Not sure if 'heating' the roots is a good solution. What plants are they? How tall? What are the recommended lowest zones?
Around 32F could be too warm for the roots?
I would rather wrap some insulation around the pot, or surround it with chicken wire 'cage' filled with dry leaves/mulch - and keep them outside.

I think worst is repetitive freezing/thawing, and wind. Well draining soil will help some with that (won't retain too much water sitting in the pot).
In any shelter, you eliminate wind.

Looking for similar info, I found that plants in pots should be about 2 zones lower than planted in ground - this is for plants to overwinter outdoors in unprotected containers.

I am in zone 5 and overwintered (past 2 winters) over 230 potted plants - very few didn't make it and those were the plants in dense soil - I even remember water sitting in pots after unusually warm temps followed by 'proper' colds/freeze for that part of the season.
I did surround all pots with dry leaves, and mulched.
If small enough plant and/or space available, you could have pots lying on the side. If any excess water, it will not pool around the roots that much & will drain better from that pot. And don't leave them in strongest sunlight available - that could contribute to some thaw/freeze cycle.

Rina

This post was edited by rina_ on Fri, Dec 26, 14 at 11:50

Yes, Rina is saying what most do. The root ball freezing is common even in ground. Some plants like strawberries and blueberries have light surface roots that often freeze. Not really a big deal there. The freeze thaw is the worst, drying winds, being too wet. Don't let them get bone dry. Slightly moist is best. When roots are frozen they cannot move water to the plant tissues, and drying winds can desiccate the plants. In the shed this is avoided. You don't need to wrap in leaves. This helps keep them frozen outside. As in pots often the sun warms the pots, they thaw, and freeze again at night. Best to just keep them frozen, let them freeze, cover in leaves to prevent the sun from thawing. In the shed, this too is not an issue.
The 2 zones is for unprotected pots. I have zone 7 plants in pots in my unheated garage, yes in a shed they would probably die being zone 6 here, but my garage is like zone 8. Warmth from the house keeps it warmer than outside in there.
As far as using a heating pad, that would dry roots a lot and may kill them if you don't watch it. That would work too. You either have to keep them warm, or keep them cold. Again it's the freeze thaw that is the enemy. Yes use a thermostat.

Drew is right (and I edited my post since I didn't make it clear about adding insulation/leaves - was meant for keeping them outside).

I still think that you shouldn't heat roots. It would be hard to heat them only enough - how would you know which plant could take what temps? Since they are hardy, they would need some colder periods anyway.

Rina

Yes, they do need chill to flower and fruit, but most don't know I guess? Chill is between 32F and 45F above or below does not count for chill hours, so set at 32 degrees would actually give them more chill hours, not less.
I agree though, it's a lot easier keeping them cold, and harder to mess up.

Thanks for all the advice. Keep in mind, I'm not talking about "heating" to 70ð or anything. My thought was stick a thermostat in there near the bottom of one of the pots, and set the thermostat as low as possible. While most heat mat thermostats don't go that low there are some waterproof thermostats I found that would work that can be set for 32, 34, even 30ð.

I think that, by placing it at the bottom of the pot where the roots are located that, in the ground, are deep enough to be less likely to face severe freezing temperatures in my climate. Setting the thermostat as low as possible and having it only kick on if it actually gets to freezing or below, should, in theory, simply keep them from getting too cold at any one point in time. I guess I should add that I lost a couple of Japanese maples in containers last winter in the exact scenario I outlined above, which is why I'm concerned about it now. My thought was it was mostly due to root damage, because the air temperature in there would not have been cold enough to cause damage to the shoots/twigs of the maples.

It should work as I said. Suprised though you lost some the other way, as I have never lost any. I have about 12 pots in the garage now, too many to heat! But the garage is warmer. Yeah I would use the heating mat, hopefully it won't cost too much, but heating elements in general suck power. Also make sure not to let them dry completely out. That will kill them too!
When i think about it maple roots in ground do not freeze, they are deep. Whereas strawberries do, probably would do fine in the shed, but maples no, yes, heat them.

Reread your op and I'll let those who are familiar with your trees answer your questions. ;-)

Have a good day!!

Laura

This post was edited by loveplants2 on Sat, Dec 27, 14 at 14:57

I myself would not raise them as often it is the ground that is warm and radiates heat.In winter the ground is a lot warmer than the air, so I think that would be a bad idea myself.

Ideally, you would be able to maintain your root mass at 30-42* after the plant is in deep dormancy for the entire winter. If you have a protected building, that can be close to doable, even if air temps drop much lower, but it might take some effort. It would be helpful if you mentioned how many plants you're talking about and what they are, as well as how well the building you have available is sealed insofar as air exchange with the outside air is concerned, and whether or not it's insulated.

Al

If you can maintain 30-42 degrees in a shed/garage/etc, would there be any problems from lack of sunlight (for conifers specifically)?

Thanks for the quick response al. Oh and thanks for your voluminous contributions to the forums, especially on soil. I got a dwarf connifer that already looks less healthy than it did a couple weeks ago and I bought some hoffman's bonsai soil mix (only because I don't have the time or experience to make your mix myself right now) and I'm really hoping that reliably draining soil and keeping it in just the right place will save my little tree. Even if it doesn't , all your writings on soil properties have been so enlightening for me and I appreciate them immensely!

You're very welcome ..... and thank you so very much for the kind words. I hope you fare well with the spruce. They're pretty sensitive to wind in the winter, so please make sure you give it some protection - especially if temps dip low enough to lock up water (in the form of ice). Happy New Year to you.

Al

Yup, after reading this thread I've decided to make putting it somewhere with a fairly constant 30-40 degree temp until winter is over. I've been keeping it out of the wind on my slat glass window sun porch but now that it's getting into the teens at night, and after doing more reading and thinking, I realized that the porch just accentuates daily temp extremes by warming up drying the day more than the outside and then plummeting to near-outside temps when it's coldest outside. I've got an un-attached shed and a garage that's not climate controlled (both of which reduce day-night temp fluctuations instead of exacerbating them like the sun porch) and I'm going to make regular watering in the gritty soil and keeping it 30-40~ degrees a priority.

Happy new year to you too :)

Yes, sounds better. Good luck!

While you're over-wintering in cold conditions (30-42*) you won't need to water very often. Every month or so, I toss a little snow on top of plants over-wintering in the garage.

The roots of maples in situ are actually very shallow for most of the year. When conditions are favorable, up to 90% of the plant's root mass can be found in the top 3-4" of the soil. As summer and dry conditions approach, the roots in the upper soil levels die back and are replaced by roots growing deeper in the ground. The roots of many species of maples do freeze solid, depending on where they are growing.

There are 3 kinds of water to consider when we talk about roots of temperate plants freezing. Freeze/thaw cycles aren't an issue unless the low temperature is a killing low. I'll explain.

Water in soils can freeze solid with no harm to the plant, other than the possibility of desiccation if the water is locked in ice for an extended period. Intercellular water (water in tissues found between plant cells) can also freeze without harm to the plant. It's intracellular water we need to be concerned about. When water INSIDE of plant cells freezes, cells are killed. Fortunately, in most temperate plants, a build-up of solutes occurs in the cell solution as the plant is pushed toward dormancy by photoperiod and chill. These solutes act as antifreeze and allow the plant to tolerate temperatures well below freezing w/o the water inside of cells forming ice ..... which is why freeze/thaw cycles are no big deal unless temps drop low enough that intracellular water turns to ice.

Al

Actually growing many fruit trees, the freeze thaw is a huge problem. Even for trees in ground. Especially in young trees. Fruit trees have a hard go of it. It's best to paint trunks with a combination of white interior latex, and joint compound. The best mix. Dilute paint with 50% water. I'm not sure this is needed with maples? The white helps the tree not absorb heat, keeps it colder, so the freeze-thaw is less of an issue., It is such a problem every fruit tree nursery advises you to do this.
The joint compound is more to keep borers out. It works very much like the clay product used to protect fruit from codling moth and other pests.
Your maples died for some reason, not sure what that was? But better safe than sorry. I would avoid the flux in temps, as this is exactly what causes problems to plant tissues. Horticulture 101. Every plant has shallow feeder roots. Even trees.
One of the problems with discussing what to do for winter, is what we are considering. Each species is different. Japanese maples like the shade. They have a hard time in full sun. Some shade for them is best.They are more susceptible to the freeze-thaw cycles than most trees. Also some are only hardy to zone 6. And even though I'm in zone 6 last year we had zone 5 temps. Most are hardy to zone five but a few cultivars are not.
I myself have one Japanese maple I planted about 35 years ago. It is now 15 feet tall. It is not an easy tree to keep happy. It has been a struggle keeping it alive. It seems to have run into every problem these trees can have, but has recovered every time too, so maybe not that hard to grow. The worst was a fungal infection. Often trees become weak and insects and fungi love compromised plants. Often the pest can kill the tree, but really what killed it was the fact it was compromised in some way, too much sun, not enough water. the tree's natural defenses are down, and opportunists will jump onboard.

This post was edited by Drew51 on Fri, Jan 2, 15 at 2:53

A different topic, and a reminder we're talking about plants in pots. You confuse freeze thaw cycles with an extended warm period that pushes the plant out of quiescence and gets sap flowing, with subsequent freezing at temperatures much higher than 'root-killing lows' doing the actual damage.

You're discussing what happens to the trunk after sap movement is initiated and I'm discussing freeze thaw cycles within that 30-42* range I mentioned in the opening paragraph above, along with the impact these cycles don't have on root viability within that temp range. Once high temps have stimulated the plant to growth, it needs protection, but that's easily remedied - put the plants on a wagon and move them outdoors or under cover as temps dictate. I overwinter maybe 150 trees in my garage during the winter, the soil freezes and thaws regularly - all winter long with no obvious issues; this, after about 25 years practice. My biggest enemy is killing lows (last year, my better half left the garage open overnight in -12* temps and I lost many valuable trees - mostly those hardy only to zone 7 & 8), and forgetting to water on a timely basis (at least once per month).

Al

This post was edited by tapla on Fri, Jan 2, 15 at 11:53

Yeah last year was bad for lows. I lost a 25 year old Lavender plant. It was quite beat up anyway. I put 4 different cultivars in last spring. A pink flowering one too, pretty cool. I also lost a lot of blackberry canes. The crowns lived except for one which did die. This year I covered with leaves, burlap and sprayed them with wilt stop too. The frozen roots is what killed the canes. The roots were fine frozen, they just didn't work, so the canes dehydrated. So far this year ok, but lot's of winter left. My trees are in a good spot for winter. The sun does not hit them much, my house blocks the low sun. Trunk damage is in winter btw. Then you have the voles too, the torture never stops. Last winter I lost 2 trees to the cold, one for sure was trunk exposure to the sun during the day. I didn't paint one, and it died. It was in the front, southern exposure, no house shadow to protect it. I knew better, my bad. The damage to the truck was very obvious in the spring, splits, etc, it was bad. All peach and plum fruit buds died too on all trees. Sprayed with wilt stop this year, and they are loaded with fruit buds! They can take to -10, but we got down to -14F last winter here. I hope it stays above -10, if so i should get about 400 peaches and plums.
One of my peaches has a fugal canker too, I'm trying to decide if I should pull it, or try and save it. A bummer, an excellent unique peach tree. I will try again on a different rootstock. Indian Free, what an excellent peach. Tastes like cranberry and peach mixed together, tart, but also very sweet. My favorite peach. It has red flesh, beautiful!
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Here is my Japanese maple, 35 years old or so. Nothing to see this time of year. Around it are strawberries as a ground cover. Covered with pine straw and hay right now.

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Never seen one of those peaches before... interesting!

When we're talking about trees going dormant, how long does it take for a plant to go dormant once it's experiencing 30-40 degrees? A couple weeks of temps that never get above 40? More or less? And once it goes dormant, what amount and duration of warmth does it take for them to become active again? Can a tree that's gone dormant and then become active again go back into dormancy, or is it 'up' for at least a few months?

Fruits come in every color, I grow white blackberries, white strawberries. Purple snap beans, yellow raspberries, pink blueberries, orange or yellow watermelons.
Most are fantastic tasting unreal flavors, just amazing

It depends on plant what triggers dormancy. For most it's day length not temps. Many come out from temps though. Like blueberries break dormancy when temps reach 45F.

Yes trees can come out and go back in. This has happened to me. I ordered trees from CA and they shipped them the first week of March bare root, but they were coming out of dormancy. We had freezing temps here, but ground was not frozen. I talked to MSU and they advised me to plant them. They had flowers even. All foliage died, they went back into dormancy and broke dormancy in late April, as they should.
I can't speak for all trees but peach trees prep for dormancy in early August, forming next year's fruit buds, and they slowly become less active. Often though not losing leaves at all till freezing temps. Strange trees!
As long as they are not growing a lot, all is fine. I stop fertilizing by July 1st. Some plants August 1st. Raspberries flower and fruit till a hard frost, so they get fed late in the year.

Again in my experience it's not temps that trigger dormancy it's day length. They are not day neutral.
Some plants are though. Day neutral that is. Like some strawberries. I removed frozen fruit off some.

This post was edited by Drew51 on Sun, Jan 4, 15 at 2:10

oops, repost!

This post was edited by spotch on Sun, Jan 4, 15 at 20:51

It's actually the increasing length of nights (not longer days) that acts as the primary trigger that moves plants toward deep dormancy, with chill sealing the deal. A period of chill within a specific temperature range then releases the plant from its dormant state. Subsequently to dormancy is a period of quiescence with the plant resting for as long as cold prevents active growth. During this quiescent period, the plant can be stimulated to growth at any time by several days of warm temps >42*. Once growth is initiated, freeze damage is a much greater threat due to the plant's loss of cold-hardiness.

Al

This post was edited by tapla on Wed, Jan 28, 15 at 12:13

The western side of the Midwest (Kansas) is experiencing very warm temps, looks bad for the farmers they are very worried. Like 50-60 F for weeks now.No doubt the trees are losing hardiness quickly! I'm waiting for a bunch of scion wood from Kansas today or tomorrow. . Excited to add a number of peach cultivars to my current trees. Also expecting some plum scion too in late February. I have to hold these for some time before I can graft. I wasn't going to buy anymore trees either, but Nadia the Cherry-Plum interspecific hybrid is available and could not resist. An Australian cultivar. I also am adding another plum tree
Here's a fruit from Nadia
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