Answer: I apologize for the delay in answering your question.
The clematis you have is a lovely one. It can be container grown in cold winter areas such as yours only if you are able to move it to a sheltered spot over the winter so that the roots can be protected from the extreme cold. The larger the container the better, sixteen inches deep would be a rule of thumb for minimum depth, a half barrel size or bigger would be best. The larger container makes it easier to keep the soil evenly moist as well. These plants tend to grow very slowly the first year, grow a bit the second year, and then come into their own by about year three.
As with any larger, longer lived plant, the root system of the clematis is ultimately quite extensive, it is also a bit fragile. For this reason I would not recommend planting other things in the same container with it. The clematis needs to be able to root as best it can during the establishment phase and planting around it could be a disturbance and introduce competition for the water and nutrients as well as the space in the container. To give you an idea of size, in the garden, in the ground, clematis are typically allowed a three foot wide patch for their roots so most containers will cramp them somewhat.
From your question I gather you are looking for all season color in that location. You may find in the long run that you would be happier with some colorful annual vines instead. I say this in part because of the desire for a long bloom period and in part because of the difficulties inherent in growing a clematis in a container in a cold winter area. Also, most annual vines are big plants and are strong competitors; I would be afraid they would outcompete and smother your clematis altogether if planted in the same container. Perhaps they could be grown nearby instead to try to avoid some of these potential problems.
You might try an assortment of annuals such as nasturtium, morning glory, cypress vine, moonflower, purple hyacinth bean, and so on. You could also plant some shorter continuously colorful annual flowers at the perimeter of the container to provide early season color while you are waiting for the vines to come into bloom.
Finally, pruning instructions for your Candida clematis. This vine will bloom on both old and new wood, so routine annual pruning can be done in one of two methods.
One method is to prune back just some of the stems to about two to four feet in early spring. This will stimulate new growth and bloom will occur on the new growth later in the season. Meanwhile, the remaining older stems will bloom earlier in the season. This method will prolong the flowering period for you but you will have fewer flowers at a time.
Alternatively, you can cut the whole thing back to about three feet in the spring and a more concentrated bloom flush will occur in mid summer.
Each spring you would also remove any winter damaged stems, cutting down to good wood. For new plants, the International Clematis Society recommends cutting all new clematis back quite short to a good set of buds the first spring to encourage sturdy long term growth. Routine pruning would begin the following year.
I hope this helps you work with your clematis and get the results you are hoping for.
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