Is it necessary to take precautions to protect your clematis from Old Man Winter? The answer to that question can be determined by the following two factors: the severity of cold where you live (i.e. your USDA Zone) and/or which group each clematis you are growing belongs to. I have listed some guidelines below for clematis winterization to help you decide whether you need to protect your babies.
The Large Hybrid Clematis and Viticellas
The majority of our garden’s clematis usually fall into the Patens, Jackmanii or Viticella Groups. If you live in USDA Zones 9 to 12, you most likely will never have to worry about freezing conditions, so there’s nothing you need to worry about for these clematis. However, if you live in USDA Zone 4 through 5 (possibly Zone 6), this is the time to start getting them ready for their winter hibernation (see Winter Protection below).
The Disappearing Clematis
The Heracleifolia, Integrifolia, Texensis and Viorna Groups are the magicians of the genus since they like to perform a vanishing act by dying down to the ground each winter and then reappearing in spring. They should be fine left on their own if they were originally planted properly which is 3” or so below the soil surface.
Frost Sensitive Clematis
Because of their sensitivity to the cold, evergreen clematis (those in the Armandii, Cirrhosa or Forsteri groups) are grown in the warmer locales of USDA Zones 9 to 12, which is usually above 30°, so they normally do not need any special protection. However, occasionally there can be an extreme climate change (such as temperatures dropping below 28°-30°) so this is when it would be prudent to cover them with a frost blanket, a soft fabric blanket or some burlap. One trick that citrus growers have been known to use is to wrap their plants in miniature Christmas lights for warmth until the danger of frost has passed, which should work for clematis as well.
The clematis in the Montana Group, which can be grown in USDA Zones 7 to 9, are a little less frost sensitive than the evergreens above and are better equipped by Mother Nature to endure a little more inclement weather. If you know a frost is imminent though, it wouldn’t hurt to take precautionary measures.
Since I live in sunny California I can only give you advice that I have read or heard from other gardeners that have had to deal with winterization in colder locales. Obviously, if you live in an area where you have never had to protect your clematis in the past, there is probably no need to start now, just keep on doing what you are doing.
Timing is crucial, so it is important to wait until the ground is frozen before you start tucking your clematis in for the winter. At this point you’ll want to add a substantial layer of mulch to form at least a 12-inch mound or more over the base of the clematis’ stems. This is done to insulate their roots and help protect them from drying winds and extremely cold temperatures. Your mulch can be made up of fine bark, chopped leaves, leaf mold, grass clippings, etc.
Don’t Jump the Gun
In spring, as the weather warms up and once the soil has thawed, you can begin to gradually remove the mulch from around your clematis. It is important not to remove the mulch too early or you can risk damage from a late or sudden cold snap. Helpful hint: if you happen to be growing forsythia in your garden, watch for its cheerful yellow flowers to bloom. This is usually a good sign that it will be safe to remove all the much.
Clematis Winterization for Containers
If you are growing your clematis in containers, you should relocate them into an unheated garage or shed where temperatures will remain between 25° and 40° during the winter. Give your clematis a thorough watering. Rewater only when the soil surface feels dry. Return the containers to the outdoors when the weather warms up. If you have containers that are too large or heavy to move, treat them as you would clematis growing in the ground by adding as much of the mulch I mentioned earlier that you can around the base of the plant.