Wintering Cold-Hardy Plants in Pots

Thanks to a person that sent me an email asking about what to do with his cold hardy Japanese maple in a pot sitting on his porch! It reminded me of a question that I got a lot.

How do I keep a cold hardy plant in a pot for the winter?

Thanksgiving and Christmas is a time that a lot of people have cleaned out their pots of dead annuals and want to replace them for the holidays with some sort of evergreen. My experience with buying and decorating with evergreens is that people and myself do not take care of or water like we should. The plants that we use in pots are cold hardy here, but as we talked before, there are other hurdles besides cold temperatures that could affect how the plant does.

Plant hardiness zone guide lines come from how hardy a plant is when planted in the ground. Plants in pots have their roots somewhat exposed to the elements. When a plant is in the ground the ground will freeze from the top down. The warmth in the ground will radiate upward. In a pot the roots will freeze from all directions- the top, the bottom and from the sides. Winter watering is important also because the plant will use the moisture as a insulation as well.

If you have plants in pots what is the best and easiest way to keep them?

The bigger the pot the better that the plant will winter as long as you water at the right time. A 14 inch pot is better than a 12 inch pot but not as good as a 16 inch pot. You get the idea.

If in the case of the person who has a Japanese maple he is trying to winter in a pot, I would suggest that this person bring the plant and pot into a unheated garage or shed in the coldest part of the winter, usually those months are December through February. The cold weather plants still need cold to get the plant to go dormant and can take some cold until the night time temps start to dip into the teens on a regular basis. Bringing it into the unheated garage or shed will change the zone you are trying to grow by a couple of zones. Inside the unheated garage the temperature is cold enough to keep the plant dormant but warm enough from freezing too much plus inside you don’t have the wind desiccating the branches or soil. Once brought inside the plant will still require a good soaking of water once a month.

The plant is now dormant and won’t need any sunshine so I wouldn’t worry about that. If the temperature looks good in the spring bring the plant back out and let the plant wake up naturally. You don’t want to have the plant push out its first flush and then set it outside. It would probably kill that first flush. It’s all about the timing. I leave my Japanese maple outside. Mine is also in a pot but the location that I have it gets a little of winter protection from the wind. A large shade tree gives it shade in the summer, but with the leaves off in the winter the sun hits that area keeping it warmer that if it were on the northside getting the cold wind and no sun.

Whether it be plants brought in or left outside, other things you could to is spray them with a antidesiccant. Those products will help the needles or branches retain their moisture and not dry out as fast. Some suggestions being Wilt-pruf or Wilt stop. These are not chemicals and they work very well. If the plant is outside, you could protect the root ball by healing in around the pot and roots. Keeping them outside out of the wind from the northwest will also help plants winter over. The wind and not the temperatures could be your biggest enemy.