1) Perhaps, though probably not from temperatures. Plants in containers are more susceptible to dessication and other complications related to winter sun/wind exposure. Siting containerized plants that don't die back to the ground in the shade where they will be sheltered from wind is a good practice.

2) Plants don't experience a chill factor like humans do. Many containerized plants sited in full sun can be stimulated to grow after several warm winter days. Not only does this greatly reduce cold-hardiness, but the rising sap can also freeze and cause irreparable damage. Once a plant has entered the dormant phase of its growth cycle, it's to the plants advantage that it remains dormant until danger of damaging frost has passed. The tremendous mass of the earth doesn't respond quickly to the sun's direct rays, but a container, especially a dark colored container, can gain enough solar energy to raise temperatures 50* or more in a single day of warm temperatures and exposure to the sun.

Al

Thanks Al... so for my container clematis which are supposed to freeze to the ground in winter, I should bot have to do anything but for my "evergreen" passion-vine, I may want to move it to a shady wind sheltered location but I still don't need to wrap the pot right?

And for those pots going into the ground along the north side of the house, how close to the foundation should they be?

I am thinking about incorporating some sunken (bottomless??) pots with round stepping stone covers along the pebble path on my north side so I can just remove the stepping stones in the winter and slip the potted plants into the sunken pots. Would pebbles work for mulch or should I stick with pine bark on top of the pots once I stick them in the ground?

"I am thinking about incorporating some sunken (bottomless??) pots with round stepping stone covers along the pebble path on my north side so I can just remove the stepping stones in the winter and slip the potted plants into the sunken pots. Would pebbles work for mulch or should I stick with pine bark on top of the pots once I stick them in the ground?"

What a great idea! Is that a product of your own thinking? Bark, or something that is a poor conductor would be a better mulch. You want it to help retain some of the geo-heat entering the container from the earth. The dead air spaces in the bark, and the bark itself are both good insulators.

Your plan for the passion vine & clematis sounds good to me.

Pots on the north side should probably be within a couple of feet from the foundation to gain an appreciable benefit from the added warmth.

Good luck!

Al

Thanks, I can't take all the credit for the idea: my original plan was to bury the pots along the south-side of the house and just dig them back up when I needed to move them but someone on one of these forums suggested the "pot-in-a-pot" to make it easier. Apparently some nurseries do that. When I found out they need to be on the north side, I had to adapt the plan since I already have a 4.5' pebble path adjacent to that side of the house.
Nikki

There are several different problems that must be addressed when overwintering plants. Plants in containers have all the same problems as plants in the ground and some additional ones as well.

Some containers will break when the moisture in the soil freezes and expands. It is very difficult to prevent terra cotta pots from cracking if plants are left in them during winter.

The root ball of plants in containers is exposed to colder temps that the same plant's roots would be exposed to if planted in the ground, so extra protection may be needed. For this reason, many people dig a hole and bury the pots in the ground during winter.

Plants with shallow roots planted in the ground and any plant in a container will suffer from fluctuations in temperature. This can cause damage when the soil repeatedly freezes and thaws. Brief warm spells can cause plants to bud out too early and be damaged by late freezes, which is why southwestern exposures are often bad.

The limbs of plants can be damaged by the weight of ice and snow during the winter. For some plants, this can be prevented by tying the limbs into a bundle during the winter.

An exception to the rule of moving plants to the north side of a house might be for some evergreens. For 20 years I've grown gardenias because my wife loves them, even though they have not been quite hardy enough for our locations. Because they are evergreens that are only marginally hardy, their leaves suffer a great deal of moisture loss from cold dry winter winds. In the southeast, our cold winter winds come from the northwest mainly. To keep our gardenias from from getting killed back each winter, we have either planted them on the east side of our houses, or moved their containers to the east side during winter. The north/northwest side will still work well if there is something, such as a hedge or fence, that protects the plants from the the cold north winds that desiccate the leaves and stems. Whenever we've had extreme temporary cold snaps for a few nights, I would spray the leaves with products that seal the leaves to lessen moisture loss and cover the plants with a quilt for a few nights. I bought several cheap used bed comforters from Goodwill and yard sales and keep them to use as temporary covering for plants. These also work well to protect my tomatoes from late frosts in the spring and early frosts in the fall.

Ironically, covering plants with snow can reduce damage from extreme cold weather. Snow helps to reduce temperature fluctuations and prevent desiccation from dry cold winds. Snow also insulates plants from air temps below the 32 degree freezing point of water, which I don't understand well enough to explain even though I know it's true.

Empty in-ground pots covered with stepping stones/pavers! That IS genius!

I second the recommendation on Bark as the filler.

Josh

Thanks Josh...I have my moments ;-)

Thanks for the tips gtippitt, I see you are in my zone. I have had my Gardenias in the ground all along the northwest side of my house (which happens to be the front) for 3 years. I decorate them with Christmas net lights (warm ones) over the winter - last winter was the first time that they really suffered any freeze damage but they all sprang right back so I won't worry about them at all. Good tip about tying up the branches to prevent breaking due to weight of snow/ice.

I have heard that about snow too, we really don't get snow that sticks around long enough here though.

Nikki