Cold resistance - no easy matter

In mid-January, it was for the first time in four years really cold in Putt Myra forest garden, with temperatures down to -30 ° C. For many of the more sensitive plants we've planted - such as walnuts, chestnuts and Korean silver bushes - was the first real cold snap. It is too early to see whether they have done or not, but many questions about plant cold resistance awakened course. What is it that determines the thermal resistance actually? How can I as growers influence? And how can I judge that a plant has vintrat properly or not?

Nowadays I got used to that there are no easy answers in the woodland garden world, but it would be so complicated when I started reading about plant hardiness, I had still not expected. There are over 10.000 scientific articles dealing with the subject, but fortunately there are a few articles where the current state of research is summarized [1-3]. In the coming weeks I will try to sort out many of the different aspects of cold resistance available. In this post I will start with some general points, then it becomes a depression in the woody plant hardiness and finally a post about how herbaceous plants can handle the cold.

Cold resistance is undeniably one of the most important aspects to take into account in our harsh climate if we are to succeed in our timber gardening. In the literature defined thermal resistance as "a plant's ability to adapt to and endure freezing temperatures" [3] and that is the definition I will assume in this series of posts. Many different factors affect how hardy a plant in its entirety, for example:

• bark and wood's resistance (which need not be the same),
• when the plant goes into hibernation (which is influenced by both genetics and biotope),
• when the flower dot occurs,
• how cold-resistant buds are,
• roots hardiness,
• the difference between the thermal resistance of the year and previous year's wood and
• how strong the correlation between the production of fruit and invintringen is (there are various different plants).

Based on this list, it becomes obvious how complicated it is to assess and influence a plant's winter hardiness. Interestingly, all plants are equally sensitive to the development of ice within their cells. Frost builds up within the cell, the cell dies and the plant will freeze damaged. At worst, dying the whole plant. Different plants have developed a variety of strategies to prevent this. Generally speaking, there are two main strategies: To avoid to freeze the tissue at all, and that develop tolerance against freezing. Often, plants use both of these strategies, First, they try to prevent the tissue freezes and once it has happened, they try to endure freezing in the best possible way, so that there will be no damage.

To avoid freezing makes the plants in many different ways. Quite well known is that the plants increases the sugar content of the tissue to bring down the freezing point. Other plants use enzymes and proteins to delay freezing on cell level. The delay is important in autumn when the plants are still being overwinter in, thus preparing for the inevitable, namely that the tissue eventually will freeze. More importantly, these delay mechanisms in the spring when the plants begin to wake up from hibernation, but when it can still be stone cold in the nights.

Once it has become winter plants must use other tricks to survive the cold. For even if the plants manage to keep their cells free of ice, it is very stressful for them to have ice in other parts of the tissue. The ice vapor pressure changes in the tissue and enables the unfrozen water in the cells would pass through the cell membrane from the cell to equalize the pressure. So really means iciness severe drought stress in plants. Again, there is a great diversity of strategies that the plants have been chosen to prevent dehydration. Some plants may choose where it will be formed ice somewhere, which usually occurs in parts of the tissue that is quite resistant. Others try to tamper with the physics and produce proteins in the cells that prevents water loss in different ways by influencing the vapor pressure. That the drying be less does not mean that it ceases completely, which also explains that cold duration impact resistance. They really hardy plants form something resembling glass between cells using sugars and can thus minimize the drying up even when it gets really cold. In laboratory experiments, for example, the apple variety 'Antonovka' demonstrated its capacity -60 ° C and firs can even survive 196 ° C [1]!

In the next post: Den livsviktiga invintringen.

Literature

[1] Wisniewski, M., C. Bassett, and L. Like, An Overview of Cold Hardiness in Woody Plants: Seeing the Forest Through the Trees. HortScience, 38(5): p. 952-959, 2003.
[2] Alden, J.H.R.K., Aspects of the cold-hardiness mechanism in plants. 1971.
[3] Like, L.V.W.M., Understanding plant cold hardiness: an opinion. PPL Physiologia Plantarum, 147(1): p. 4-14, 2013.