Winterize potted plants

snowling888April 23, 2010

I'm running out of space. I've started to planting in pots. I've researched online. Everyone says plants should be 2 zones colder in hardiness and put close to the foundation. Is that ture? Some even say put them in the shed. My basement is 3/4 finished. I have no garage. I check the shed's temp, it's as cold as outside. I have a deck warps around the unfinished part of the basement. Last winter I winterized potted peonies under the deck. It was a success.

So which one is the best location for winter protection, close to basement, the shed, or under the deck?

Thank you!


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tapla (mid-MI z5b-6a)

Unprotected plants in containers should be hardy to two zones lower than your zone to be safe. Plants to a zone or two warmer than yours can be over-wintered in your garage (as long as you use some care & don't leave the door open wide during periods of extreme cold). If you mulch your containers heavily, or bury them against the foundation of a heated building on the north side, you can usually over=winter plants rated to a zone warmer than yours. If you bury the container in your garden or beds away from any warming effect of a heated building, you can USUALLY over-winter plants listed as hardy to your zone, but dessication takes lots of plants set into the ground away from shelter during extended periods absent precipitation.

W/o knowing how open to wind and air movement, so we can judge how much heat from the earth the deck is able to trap, there is no way to judge how effective that strategy might be. The same is true where the shed is concerned. You're pretty safe to a zone warmer if you bury the plants against the foundation and mulch - but make sure they don't dry out.


    Bookmark   April 23, 2010 at 2:19PM
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mainegrower(Z5b ME)

Al, here's some thoughts about about wintering plants in containers I've had for some time. I'm familiar with the two zones hardier advice, but it's never really made much sense to me.

Hardiness designations for all plants (except those sold by unscrupulous vendors such as the infamous "Z5 hardy" crepe myrtles)are based on the hardiness of the top growth. A great deal of research has shown that the roots of most plants are much less hardy than the tops. A plant designated as Z4 hardy can be killed outright if its roots are subjected to temperatures not much below freezing. Roots in the ground are protected by the thermal mass of the earth. They almost never experience sub-freezing temperatures. If they do experience such temperatures either in containers or in the ground, the plant will be killed. We had a very dramatic example of this in Maine some years ago. For a ten day period in a snowless January extremely low temperatures caused the ground to freeze much more deeply than normal. (I know of several people who had to cope with frozen septic lines despite code requirements that the lines be 4 feet below ground). The loss of plants was dramatic. Hellebores, for example, were pretty much wiped out despite their Z4 designation and the fact that the absolute lows were only in the -10 to -15 range. It was the cumulative effect of cold without any warm ups that froze the ground so deeply and killed plant roots.

That's a rather lengthy preface to my belief that you need to protect the roots from freezing and not worry about the hardiness zone of the top growth, within reason. If you live in Z6, there's no reason to believe a Z4 plant will survive if its roots freeze. Conversely, a Z6 plant will survive a normal Z6 winter in a container if its roots are protected.

If you can protect a container with 2" styrofoam on the bottom and sides of square containers, and several layers sheet foam (cheapest is the stuff they use under floating Pergo type floors)on the top or wrapped around urn type containers, all but the very smallest containers will not freeze despite the subfreezing temps. Lots of other ways, straw bales, dry leaves, burying (as you noted), etc. to insulate. The key thing, whatever method you use, is keeping the roots from freezing.

We always talk about frost entering the ground or container, but the opposite is what actually happens - warmth leaves. You can try this experiment. Take a 2'x2' piece of 2" styrofoam, place it on the ground and weight it down. Check under it in the coldest part of the winter. The ground beneath will be unfrozen or frozen to a depth of less than 2". A carpenter I know specializes in decks. He hasn't dug a 4' footing for years. 12" down, 4-6" of gravel, then 2'x2' 2" styrofaom, then a precast post holder, then the post. All the decks are still standing and no frost-heaved posts.

    Bookmark   April 24, 2010 at 3:25PM
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tapla (mid-MI z5b-6a)

I've made exactly the points you are making many dozens of times here & in talks about container culture, so you'll get no disagreement from me. Everything I wrote upthread was with root temperatures in mind.

One of the things I often tell people is that they can usually keep plants in containers that are only hardy to 2 zones warmer than their zone by keeping them in an unheated garage and covering them with an overturned cardboard box to help trap geothermal heat conducting through and radiating from the floor.

Getting back to the foam issue. It does no good to insulate a container unless you are using the insulation to trap an extraneous source of heat. E.g., insulating a square container on 5 sides (top open) is going to be counterproductive whether the container is resting on the ground, or is sunk into the ground, or is resting on the ground and mulched. The reason is the insulation prevents extraneous heat from entering the container. The soil in a container insulated on 5 sides will get colder than a container with no insulation that is set into the ground, or a container insulated on 4 sides and with the bottom left uninsulated that is resting on the ground or is sunk into the ground. The reason for this is because heat can conduct upward through the uninsulated container bottom and the insulated sides help trap it in the container. Additional mulching of the soil surface would also be helpful because it will create dead air space - a very effective insulator.


    Bookmark   April 24, 2010 at 3:58PM
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mainegrower(Z5b ME)

I'm not disagreeing, but I still don't see what difference it would make if a plant were Z4 hardy or Z6 hardy if it were in a garage and covered with a cardboard box. (A box made from rigid styrofoam might work even better.) If the goal is to protect the roots from freezing, wouldn't the hardiness designation of the plant's top growth be pretty much irrelevant?
Insulating a container bottom would prevent any heat from the ground moving upward into the container. In the case of large containers, however,wouldn't insulating on all sides, bottom and top prevent the migration of any warmth inside the container to the outside, colder environment?

    Bookmark   April 25, 2010 at 6:26AM
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tapla (mid-MI z5b-6a)

It's ok - even if you were to disagree, it's a good discussion. ;o)

Yes - how hardy the top is is pretty much irrelevant, but imagine you have a container resting on a garage floor and it's 30* in the garage. You open the door to the outside world which happens to be 0*. All sides of the container and the top are exposed to the extreme cold as ambient temperatures plunge to near 0*. The only source of warmth for the container is what conducts through and radiates from the floor. If you lift the pot and set it in a bench, or set the pot on a piece of foam insulation, you've largely eliminated the extraneous heat source, so the soil temperature will quickly equalize with ambient temperatures.

Now imagine the same scenario - a plant directly on the floor, but covered with a cardboard box. The door is opened to 0*, but the box is very effective at trapping heat from the floor. Instead of temperatures equalizing at somewhere near 0*, it's not out of line to think the temperature inside the box will not go below 20*.

The finer roots of many plants listed as hardy to zone 6 will not survive actual temperatures of much below 22-25*. Here in MI, even plowed fields with no snow cover rarely see 6" soil temperatures drop as low as 25*.

In the case of large containers, however,wouldn't insulating on all sides, bottom and top prevent the migration of any warmth inside the container to the outside, colder environment?

No. With an example: Put a covered foam cup of water inside a very efficient cooler and put the cooler in a freezer set at 0*. Within a day (or so) the water is frozen and the temperature of the ice and air surrounding the cup is 0*. The insulation around containers can't contain heat indefinitely, it can only increase the length of time it takes for the temperature to equalize with ambient temperatures. Put a heated curling iron in the cooler and seal it up (trapping extraneous heat) and the cup will not freeze as long as the curling iron is on.

People often think that because heat rises, that setting plants in a garage on a table will keep them warmer, but not so. When they raise the plant off the floor (or insulate the bottom) so heat cannot conduct into the soil, the soil equalizes with ambient temperatures because you've eliminated the extraneous heat source. It's not frozen soils or frozen roots that necessarily kill plants, it's root tissues being exposed to killing lows so intra-cellular water freezes, and soils become much colder when lifted above the heat source and exposed on all sides to cold air.

Imagine the garage scenario again - doors open and 0* outside. You have a 1" piece of foam insulation that is exactly the size of a box opening. Rate these plantings from what you feel would be the coldest soil temperature to the warmest. Substitute 'foam box' for 'cardboard box' if you wish. The end results will be the same.

A) For this planting, you simply set the foam on the floor and set the plant on the foam.

B) For this planting, you set the plant on the garage floor & call it good.

B) For this planting, you set the foam on the floor and the plant on the foam and cover the plant and foam tightly with a cardboard box.

C) For this planting, the plant rests on the floor, the foam is discarded, and the box is placed over the plant.


    Bookmark   April 25, 2010 at 11:48AM
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Wow...Great info as always!


    Bookmark   April 25, 2010 at 3:03PM
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mainegrower(Z5b ME)

OK, to start at the end, I do agree that A) would provide the least protection and the fourth (labeled C) but actually D))the most. Still... (And at considerable risk of revealing how little elementary physics I actually remember).

Our basic goal is to protect the roots by insuring they are not exposed to temperatures so cold that they will rupture cell walls - call it 25 degrees. Let's say we have a 2'x3' container 1' deep. Since warmth always moves toward cold, we are trying to, as much as possible, slow the loss of warmth already present in our 2x3x1 container.

The warmth moves by conduction, convection, and radiation. Once the surrounding air temperature is less than that of the mix in the box, all three forces will come into play. If the top of the box remains open heat loss by radiation from that surface will be the greatest through both radiation and convection. Loss through the sides will take place, but will be less because the walls of the container will have some insulation value. The bottom of the container will also lose heat, this time by conduction, as long as the mix is warmer than the surface beneath the container.

When we place the cardboard or foam box over our container, we reduce heat loss through convection and radiation, but the loss by conduction through the bottom remains exactly the same.

Once the mix in our container cools below the temperature of the surface it sits on, the radiant process reverses itself and the mix in the box will gain heat rather than giving it up.

So what the box does is slow radiation and convection (probably the key element in the opening the garage door to 0 temps example.)It does nothing about conduction. What we're counting on is that the gain in radiant heat from below will outweigh our losses from radiation, convection, and the initial conduction of heat from the mix to the floor. (The floor, incidentally, would have to be in direct contact with the ground (I think). Garages on piers would not work.)

As a practical matter, it would seem to me that the best plan would be to group all the containers as close together as possible. Make a box with a lid from rigid insulation board that would fit around all the containers together. Seal joints with tape and gaps where it sits on the floor or ground. The larger area of ground contact should (I think)insure a greater degree of heat transfer to the containers and the air around them. Being inside a building would help reduce convective loss. The gain in radiant heat, however small, would be just enough to keep the mix temperature above the critical point. We hope.

The next fascinating topic ought to be about how plants with extremely shallow root systems - rhododendrons come to mind - are able to endure very low temperatures in the root zone. Some species of rhododendron are native to areas of Siberia where winter lows routinely reach -50.

It is always good to stretch you mind, so thanks, Al.

    Bookmark   April 25, 2010 at 3:36PM
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tapla (mid-MI z5b-6a)

OK - we're on the same page.

I probably have something of an advantage in that I already know that placing a cardboard box over trees on a garage floor when ambient temperatures are very low is enough to keep the soil mass from freezing. I have been stupid enough to leave my garage overheads open during single digit nights, but lucky enough to have placed my pomegranates, apricots, and other plants too tender for my zone on the floor and covered them with boxes.

FWIW - you can't just seal the boxes up. You need a certain amount of air circulation or fungal issues will raise their ugly heads and steal your plants from you. ;o)

If you want to look into the reason some plants can survive extreme temperature lows, look into 'supercooling'. Some plants are able to combine cellular dehydration and supercooling of their cell's cytosol and vacuole fluids by the synthesis of certain proteins that prevent ice nucleation. IOW - in some plants, what water remains as bound or intra-cellular water remains liquid at temperatures as low as -50*.


    Bookmark   April 25, 2010 at 5:42PM
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mainegrower(Z5b ME)

Al: I don't in any way doubt your experience, but it really is amazing to me that something as tender as a pomegranate can be wintered in a cold climate with no more protection than a cardboard box!

In my own case, I have three - perhaps four - options for wintering three cedar containers (each about 24"x18"x10") which will contain Z6 hardy Japanese maples, dwarf azaeleas and some dwarf conifers. Each option seems to have its own advantages and disadvantages, so I would really value your opinion.

Option 1: Move the containers to the "barn" floor and cover with a box. Disadvantage - no direct ground contact as the building is on posts but the underfloor area is not exposed to wind, etc. Advantages - Much warmer temperatures than outdoors. Roof faces south east and gains a good deal of solar heat.

Option 2: Place containers on stone patio and cover with a mini poly house covered with opaque white plastic. Adv. - ground contact. Disad.- Much colder, must be secured against wind, etc.

Option 3: Same as 2 except on the ground.

Option 4: Remove all the plants after they have gone dormant, pot and put in the cellar which goes no lower than 37 no matter what the outside temperature. Adv. Safety for the plants. Disadv. A great deal of work, and I'm not sure what the effect of annual potting and repotting would be.

Should mention containers all have "feet" which raise them above the the ground by 1.25".

I do appeciate your expertise and your willingness to share your experience with others, so thank you in advance.

    Bookmark   April 27, 2010 at 5:33AM
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greenman28 NorCal 7b/8a

I'd remove the plants and re-pot every 2-3 years (with the species you mentioned).
The re-potting will vastly improve the vitality of your plants, especially as time goes on.
During the off-years, I'd probably haul the containers to the barn (as long as it doesn't get too warm in there!).
You want your plants to "sleep" through the winter without waking up.


    Bookmark   April 27, 2010 at 10:55AM
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tapla (mid-MI z5b-6a)

It would probably be wise to establish some parameters we need to stay within. My thought is that you probably want maintain soil temps no lower than around 28*, and no higher than 42* for as long into the spring as possible. You might be able to go a little colder than that, but roots die incrementally, with the finer roots being unable to withstand temps as low as their older, more lignified counterparts; so, even though the organism as a whole might pull through at lower temperatures, you still have an interest in preventing temperatures from flirting with killing lows.

On the other end of the spectrum, if the soil gets much higher than 42* for any length of time, it will stimulate the plant into growth. Once this occurs, the plant takes a giant step backward in its resistance to chill, and the top becomes susceptible to freezing. If this doesn't kill the plant outright, it severely sets it back by reducing energy reserves required to push a second set of leaves. Rising sap can also freeze in the stem causing it to split and kill or badly deform the plant. You can also see the interest you have in keeping temps from getting too high.

Plan 1 sounds like it would work ok, but I can't judge how well it fits in that 28-42* range. Is there a chance you can remove them from their containers and bury them against the foundation of a heated building on its N side & mulch?

The cellar would work if temps stay low enough, but it would be very unusual for a cellar to stay cold enough, long enough to prevent the onset of growth in late winter - which would be a considerable problem - particularly for the dissectums.


    Bookmark   April 27, 2010 at 11:02AM
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mainegrower(Z5b ME)

Many thanks for the thoughts from both of you.

I suppose the only way to find out is to try. The barn floor option is the most attractive since it involves the least moving of the containers. I doubt if I'll gain much heat from the floor, although it is a full 2" thick and probably warms up quite a bit each day.

I've used the cellar to force bulbs for many years, so I think the temps probably stay cold enough. We heat almost entirely with a first floor woodstove, so there's no heat emitted in the cellar at all. There's no way the containers are going to be manuevered down the narrow circa 1810 cellar stairs, so that option means removing the plants.

I do understand about the temp range and think it can probably be achieved with option 1.

Thanks again.

    Bookmark   April 27, 2010 at 11:16AM
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>>> We always talk about frost entering the ground or container, but the opposite is what actually happens - warmth leaves.

It's very easy in the discussion about container protection (or heat gain/loss in general) to only consider one side of the equation.

Great topic...

    Bookmark   August 7, 2010 at 1:51PM
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