You may find this copy/paste job of some interest. If you still have questions, please ask. I've seen general cold hardiness limits published for various perennials (trees & shrubs) in some of the texts I have on plant production methodology, but nothing I recall on the net. 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

A couple years ago I heard of a cold hardiness trick that I saved in my files. I've never tried it because I'm in a pretty warm zone, but with yesterday's temperature drop (went from 59F to 3F in 24 hours) I may wish I would have tried this. Passed down from an old fruit grower: In the late fall on a day of 35F or warmer, spray the tree with rubbing alcohol followed by a spray of food grade calcium chloride. The old timer claimed it would increase the winter hardiness exponentially. Claimed it could be done again in the spring, two weeks before bloom to increase frost tolerance of flowers. Don't know if this really works. If it does, it probably lowers the freezing point of the sap, the same way urea does. Don't know what concentration calcium chloride the gent used. Potassium Dextrose Lactose (KDL) is a product specially manufactured to improve frost tolerance of blooms. Supposed to offer about 3 days protection. After that, it is metabolized by the plant/tree. Supposed to increase frost tolerance by up to 5F. I used some this spring, and recommend the lower of the two rates on the label. Even at the lower rate, it tended to want to burn the flowers a little, after a couple of sprays. On obtaining urea. You might try a local farmer's coop. Ours sells it by the 50# bag, or bulk. I've seen people use it as ice melt in a pinch (years ago) although I wouldn't recommend it. For fertilizer, it doesn't take much, it's 50% N. For an area of 100sqft., you'd only use about a 1/2 lb.

I'll back up pineresin on this; snaxs's cladogram does not say the same thing as the TOL cladogram. The two trees are not the same, with one showing a lack of resolution (trichotomy) where the other shows resolution (dichotomy). Again, little point in arguing with somebody who keeps insisting they can read a cladogram when they apparently can't. Meanwhile I'm still trying to figure out what that blinking "Resin's water lilies" refers to.

It won't be a chemical you add, it will be genetic modification. Scientists have already tentatively studied this and know the chemicals involved; tracing, cloning, inserting and amplifying the genes would not be impossible these days but still not anywhere near dirt cheap enough to motivate some party to do it for any reason other than agriculture. There was a scientist/horticulturalist doing work related to this in DC but in conversations with him the impression you get is he moved on and sold his lab equipment because it would be hard to successfully commercialize. His investors didn't think he was a big enough party to pull it off so didn't give him anymore money, but the big parties (Syngenta, etc) will only be interested in agricultural applications for the foreseeable future. So...zone 7 hardy Date Palm? Expect it to be a college student's toy/pet project in 25 years, maybe; not a Monrovia Nursery project in the next 5. Horticulture is a very bottom line business, and is getting more so. Their genetic research will be focused on making plants cheaper to produce...not necessarily interesting for us. This post was edited by davidrt28 on Tue, Dec 16, 14 at 4:08