Have 17″ x36″ planters 17″deep patio planters with clematis in each. Very small drainage holes in bottom.
Live in zone 5. Clematis planted this past spring. They did very well. Climbing up Pergola posts.
Not sure how to winterize them.
Thank you for contacting the Toronto Master Gardeners with your Clematis question.
I cannot imagine anything more beautiful and relaxing than sitting on a deck surrounded by not one but two flowering Clematis. They can certainly be over wintered in their planters with a bit of care and preparation.
Clematis grow in zones 4 to 9. The rule of thumb for over wintering perennials and shrubs in planters is that the plant should be hardy one to two zones colder than your zone, which in this case is 5.
To ensure a greater chance for survival of your two Clematis, you will need to deal with two areas of possible concern: root rot and freeze/thaw cycles during the winter.
1. Root rot: Clematis roots to not like to sit in water or soggy. The soil needs very good drainage. You mentioned that your planters have very small drainage holes. If possible, you should try to enlarge them, perhaps with a drill. Raising the planters off the deck with pot feet or pieces of wood or flat stones will benefit not only your plants but also protect your deck. The drainage holes can easily become clogged by a thin sheet of ice in early winter that will form under the planter if it sits directly on the deck.
Freeze/thaw cycle: this refers to the freezing and thawing of the soil during the winter. As it gets below freezing, the soil will freeze, a natural ocurance whether in the ground or in a planter. If your planters are positioned in the sun, which I am assuming they are since you are growing Clematis in them, the sun beating on them on a sunny day even on a very cold weather will cause the soil to thaw and, of course, freeze again over night. This breaks the fine roots of your plant.
There are a number of things you can do to avoid these cycles. You will want to insulate your planters. Sheets of styrofoam positioned around the planters would be helpful. Or you can place bags of leaves or mulch around the planters to protect them from the heat of the sun. In addition, you should add a four inch layer of mulch on top of the soil. Many people move their planters to a shady area to ensure the sun does not reach them.
With any luck you will have lots of snow which is nature’s blanket for our gardens.
A previous Toronto Master Gardeners’ post provided a link to a blog about winterizing containers by Larry Hodgson who is a horticultural blogger living in Quebec City. I am including the link here.
https://laidbackgardener.blog/2017/10/18/overwintering-hardy-plants-in-containers/
Good luck. I am sure you are looking forward to a profusion of Clematis flowers next year.