Micropropagated plants (tissue culture) and plants from cuttings (vegetatively propagated) don't produced the equivalents of tap-roots (i.e. symmetrical lateral roots with a sinker immediately below the root-shoot junction, i.e. tap sinkerÂ), so it's not something to be concerned about.

It's unlikely that trees heeled in entirely would benefit from the taproot, even if they had one, because they would be subject to extremely high humidity levels & dessication would be a non issue. To the degree that it MAY be possible for new shoots to emerge from a deep taproot (doesn't seem likely, and there is none anyway) if the tree & shallow roots were to succumb to freezing, a tap root 'might' be of some benefit to some tree's survival.

Al

Tapla,
By "dessication" I meant of the trunk and limbs especially if left uncovered after a few years of the "Italian winterizing" treatment. I keep hoping for an easier way to grow figs in this cold zone.
Cath

Ahhh - I thought you were referring to trees you'd heeled in.

A little background before I comment: There are actually three kinds of water to consider when we discuss "freezing" or cold-hardiness. The water held in soil - When this water freezes, and it can freeze the soil mass solid, it doesn't necessarily kill the plant or tissues. 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 living 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 bound water, the frozen cells and the tissues they form die. This is the freeze damage that kills plants.

With this in mind, we need to look at what state the water inside the plant is in when it is cold enough to freeze the ground. When temperatures drop much below 25*, the unbound or inter-cellular water, water between plant cells is frozen. In this state, it cannot move into cells to replace bound water lost to diffusion and evaporation, so deep roots would not be an advantage. Water has to change phase, back to a liquid to be mobile in the plant. When low temperatures and high winds are a concern, we must wait until the water in between cells thaws for moisture to be replaced via capillarity. No matter how deep the roots go, moisture will rise only until it freezes in tissue & then it stops, so frozen wood is a sort of roadblock or dam. When the plant thaws, unbound water can again move into cells and the plant's water supply can move upward only through tissues that are unfrozen.

If the temperatures drop so low that bound water (inside cells) freezes, the point is moot because the tissue dies.

Al

Tapla,
Okay, but then by what mechanism do some plants survive well below 25*F?
Cath

There is sugar and salts in the sap wich act as an antifreeze.

Commonly, each species of plant has a general range of cold-hardiness. Within species and cultivar, 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 actually three kinds of water to consider when we discuss "freezing". The water held in soil - When this water freezes, and it can freeze the soil mass solid, it doesn't necessarily kill the plant or tissues. 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 living cells 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 even take advantage of another process to withstand very low temps called intra-cellular dehydration.

The roots of your trees can stay frozen for extended periods or go through multiple freeze/thaw cycles w/o damage, so long as the temperature does not fall below that required to freeze intra-cellular water. If roots remain frozen, but temperatures remain above killing lows, dessication is the primary concern. If the tree is able to take up water, but temperatures are too low for the tree to grow and make food, stored energy becomes the critical issue. Dormant and quiescent trees are still using energy from their reserves (like a drain on a battery). If those reserves are depleted before the tree can produce photosynthesizing mass, the organism dies.

There are a number of factors that have some affect on 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

No one can give a definitive answer that even comes close to accurately assessing the temperature at which bound water will freeze that covers the whole species. Unbound water is of little concern & will usually freeze somewhere around 28*.
Some material will be able to withstand little cold & roots could freeze/die at (actual) root temperatures as warm as 25-27*. Other plants may tolerate much colder actual root temperatures - as low as 10*. There's just no way of knowing unless you have a feeling for how cold-tolerant the genetic material the plant was derived from might be, and finding out is expensive (from the plant's perspective). ;o) Another example of this genetic variance is that trees found growing and fruiting well closer to the equator need no chill time, while other trees, derived of genetic stock from a more northerly provenance may need a period of chill to grow with optimum vitality in the subsequent growth period/cycle.

It's wise to remember that root death isn't instantaneous at one particular temperature. Roots succumb to cold over a range of chill with cultural conditions affecting the process. The finest roots will die first, and the slightly thicker and more lignified roots will follow, with the last of the roots to succumb being the more perennial and thickest roots.

Since any root death is a setback from an energy allocation perspective, and root regeneration takes valuable time, it's probably best to keep actual root temperatures in the 25-40* range as long as we can when the tree is resting, even though the organism as a whole could tolerate much lower temperatures. Even well established trees become very much like cuttings if all but the roots essential to keep the tree viable are lost to cold. Regeneration of roots is an expensive energy outlay and causes the trees to leaf out later than they normally would and shortens the natural growth period and reduces the potential increase in biomass for the next growth cycle and perhaps beyond.

Al

Tapla,
Thanks. And your last two paragraphs exp;ain the experience of northern fig growers. In-ground figs bear later than potted figs and are smaller than they would be if grown in warmer climates.
Cath