Because I have so many containerized trees, I don't want to go through the effort of burying them in the garden & beds to over-winter, so I house them in my unheated garage. The only trouble I have is in the fact that the deciduous trees always leaf out too early because of high soil temperatures (compared to the garden & bed soil temps).
Yes - there is danger to trees because of low root temperatures. Each species of plant has a general range of cold-hardiness. Within the species, cold-hardiness is genetically determined. That is to say that a plant that is propagated from cuttings or tissue culture will have the same ability to resist cold as the parent plant. Plants cannot "develop" a greater degree of cold-hardiness by repeated or prolonged exposure to cold, even after 100 years (trees).
If we pick any plant at random, it may or may not be able to withstand freezing temperatures. The determining factor is the plants ability to prevent freezing of bound water. Bound water is the water inside of cells.
There are three kinds of water to consider. The water contained in soil - when this water freezes, and it can freeze the soil mass solid, it doesn't kill the plant. Then there is free or unbound water, also called inter-cellular water. This is water that is found in plant tissues, but is outside of plant cells. This water can also freeze solid and not kill the plant. The final type of water is bound water or intra-cellular water. If temperatures drop low enough to freeze this water, the cell/tissue/plant dies. This is the freeze damage that kills plants.
Fortunately, nature has an antifreeze. Even though temperatures drop well below freezing, all plants don't die. In hardy plants, physiological changes occur as temperatures drop. The plant moves solutes (sugars, salts, starches) into cells and moves water out of cells to inter-cellular spaces in tissues. These solutes act as antifreeze, allowing water in cells to remain liquid to sometimes extremely low temperatures. The above is a description of super-cooling in plants. Some plants take advantage of another process to withstand very low temps called intra-cellular dehydration.
A generally accepted guideline to hardiness in containers is add 1 zone. In other words, a plant hardy to 5 might only be hardy to 6 in a container. Again, this is a guideline. There are a number of factors that affect the cold-hardiness of individual plants, some of which are length of exposure to seasonal cold, water availability (drought stressed plants are more cold tolerant), how recently planted/repotted, etc.
Trees never "get used to the cold" or become more cold tolerant because they might be exposed to increasing cold each year. Every tree has the exact cold tolerance that it was genetically programmed with for its entire life. Trees that are vegetatively reproduced (cuttings, e.g.) will have the exact same degree of cold tolerance as the material they were propagated from, for the life of the tree. However, young roots are not as cold-hardy as older, more lignified and perennial roots, so young trees are somewhat less cold tolerant than older trees.
Roots succumb to cold incrementally. The fine rootage dies first & the tough old lignified roots last. From this, you can see that even though a tree may live if exposed to, say 10* temps, it's likely that it will have suffered extensive root death and will wake up in the spring acting much like a large cutting. Cold damaged trees leaf out late, grow with reduced vitality, and exhibit less increaser in biomass than their protected counterparts, so whenever we can, it is prudent to try to keep our over-wintering temperate tree's roots above 25*. Interestingly enough, even in the coldest MI winters, our 6" soil temperatures rarely fall much below 26-27*, and there are soils in the cedar swamps of MIs upper peninsula that have never seen frost. The point is, just because the air is cold, it doesn't necessarily mean the roots of in situ trees are equally cold. Consider that point when you decide on winter quarters for your over-wintering pals. ;o)
Al
This message was edited Feb 23, 2008 7:14 AM
