Answer: According to Word Hippo (http://www.wordhippo.com/what-is/the-plural-of/clematis.html) the plural form of clematis is clematises. According to Wiktionary (http://en.wiktionary.org/wiki/clematis) it is clematises or clematis.
Here is how two renowned clematarians weighed in on the position: In Ernest Markham’s 1951 book titled The Large & Small Flowered Clematis, And Their Cultivation In The Open Air he wrote, “It will be noted that I have used the word clematis both as singular and plural. For this I offer no apology but the explanation that I have long considered the more usual clematises to be cumbrous if not an ugly word.”. In John Howells’ 1998 book titled Trouble Free Clematis: The Viticellas he wrote: “Plural is clematises but its awkwardness has led to its being discarded. Therefore ‘clematis’ is used for one or more than one plant.”
Since the singular word of this fabulous genus already opens its own can of worms with its many pronunciations and spellings of the word (https://www.clematisqueen.com/content/how-do-you-say-spell-clematis), I personally don’t feel we need to add any more obstacles to its name. So, I would like to encourage you too to use clematis for both singular and plural uses. If you are in doubt about this view try saying “clematises” three times fast and I’ll bet you will agree with Mr. Markham’s assessment that it is an ugly word better off abandoned.
When I asked my husband Ken what he thought the plural of clematis is, he said with a straight face that “Obviously, it’s two or more clematis”. So, it is my hope you will grow at least two or more clematis in your garden.