Can Evergreens be Overwintered in Pots?

In southern Ontario, the short answer is YES! Not all evergreens or pots are created equal. Read on to get the scoop on how to keep ‘Christmas trees’ alive year after year.

For starters, don’t get a mini lemon cypress tree with the hopes of it surviving our winters.

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Can Evergreens be Overwintered in Pots?

Yes, evergreens be overwintered in pots in Southern Ontario. If they have the right conditions, and the right evergreen is selected, you can enjoy this plant for many years in a container.

Not all evergreens are created equal when it comes to overwintering a potted Christmas tree in Southern Ontario and not all pot setups will suffice. Here is what you need to know in order to be successful:

1. Not just any tree will do.

   We need a hardy variety, one that is already suited to our growing zone at the minimum, but ideally you want to aim for something a few zones hardier. In Toronto we are a zone 7a according to updated hardiness maps. For a solid shot at this we want to aim for a tree hardy down to approximately a zone 4.
   Why? When a plant is in the earth, it has access to much more insulation from freeze-thaw cycles but it also has access to residual heat from the earths core. Once we take those insulating factors away we strip the tools a plant has access to which make it thrive in our zone. So we need a plant that is OK with more extreme conditions.
   The same applies if you are doing any balcony or rooftop gardening. As you elevate your garden, you are exposed to greater extremes in heat, cold and wind and should aim for plants that are 2-3 zones hardier than ground level if they are to have a shot at surviving.

   We cover 7 Christmas tree style evergreens perfect for containers later in this post.

2. Size

   Both the size of the tree and size of the pot matter. The bigger the pot the better, as you will be providing more insulation around the roots in the form of soil. The size of the tree matters a touch less but is still important. A smaller tree will have less roots towards the edges of the pot, a younger tree will be more adaptable then a mature tree and a large tree will be more prone to damage from wind. But in the end the tree needs to be matched to the pot on a case by case situation.

3. Material of Pot

   Freeze and thaw cycles are what we are trying to combat and protect against. Certain materials have more stability and more insulating factors than others. My solid oak whisky barrel halves offer a heavy base to prevent top heavy blowing over, some insulation from the wood and the slats have some room for expansion and contraction so that the planter doesn’t split apart.

   Concrete is an excellent option as it resists cracking and is well weighted to resist winds. Unfortunately, it’s location requires a bit of permanence as once full it is not easily relocated.
   Metal planters, lined with rigid foam insulation are often successful with overwintering. Depending on the type of metal these can be lightweight or quite heavy and there are many sizes to choose from. The drawback to these is the cost.
   Ceramic or Terra Cotta pots do not overwinter with plants very well in our zone and are prone to cracking as they absorb water.
   Plastic pots offer very little insulation or weight to combat winter winds and depending on the type of plastic, UV rays will break it down causing it to become brittle and crack.

   All that said, my little potted evergreen is heading into its third winter… and is in a basic nursery, black-plastic pot. It gets upsized as it grows, but otherwise is quite unassuming in size.

4. Location

   This is the trick for getting less then ideal pot size/materials to work for you; Micro-climates.
   If your tree is primarily for holiday displays and can then be tucked in for the winter like mine then you’ve got a great shot at making this work.

   How do you make use of your gardens micro-climates to protect and insulate your tree?

   -Display the tree in an area sheltered from west and north winds.
   -Use surrounding elements to radiate the suns heat after it is no longer shining. Ex: place the tree in front of a south or east facing brick wall.
   -If there is snow, mound it up around the planter. Snow is an excellent insulator helping to regulate the fluctuations in temperature.
   -If you can, bury the pot within your garden. It will stay “potted” and can be relocated the following spring but reap the benefits of the earth’s insulating factors.
   -Tuck it between large grow bags that are full of soil. I use large 55 & 65 gallon grow bags for vegetables and tucking the pot between these has served me well. The additional walls of soil is similar to increasing the size of the pot and adds a layer of insulation. The flexible nature of grow bags and soil allow it to fit the contours of the trees pot.
   -Tuck the potted tree into a sheltered sunny corner and mulch heavily, over and around the pot. This has also worked well for me. Straw is a fantastic economic option.

5. Do not fertilize

   Over the winter growth slows and you want to encourage that. If you fertilize in the fall/winter you can trigger growth when the plant should be conserving energy to survive the coming months.

6. Continue to water

   A potted tree will dry out faster then an in-ground tree and has less opportunity to capture snow due to its reduced root spread. Do not allow the planter to become soggy but do give the tree an occasional drink. How much will depend on size of tree, size of pot, how much wind it is exposed to and type of tree.

7. Transplanting

   Ideally you want to do this in early to mid fall (or earlier) so that the roots have chance to settle in and it can bounce back from any potential transplant shock before the cold of winter hits. With this timing the tree will have the opportunity to build a strong base to support itself through winter.

A potted Japanese Umbrella Pine ‘Sciadopitys verticillata’ decorated for Christmas

So if you have a pot, and you have a location in mind now you need a tree!

Trees to avoid planting in containers

1. Boxwoods.

   I know, they make all the lists and are usually listed number 1 as they are ‘easy’ and adaptable. But southern Ontario is dealing with boxwood blight and although there are more resistant varieties (“Green” such as green gem, green mountain etc), I would recommend avoiding.

2. Cypress.

   Although adorable and easy to come by around Christmas, they are barely hardy in the most southern areas of Ontario. Once you move north of the 401, they are no longer hardy.
   Dwarf Falsecypress. I’ll lump these in here too as they are hardy to zone 5 but aren’t hardy enough for long term container use.

3. Anything without the term little or dwarf.

   Although I like to break my own rules. My Japanese Umbrella Pine, Sciadopitys verticillata, is an example of this. It has a very slow growth habit but it’s mature size is 30 feet tall! It is such a slow grower, and also a unique specimen (a fossil tree like gingko’s) that sometimes you need to experiment!