Whenever I do a lecture or a public event I am invariably asked “When do clematis bloom?” (i.e. what season will they be flowering?). My answer is that it is hard to predict exactly when because clematis are like roses; some are capable of only one crop while others can produce many crops over an extended period of time.
Even though the Clematis Queen does not have a crystal ball, I can tell you there are two main reasons that affect timing. The first is the USDA Zone in which you are located because this influences how much cold (or lack of) you will receive in your garden. The second is which particular clematis you select from one of the 18 different Clematis Groups (to find more information about Clematis Groups visit: J. van Zoest Clematis http://www.clematisinfo.nl/1280_1024/) because within these groups there are inherent factors that affect how they will grow in your garden. For example, some groups require a frost-free location while others will not bloom without a winter chill. To a lesser degree the following factors can have an impact on blooming times: the location of the planting site in your garden, how much light, how much irrigation, regular fertilization, the strength of the cultivar or species, etc. Each and every one of these aspects will have some impact on when and how often your clematis will bloom.
Gazing Into Your Crystal Ball
So, when you read a plant tag or a book that proclaims a rather exact time frame such as which month(s) a clematis will bloom, and yours is not performing as stated in print, you may be disappointed. Just remember clematis cannot read! I think those in the horticulture industry would be doing the gardening public a big favor if they would adhere to a less exacting time period. I predict that as the global climate changes we will see many more parts of our country witnessing clematis blooming at times not previously experienced.
The bottom line is that it is the clematis’ job to bloom so that they can ultimately produce more seeds which allows the genus to proliferate year after year. Mother Nature (i.e. how much heat, chill, rain, etc.) is constantly changing, so this means that there is no way one can accurately predict “precisely” when a plant will bloom. Just ask my countless “summer” blooming clematis that bloom every year in “spring”!!! Guess what…I do not question why this happens. Instead I am just thankful that they bloom in an area of the country were some misguided souls still do not believe they can.