-866-449-3344 in USA

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