I really don't think anyone can tell you for sure and anyone who does probably is just full of it ...It likely depends on what type of soil and wetness of such you have potted in and as you said how long it is at that temp ... .... I have read that same 14 degree info but think it is likley a guess .and feel that is VERY COSERVATIVE.. Remember the RS is the hardiest part of the tree...winter dieback on planted trees even from 20 below is almost always on the grafted tree while the root stock is unharmed.
I had newly grafted quarts in an unheated garage last winter with temps outside of -10 to -15 (prob, 0-5below inside) frozen solid for weeks and they did fine. I personally ...even though i can't swear to it ...don't think you have anything to worry about... I think folks worry just a bit too much about their babies and there really is very little ( I have found NONE) real scientific info ( double blind studies) on hardiness of Acer Palmatums zone specific info or wintering in ground or pots ...Till I see that I would take anyones "thoughs" or guesses here with a grain of salt including mine... which is purly homeopathiic ( self observation) in nature as are most all other " so called" info or alledged facts folks state on this in this and any other forum. David

The root system is the least hardy part of the tree. And the new roots on the outside are less hardy than the old woody roots on the inside. All plants have a minimum temperature below which even a few hours of exposure will produce damage. When environmental conditions have an influence it is to make the plant less than at its full level of hardiness, a cool dull summer not maturing growth fully on a plant adapted to a hot summer climate for instance.

Mature roots of Acer palmatum 'Atropurpureum' being killed by 14F/-10C was reported in the 1987 (1991) Lacebark Inc. book Establishment and Maintenance of Landscape Plants by Carl E. Whitcomb in a table combining findings from these two articles:

Havis, J.R. 1976. Root hardiness of woody ornamentals. HortSci. 11:385-386.

Studer, E.J., P.L. Steponkus, G.L. Good and S.C. Wiest. 1978. Root hardiness of container-grown ornamentals. HortSci. 13:172-174.

If you want to be sure of keeping your potted Japanese maples intact you will not leave them out to freeze up like bricks. Any method that effectively insulates the roots from killing temperatures can be used. Often this consists simply of plunging the pots into a loose material like sawdust before frigid weather occurs.

Relying One study as an absolute fact combining 20-30 year old info is in my mind hog wash. In addition, yes you may have some root damge, but the final question was would it spell doom and I would still say probably NO. It is a bit rediculous to make any absolute statement when potting coditions size of pot, wetness and potting material varies so much and can insulate in very differnt ways ... certainly a 5 gal would give likely as much insulation as a newly planted jm in the ground...then you have exposure to wind or protection of the pot from such exposure. I just don't buy this "Theory" there are much too many variables to make blanket statements...I do agree the safest way is to bury the pots in mulch or sawdust ...but how necessary it is....is an open question David

There are so many variables affecting root hardiness. My plants are all potted. My practice has been to put the pots into trash cans and insulate with leaves, straw etc. I wondered how much temperature protection this provided. This winter i placed a sensor in the can with the pots and the readings were never more that 2 degrees different than exposed temperature. My plants have been getting this treatment for close to 20 years and i've only ever lost a few. They have been frozen like a brick and still flushed strongly. Temps have been close to zero F on several occasions. My conclusion is that absolute root hardiness is site and plant specific.

It is equally ridiculous to make vague assertions that are merely based on speculation rather than science. Size of pot, potting media and moisture levels have virtually no bearing on the matter. Plants are preprogrammed with a tolerance of cold that IS absolute. If the temperature drops to a point where intracellular freezing occurs, plant tissue dies. When that plant tissue is root tissue, the plant dies. Although intracellular freezing of the shoots (top growth) of hardy woody plants generally does not occur until somewhere around the -4F range, roots are far more sensitive to cold and will experience damage in 20-14F range. Inground root systems are seldom exposed to that degree of cold - unless you live in the tundra - as the soil mass provides a high degree of insulation (soil doesn't freeze, only the moisture in it freezes). The roots of containerized plants do not benefit from the insulation factor of any substantial soil mass and are far more vulnerable to sharp drops in temperatures. I'd avoid any exposure for any length of time to temperatures below 20F for any type of woody plant in any size container.

Even intracellular freezing of top growth can occur at higher temperatures if the temperatures fluctuate rapidly. This is what many JM growers experienced last spring with the abrupt and severe cold spell after more normal late winter/early spring and the break of winter dormancy. In many cases the top growth was severely damaged down to the graft point while the roots remained viable.

If you have questions about the science behind this, do some research on the bonsai forums. Bonsai enthusiasts, because all their plantings are containerized, have done a great deal of study on root hardiness and exposure to cold.

Immature roots go first, then mature roots. When only the new roots are killed the top may look "fine" afterward, only growing more slowly than if these had not been killed - perhaps in the same way as if the specimen had been bare-rooted or root-pruned (as is done with bonsai).

The one instance of plants designated as A. palmatum 'Atropurpureum' (variable A. palmatum f. atropurpureum seedlings are often grown in place of the clonal cultivar A. palmatum 'Atropurpureum') having mature roots killed by 14F/-10C should be seen as an indication that care needs to be taken. It certainly does not establish that every Japanese maple has that same level of root hardiness, some may be more hardy - or less hardy.

Thanks for the input folks. I guess at this point, I will hope that the root damage is limited, and my negligence doesnt lead to losses. I really wasnÂt prepared for the additional plants acquired in the last couple weeks and should have prepared a sawdust/mulch bed for them. About the only positive from this southeastern drought seems to be a surplus of markdowns at nurseries as people are not planting and stocks were high. I probably overextended what I could properly handle. The benefit of some good deals now, I would trade for a healthy nursery business. I hope they can all ride out this tough time.

I am thinking they have only experienced below 15 degrees for about 3 hours on a couple of nights maybe I will be able to establish an insulating bed of mulch

A few hours below the lethal temperature is all it takes to kill immature roots. If the tops leaf out in spring but are not as full as expected or don't make much growth you may have lost part of the root system.

I'd like to hear from enthusiasts with experience growing containerized maples in zone 7 and below.

"Plants are preprogrammed with a tolerance of cold that IS absolute." GG do you have a citation to go with that claim? To me it seems out of context as a blanket statement

I have only been doing it for 3 years now and have lost maybe one to two small maples. I overwinter them in a detached unheated garage that on average seems about 10 deg above the outside temps in the winter.
After the 1st year where I lost my 2 smallest maples, I started to pot-up my maples in the fall to give them a cushion of potting mix between the tender young roots and the container wall. This is just a personal theory, but I figured it couldn't hurt and so far no losses since (crossing fingers).
I know my garage has been below 14 F based on the car's thermometer (I remember 10 deg at least), but the duration of those temps are unknown and also what the corresponding soil temps were. So I am really no help after all.

Bill

herman, if you just think about it awhile, it becomes pretty obvious. Why do some plants have an ability to withstand greater cold than others? The explanation is genetic adaptation. Plants native to colder climates have a greater tolerance to low temperatures than those plants originating from more temperate or subtropical locations. But there is a tolerance limit for each species and that is part of their genetic make up, just as are flower color, fruiting ability and growth habit. That is why hardiness information is provided and why some plants are better adapted to colder areas than others. Why I can grow more and varied plants in my zone 8 climate than someone who lives in zone 5. It is just basic plant biology. Increased cold hardiness can be developed or encouraged by careful selection of plants already undergoing an additional process of genetic adaptation, but it is not a fast process and takes many generations to achieve. And it is very unlikely that a zone 8 plant can be genetically altered or modified to thrive or even endure in a zone 5.

There are many scientific publications that address the genetic properties of cold tolerance for various types of plants - a Google search will turn up dozens, but many are not published for public review. You would have to order and pay for them.

I had acer palmatum Orangeola above ground in a 3 gallon pot last year with the pot covered in oaks leaved on the east side of my house behind a piece of plywood to block wind and sun. It got below 10F seveal times last year and the plant was fine this spring. I still think the you have to worry about temperature fluctuations in potted plants more than how cold it is. If you can keep them out of the sun and wind it will be helpful. David

Davids comments are why I hardily disagree with GG and others with their quasi scientific arguments ...there are just too many variables and unknowns and very little study on this for anyone to make any such empiricle statements including JM growers, Bonzai folks or others growing palmatums.... I have found the Orangeola one of the LEAST hardy of thwe dissectums so go figure.... At this point you cannot be sure of anything as far as winter haridness IMHO having NEVER said I had the answers just that others don't either ...If you want to be safe garage or out building them for the winter especially in zone 5-6...this is a precaussion based on ignorance no matter how scientific some quasi scisntific info out there sounds...but erring on the side of ignorance is better than loosing a tree David

"very little study"??? There are scores of scientists across the globe studying plant hardiness and more specifically, cold tolerance - it is obviously a topic of major importance agriculturally and economically. There is an annual congress on plant cold hardiness that attracts plant physiologists and other scientists from all over the world to meet and discuss their research and present their findings (publications of their findings and the papers presented are available if you care to purchase - they are NOT cheap). To dismiss the life work of countless plant physiologists as "quasi science" is an amazing bit of arrogance for a hobby gardener with likely no credentials other than digging in the dirt to support his/her contentions. If there were no genetically programmed limits of cold tolerance for various plant species, then there would be no reason for any hardiness designations and you, David, and all those other growers of Japanese maples in colder climates would have no reason to even entertain concerns about winter protection or viability.

Because cold hardiness of plants is genetically determined, gene identification and genetic manipulation and engineering is the primary focus. Studies of this ARE rather recent as it has only been in recent years that the technology to isolate and identify the factoring genes has been developed. However, the study of root hardiness has long been investigated as it has such a major impact on the nursery industry. Cold damage and crop losses of containerized nursery plants have significant economic implications.

I've attached a link to fact sheet for nursery operators addressing winter cold protection and including a chart that outlines which plants can be expected to experience root damage at which temperature. These are inground temperatures - you'll note the publication indicates containerized plants can be expected to experience root damage at much higher temperatures than a similar size/age plant in the ground - 20-25F is suggested as a minimum cutoff. While I doubt it will have any impact on the naysayers who base their knowledge only on their own extremely limited experience, at least it serves to support the contention that there DO exist specific minimum temperatures - well enough established that they are given the term "killing temperatures" - below which fatal damage can be expected to occur.

For further substantiation and documentation into this extensive and very intense field of study, look for articles published in the Annual Review of Plant Physiology, which should be available at larger university libraries or that can be purchased through various online sources or copies of the annual findings of the International Cold Hardiness Seminars, titled simply "Plant Cold Hardiness", available through Amazon.com. MF Thomashow of Michigan State and Tony Chen of Oregon State are just a couple of the US plant scientists that have devoted their professional careers to this issue.

GG, i agree that there are thresholds for root destruction. I just feel that they are condition dependent/highly variable. David's point regarding fluctuations seems relevant. In Philly there are potted trees all over town, totally unprotected and somewhat neglected and Provenance is known to effect hardiness and within the palmatum group cults are notorious for both tenderness and hardiness. I think that soil chemistry and composition are important too. This year I've left plants more exposed than in the past, it's not scientific yet i have 50 or so containerized plants that will supply feedback regarding my technique come spring. I will share that info in april/may.

Appreciate the lively discussion.

Table at UCSU pages linked to above compiled from work of same authors as I listed (from Whitcomb's 1978 (1991) discussion), using data published 30-32 years ago.

Variations in performance for same species noticeable in Whitcomb's tables, for instance two sets of killing temperatures of mature roots of Ilex opaca are given: 19.9F/-7C and 8.6F/-13C. Japanese maple has a big enough natural distribution for there to be some significant variation in hardiness, unless it is confined to pretty much the same environment throughout its wild distribution. With numerous other east Asian wild plants position on the mountainside relative to elevation can make all the difference, same species in almost same location viewed from above can be tender below, say, 10,000 ft. elevation and hardy above it. Another consideration is some extreme variants maintained in cultivation may perhaps be less hardy than normal in conjunction with departing from the natural structure of the tree in other ways.

GG,
I do consider variables that impact cold hardiness. Thats why I place all my potted plants on the ground so there is ground contact. I then cover the pots with oak leaves that act as an insulator and place them on the east side of the house and behind plywood. I do this so that I creat an area that is out of direct sun light and wind producing an area that has a moderating effect on temperature fluctuations.

From my experience, and that is all I have to go on and all I am trying to pass on in this site, this is very effective for over wintering plants. All I am trying to point out here is that I think there are other factors impacting a plants viability at a given temperature than just the temperature.

I was also told by an older maple expert and collector that I could gain 1 zone of hardiness by praying my plants with Wiltproof and a cooper solution in late fall which would support the fact that there could be multiple factors effecting cold hardiness.

In my view the problem with scientific studies are that they usually don't take all the variables into consideration that the average person on this site will encounter. In my situation for example, if I remove the plywoood that blocks sun and wind and leave the pots exposed to direct sunlight the pots are likely to warm up to a much greater degree than the air temps would indicate. The sun could cause some sap to start flowing in the plant expose to the sun light while it wouldn't effect the pots behind the plywood. If that night the weather drops to say 10F the plant exposed to the sun might get damage while the plant that was shaded might remain undamaged because it remained frozen.

If you want to place plants in a freezer at certain temperature points and see what lives and what doesn't I am sure you can come up with some numbers. I just don't think that for most on this sight we can be that specific in our own gardens. I think a range like we have on the zone charts is more realistic. David

Whenever the root killing temperature is reached where the roots are, for even a single night, roots are killed.

"Plants of the same species and cultivar grown in containers will be more subject to cold injury than plants in fabric containers or B & B above ground. This is because most of the roots of container-grown plants are against the inner walls of the container. However, if the temperature drops to the lethal point for the species and stays there long enough for the entire root mass of both the container and B & B root ball to reach the lethal point, both plants will be killed."

--Establishment and Maintenance of Landscape Plants

Colleges with hort. programs here have Whitcomb's books in their libraries, presumably these can be found in collections in other areas as well. Essential reading for anyone interested in this kind of thing.

Ummm.....

I notice that the people who seem to be arguing for easily killed roots (bboy and gardengal) both live in zone 8 -- while those who are defending the hardiness of roots are in colder zones.

Hello, do we see a pattern here? People who have actual first hand EXPERIENCE with those colder temps already KNOW that the trees can survive them.

Strudeldog -- I do most of my gardening in containers, up to and including 8 foot tall trees. I often lose plants to inadequate watering during the summers, and last year I lost one JM due to our record-breakingly-late freeze -- but I rarely lose woody plants over the winter. I also rarely do any winter protection -- I've got waaaaaaaaay too many containers for that! IOW, I would not spend too much time worrying about your JMs in toasty Atlanta. Perhaps protect them from wind if possible -- IMHO drying cold winds on the wood are a lot more likely to be a problem than cold roots -- but don't lose any sleep over the possibility of root death!

Hey Amazin,

You noticed there is a doctrinaire or two in the garden.

If I try a technique,again and again, and it works I'd call it hard evidence.

I can't say that roots are not killed in my pots. I can say the trees flush strongly in spring. Maybe if I could keep the temps up I'd get even more top growth because less energy was directed towards re-establishing roots. I think this is improbable because some of these plants grow wildly. However many of them haven't been here for a below zero F winter. Fingers crossed, we still have a month of possible

Thanks Amazin, and others. It seems like has been stated this is not black and white, that's what has got me second guessing myself lying in bed at cold nights on my decision not to relegate the car outside, and shuttle plants in. With my collection growing and the wife not thrilled with a cold car in the morning being major factors in my decision to leave them outside. I think the coldest I have experienced this year was that sub-zero stare from my wife to find her car parked outside one morning. I dont know what MY genetic determination for cold tolerance is, but I can tell you she exceeded it on that morning. I know I have been surprised both ways as to cold damage, and when you leave something to chance those grey areas in the black and white can be heartbreakers. If we have an Easter weekend as last year after dormancy has been broken you can sure I will moving in everything possible

Now THAT is for sure something you should do.

I was very happy last spring that I grow in containers! My kitchen and hall were stuffed full of pots for a coupla days there. The only JM I lost was the one I was too lazy to carry in, because I thought the pot was too heavy. Taught me a lesson!

I would say the best test would be to take soil temp in middle of pot half way down with each pot filled with the same medium and moisture level health and do so in several size pots with several same cultivar differnt ages JM's aged plants in each...and keep track over several winters...all trees exposed to the same sunlight...same location same wind protection and see how in the pot temp corresponds to outside temps and ...then dig up the plant each spring and see what if any root damage is done ...Then do so with over 400 differnt cultivars...Do the same for in the ground for the 400 plus cultivars and publich the freaking info for all cultivars for in the ground and in the pot or pots (if differnt sizes of pots) and publish the freakin' results Then do this for in each zone then do it again for differnt soil types and wetness both in ground and pots..This could be done with an extensive refrigeration system also in an enclosed football stadium ;>) for maybe 150 mil or so...Now those results I would look at as NOT voodoo science as IMHO is being proclaimed by several here ...BTW I know I left out some variables so you can add those to this witches brew of "more" scientific study that I do not personally wish to undertake ...
I still stand by the "many variables " theory conscerning winter hardiness and I realise the above is a bit wacked but I think it makes my point. And until a study that fits what I consider good realistic scientific data, double blind and extensive....Until then... use you own experiences and those of others in your areaas no such REAL scientific studies exist FOR PALMATUMS (I am NOT talkin' plants in general this discussion is ONLY related to palmutums all 400-1000 or more varieties) .. and always ere on the side of being conservative ...don't take chances

.And I still say most if not all zone info for Palmatums on various web sites are CYA pure and simple...not based on scientific fact personal experience or any guru who has any true scientific knowledge whatsoever . We all know thru common sense that "Most" Palmatums won't grow in zone 3 or 4 (unless contaierized and kept inside)thats a given but I stand by this statement ...We have no idea which PALMATUMS will or won't grow in the colder parts of zone 6a-5a if left outside in the winter and that it may be that there is too much variation of all conditions from lot to lot, city to city, state to state, age to age and cultivar to cultivar to ever know for sure ...but it may be possible to get some such general good "somewhat" scientic or personal experience info on zone hardiness but what is now out there at present is NOT such IMHO... David

I'm very late to this conversation, but I'd like to add to it. These are my own comments, copy/pasted from other forums on GW, so if they seem slightly off topic in areas, please allow for that fact. I've posted these comments dozens of times around this forum, and I think the info will hold up under scrutiny:

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

Al,

Very nice summary of current established knowledge on the effect of freezing on plants. As you explain, genetics determines the cold hardening techniques that plants have developed to withstand cold. Sophisticated approaches, like dehydration, allow some plants to grow in the steppes of Siberia or the mountains of Alaska.

However all of that applies to plant tissue above ground, which is the part of the plant that sees the very low temperatures and which is genetically programmed to initiated cold hardening as soon as the environmental factors trigger the process. Less is known on cold-hardening of root cells. Some say that since they never see the very low temperatures, they are not genetically programmed to harden as much as the above-ground cells and, therefore, are much less cold resistant. It also seems that small feeder roots do not harden at all whereas woody structural roots behave as above ground branches. On has to admit that much less is known about roots and root behaviour that about branches and leaves.

Another piece of root related research I have come across lately concerns the lifespan of roots. It seems that small feeder roots have a lifespan not greater than a growing season (scientists draw a parallel with leaves in the case of deciduous plants), being continuously regenerated except for a few that will develop as permanent woody roots. If this is so, cold damage to the finer roots in containerized plants may not matter so much after all.

Gomero

For what it's worth, I have a Emperor I that just overwintered in a pot with NO protection at all. Naked plastic, and it is leafing out beautifully at this time. It saw a winter low of 3F/-16C, and several nights of around 10F/-13C. I only occasionally watered when it was looking particularly dry. The pot did catch some morning sun, but I really don't think that made a difference, as the temperature drops extremely quickly on calm nights at this location. This all says to me that I'm not going to worry so much in the over-winter. Spring freezes are another story however.

Arktrees

`Spring freezes are another story however`

Of course since new spring cells (in twigs and leaves)have not cold hardened and intracellular water will freeze as soon as the temperature there hits 0C, the resulting ice crystals will destroy the cell giving this dreary mushy look we all know, just like a tropical plant.

Gomero