How do some plants accli mate? Not so, says Dr. Water does freeze in trees yet they survive. Laboratory tests show that the stem tissues of ac tively growing hardy plants die at the moment of freez ing.
In the autumn, super cooling does not cause death at the freezing point, and in midwinter tissues of living bark withstand severe cool ing provided rate is relatively slow. At the rate which pre vails in nature a drop of a few degrees a minute , there is a slow movement of water from the interior of living cells through the membranes.
Ice forms outside the proto plasts where the water is purer and hardy wood cells survive. When cooling is rapid ice crystals form with in the cells and death almost invariably occurs. Since intracellular freezing rarely occurs in nature, ques tions relating to damage caused by slow freezing are more relevant. Just how are they killed? In trying to answer this question, plant scientists have measured heat losses, studied freeze curves records of tis sue temperatures during freezing , and peered through microscopes at plant tissues in various stages.
Studies show that there is a slow continuous freezing of water in hardy stems and that woody cells become increas ingly permeable to water during acclimation. Weister theorizes that a point is reached during freezing when all the readily available water has been fro zen outside the cell and only vital water remains in the protoplasm. As the tempera ture continues to drop, vital water is pulled away from the protoplasmic constituents to the extracellular ice.
This causes a chain reaction of de naturation, release of more vital water, and death. The acclimation of hardy plants seems to be accom plished in three stages.
Short ened days and cooler nights, detected by a biological clock in the leaves, trigger the first stage. Spectral changes in the sunlight may also. Growth slows to a stop, a hardiness promoter is synthesized in the leaves and moves through the bark to the overwinter ing stem, and a rest period begins.
Stage two, in late autumn, apparently is induced by low er temperatures and frost. Deciduous trees and shrubs shed their leaves and become dormant. But although they are dormant, numerous meta bolic adjustments are made during its early phases. Dur ing its latter phases, hardy plants become very resistant to cold and there is little metabolic activity. There seems to be a third stage of acclimation in hardy woody species, induced by very low temperature, a physical phenomenon. At such temperatures, increased hardiness is achieved.
When you are buying plants, choose ones that have a good-sized root ball, especially if you are planting in the autumn. Some plants have only recently been taken from under glass or polythene and they will need to be gradually hardened before being planted in the garden.
Stand them somewhere sheltered for a while before planting out in the open. Many hardy annuals are winter hardy which means that they stand over the winter without damage and flower in early summer. If the plant comes with a name but no other information, it may be helpful to look it up in books or on the internet.
Finding out where a plant comes from in nature can tell you a lot about how hardy it is. Becoming a member of the Hardy Plant Society , including joining one of the local groups, will give you access to the wealth of knowledge and experience that members have and are happy to share with you. This information is also available as a two sided brochure for you to print out and keep or give to others :. First side. Second side.
This site uses cookies. Please see our privacy policy for more information. Why Join Us? Advice Advice Sample Questions and Answers. Speakers List Calendar. Volunteering Legacies Book of Benefactors Obituaries. The office is now open as usual, on Monday to Thursday and on Friday. HPS videos. What is a Hardy Plant? How Low? Why not? What can we do to help our plants? Getting the right plants When you are buying plants, choose ones that have a good-sized root ball, especially if you are planting in the autumn.
Look to see what information the label gives you. These ratings refer to UK growing conditions unless otherwise stated.
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