[pbs] Re: Hardiness
John Bryan
johnbryan at worldnet.att.net
Tue Dec 28 16:50:08 EST 2004
Dear Jim:
thanks for your message, most interesting. All the best to you and yours
in 2005. Cheers, John e. Bryan
"J.E. Shields" wrote:
>
> 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
> cases.
>
> 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.
>
> Regards,
> 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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