Hi Ya'll! I hope 2009 is treating you all well so far... My collection of trees is entirely in containers. I understand that trees in the ground are not likely to suffer damage until temps of 16 degrees F or below are reached. At what temp do I REALLY need to worry about my trees in pots? Obviously, when a freeze comes, I make sure the soil is good and wet, and I do take extra precautions for the 1 gal or smaller trees (like moving them inside for a night or two or covering them in place). Allowing for the tenderness of some cultivars over others, in general, how cold does it need to get to damage my trees in pots? I appreciate your input very much!
Hi Spookie, I thought for palmatum, they died when the roots froze to -14C. Could be wrong, haven't looked it up. Of course in the ground it would need to be ultra cold to hit that, but in pots... I just put all my pots back outside. About 10 little grafts, and maybe 15 waiting for spring to go in the ground (more tender than palmatums). -E
Thanks for your response, Emery! That is a great relief to me at the moment... We are about to have several days hovering in the twentys for highs and, with the wind chill, about 9 for lows. My trees are in a very sheltered location and I just put up a wind barrier and frost cloth after watering everything. I only have three trees in pots smaller than 1G, so I just brought them into the coldest room in the house. I am hoping for the best!!!
Spookie, You should be fine. We're having the same weather here - it will be 5 degrees F here to night - and I brought all my potted plants onto the porch. The wind is the real killer in these situations...
Hi Kaitain! YES! The wind here is a real terror. I am on a 11 mile wide bit of land between the Atlantic and the Chesapeake Bay, so it is wicked windy here all the time. It pretty much stinks here for frisbee, hairdoos, and tender plants. I have tried my best with the plastic wind barriers (not touching the trees) and frost cloth... I gathered all the pots together and just went for it with the staple gun. My yard looks like its been attacked by a giant 15' marshmallow! ...Keeping my fingers crossed!
The real killer is the actual temperature and it only takes a few hours below the minimum for each kind of plant. Tops are more hardy than roots, mature roots in the interior of the ball more hardy than immature roots out near (or plastered against, in the too common instance of a specimen not being kept potted on in a timely manner) the pot walls. Stock that retains its tops and grows the following spring may have lost all its immature roots and be incorrectly assessed as having come through in an "intact" or "fine" condition. Such plants will not have the quality and vigor of those that really did come through with all their parts. Whitcomb, ESTABLISHMENT AND MAINTENANCE OF LANDSCAPE PLANTS (1987 (1991), Lacebark Inc., Stillwater) reports study results indicating root killing temperatures of mature roots of Acer palmatum 'Atropurpureum' being 14 degrees F/-10 degrees C.
I'd like to see other data on the root-killing temps. If 14 F was all it took, all my JMs would be dead. I'm in Zone 7, and we almost always have temps in the teens or single digits for some period of time during the winter - often a week or more.
The subject was already discussed in another post last winter, I believe. For cold damage to plants in pots, people are mainly concerned about the abundant fine feeder roots (with diameters between 0.1 and 1 mm). They are very tender and are certainly damaged at relatively low freezing temperatures. However what is probably lesser known is that those feeder roots are short lived anyway (except a few which develop to become woody roots), typically one growing season. The woody roots have better protection. In addition one should keep in mind that if, say, a wet plastic 4 gallon pot freezes solid at -4/-5°C, then as the temperature drops further, most of the roots are protected by a block of (dirty) ice. I do not know the thermal conductivity of the frozen soil vs. pure ice, but some degree of protection is there. I would be more concerned by the part of the plant which is above ground, but here everybody can tell when a twig becomes mushy. Gomero
I'd like to know the experimental conditions used to reach said conclusion, did he expose bare rooted specimens to those temperatures? In my garden, potted palmatums have taken -14°C without apparent damage to the plant, but those maples were in pots which froze solid before the ambient temperature dropped to -14°C Gomero
The same table prepared by Whitcomb, combining findings from two studies (below) reports a wide range in results for star magnolia - death at both 23 degrees F/-5 degrees C and 8.6 degrees F/-13 degrees C having been observed. The point to get out of it is that roots aren't all that hardy, making it necessary to protect potted stock. Star magnolia is generally considered subzero hardy, with 'Royal Star' noted by Gardiner, MAGNOLIAS - A GARDENER'S GUIDE (2000, Timber Press, Portland) as having come through -35 degrees F/-37 degrees C (presumably growing in the ground). 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.
Ron - OK, that temp may be correct. I found this on another site: "While shoot hardiness of temperate plants is important when making overwintering decisions, roots rarely, if ever, will survive temperatures below -10F,". Good point Gomero. Also, the weather Spookie and I are getting is a short-lived cold front. I would doubt the pots would come anywhere close to getting to 14F if they were protected. The temperatures will be in the 20s and then drop for a few hours at night to their lowest points. By early morning they will be back up in the 20s. A very long freeze would be more concerning. Thanks,
We all agree that roots, no matter how woody, will die if exposed to sufficiently low temperatures; but this temperature is the temperature that the root itself is exposed to. In order to relate this temperature to the air temperature one would need to know the thermal conductivity of the frozen potted mix and the time the pot is exposed to this low air temperature. Only then one can tell, using the geometrical data for the pot, if a given air temperature will lead to the killing temperature in the central root root zone. As long as the value for those parameters (thermal conductivity, time and dimensions) are not uniformized, results will not be the same. Gomero Gomero
I'm in the same boat with Spookie i.e. all plants potted. I don't think I've ever lost a tree from low temps. I've had pots freeze solid from January to March and leaf out beautifully. I do suspect however that I lose some feeder roots and this does result in setbacks as the season unfolds. I believe that wind is not a factor in Z7 and probably not in Z6 either. Plants at the very edge of the JM range e.g. Z5 need wind protection. I have about 30 unprotected plants on my deck where the wind rips and "chill factors" get into the minus teens and have had no noticeable dieback or bud loss. I learned plenty from Whitcomb's books but I agree with Gomero that there are so many variables that it's not really 'scientific' to make blanket pronouncements about root survivability. Philly is looking at 5F tomorrow. I moved a few small pots to more sheltered spots, other smaller pots I've clustered against the house and surrounded with leaves. But everything is outside and uncovered. I've got about 70 pots here. +5 to 0F is about as cold as it's gotten in past winters and all has been fine.
Well, I put it off as long as I could, but moved my potted maples into the garage today. I repotted them all last week, and was hoping the temperatures would stay mild enough to leave outside this winter. But we've been having a cold fall and winter so far, and with low temperatures expected in the teens (F) for several days running this week, I decided not to press my luck. The problem with moving them inside, for me, is that my garage is not that cold. It is not heated, but my water heater and furnace seem to throw off enough heat to keep the temperatures no colder than 40s or so. Plus it warms up well on sunny days, so my maples end up starting to sprout earlier than they would if left outdoors. However, the lesser of two evils seems to be keeping them in the garage since a few degrees apparently can make a big difference for root survival. I also transplanted two maples that didn't seem to be thriving in the spots where I had them. My Red Dragon was getting too much sun where it was planted, so I moved it to a shadier spot. I moved my Beni otake to the spot where Red Dragon had been because it seems to like more sun. We'll see. One way or the other, I'll need to watch them closely and water more than usual this year. Happy New Year everyone.
Yes indeed, its Arctic Blast time here in the South! We will be in the low teens - maybe even down to 10 degrees F (-12 C) for 5 days. Its tricky with the potted plants. This year I just heaped up a bunch of leaves on my potted stock, and I think it will do as well as putting blankets out or moving to the porch. I did move my newly potted understock to the porch, tho, and put blankets on them too! Can't chance it. We bemoan this weather when it comes, but in a way its a good thing. When we don't have an Arctic Blast, the next summer the insect population rises to near plague proportions. Every cloud has a silver lining!
Gil, how do you come to this conclusion? What I mean is that I too have had plants leaf out quite well in spring, only to die a short time later. I have wondered whether it was root/tree damage in winter that was the ultimate culprit.
The only trees that I've lost to cold weather were early spring purchases that were grown in greenhouses. I bought them after they leafed out in the warm greenhouse, and placed them in my yard where late spring cold weather killed them. I lost 4 new trees last year because of this. My trees that wintered outside never seem to be damaged by the cold weather. Of course in West Oregon, we rarely see temps below 30 degrees. This year we had a ten day cold spell with lows around 10F, so we'll see if any trees took a beating.
Paxi, I can't say conclusively that I've never lost a tree to low temps. Maybe winter damage exacerbates other problems and trees die after leafing out weakened from the cold. When my plants wilt and die after leafing out I attribute it, maybe incorrectly, to pathogens. My logic is that in very cold winters I've had every tree leaf out and not have a single loss through the growing season. Here in Philly all my trees are in pots. Quite a few have been though 11 winters on my windy deck with temps to 0F. When it gets real cold(<15F) and stays that way I have put a border of blankets around the cluster of pots. When I grow small, expensive new grafts I do give extra winter protection. Last Winter at Ed Shinn's garden Dave Verkade, who grows a lot of trees, explained to me that wind IS the culprit when it comes to winter damage. I have heard that echoed by others in Z5 and cold parts of Z6.
That is interesting about the wind being the potential culprit. Did dave (or others) say what the mechanism is. That is hopeful to hear as many of my little guys have pretty decent protection from the wind. I too have attibuted wilt to pathogen but have (maybe incorrectly) assumed that roots with freeze damage during winter would be a lot more susceptible to pathogens during spring.
Yes, with the wicked wins from the North, a dry out to more tender limbs and subsurface roots. hence winter burn from high winds... newly planted bamboo succumb to this if not initially protected. Sub arctic temperatures combined with high winds, are every gardeners nemesis.
Sounds like the reverse of what we have here. Hot northerly winds off the inland, high temps 30-40C in summer. My solution is to bury all things in pots in a deep woodshaving holding bed with a wind barrier (bank). Could the wood shavings work to deter the cold??? Liz
Paxi, I was tuned into our local NPR station and winter weather was the topic. Their correspondent from St. Louis was detailing weather conditons there. It is WAY nastier there than here in Philly. Regarding the wind factor, I didn't ask Dave specifically what damage he was referring to but I'm pretty sure he was talking about dessication. I understood that if the soil is deeply frozen for an extended time in windy conditions dessication will result. Here the soil does freeze but it's unusual for our temps to stay below freezing for more than a week or 2 at a time. I think only the top 2-3 inches freeze. There are a few Z5 growers I'd be interested to hear their observations and conclusions.
you're not kidding about the weather - it will be several days this the highs top out above freezing let alone enough for any true thaw. Holding breath until spring!
Hey Gil, Here is my two cents. I live on the boarder of Z5 and Z6, in past 4 years (my maple growing experience) Ive seen some diebacks on tender shoots in the spring which I believe are caused by winter icy windburn. So just being conservative, now I pretty much over winter all my potted JMs in my unheated garage and burlap my newly ground planted trees. However, I did attempt to let some 1~2 year old palmatum seedlings that planted in a 30 X 45 X 6 tray outdoor next to a fence last year. Having snow covered and the fence to block the wind chill in most of the cold days, I only lost couple weak ones out of about 30. I think Gomero got a good point about thermal conductivity of the frozen potting mix which really matter to the roots. Regards, Joe
