Saturday, 27 June 2009

Setting the record straight, again

Following on from my indignant reaction to the "shallow root system = a tree's ability to hold out in freezing temperatures" statement.

So here is the skinny on how cold dormancy works, i.e. what a deciduous (in this country, used to refer to trees that shed their leaves in winter) tree does to prepare itself for winter beddy-byes.

Actually cold dormancy; the process by which a deciduous tree prepares itself for winter is quite an interesting topic in a geeky sort of way.

Its starts with the absorption of chlorophylls which exposes the autumn colours we are familiar with. Then as nutrients are absorbed a whole range of other processes begin including cell wall hardening and increase in respiration as sugars are stored around the plant. It would seem that the tree has shut down initially but far from it. Then as daylight and temperature decrease there appears to be a timing mechanism that prevents the tree from emerging from winter too early. It would be disastrous for the plant to produce leaf just to be lost to a frost so the tree needs a degree of certainty that this is not going to happen. So a substance in bark and stems called phytochrome detects light levels, as these increase along with soil temperature and as long as at least 300 hours of low temperatures has passed, the tree will emerge in Spring. Amazing stuff.

There. Before anyone else starts spewing a whole load of crap.

Thanks to bonsaibanter who is a fantastic source for horticultural facts. (Get the hint? Keyword = FACTS.)


  1. Very interesting, so what happens when you collect a tree which is surrounded by snow at -7ºC and you take to it's new "home" at 16º, 20ºC temperatures. What would be the best solution for survival keep it in shade or full sun?

  2. My gosh, you've asked a good one there!
    I will have to go away and find someone who can give a rational answer to this one...
    Any readers out there who have a (scientific / horticulturally correct) clue?

  3. Need to make some assumptions here.
    First that the tree is deciduous.
    Second that you are asking if the tree would emerge from dormancy early as a result of temperature change.
    Third that you are collecting the tree fom an alpine region and moving it to a lower altitude at the same latitude; ie daylength remains the same.
    If it is leafless and dormant then it should emerge from dormancy naturally once day length and temperature increase. Full sun should not matter as long as you have collected it with sufficient root mass. As with any collected tree a degree of after care will help during the first year; well draining soil, water and sunlight. Bottom heat helps as does increased humidity around the foliage.

  4. Thanks for your detailed reply. In this case they are Pine trees and Juniperus that are at 1800mts and come down to 300mts. These trees spend Winter under a meter of snow so my concern is the shock they get due to mainly the change in temperature. I many times have read that people leave them in the shade the first months and some people say that they need a mimimum of hours in the sun. I soak the roots with Supethrive and then with Micorrhizas and spray the foliage with root promoters the following weeks. Again many thanks for you scientific answer! Which is not easy to find in our Bonsai world.

  5. The case for conifers are slightly different. They are adapted to cope with colder conditions by their cell structure and leaf forms; long, narrow needles or small waxy leaves. This also means that duing the right conditions in winter they will continue to grow. It also means that in the event of a frozen rootball and bright sunlight the tree could become dessicated as water is lost throught the foliage but not taken up. So there is risk lifting a conifer from frozen ground and movinbg it to brighter, warmer conditions. In this instance shade and a gradual temperature change would be better. Mist the foliage and cover with a large polythene bag to help raise the humidity around the foliage. The benefits of Superthrive and root promoters on foliage are practices too long to discuss here. Better to concentrate on providing the correct environmental conditions for cell division; warm, moist, aerated conditions.

  6. Many thanks Mike for your help!