Do not cry! It will definitely come back. Roots are the most important part of the Clematis.

Personally, I'd move it now. First, because it is much easier without the top growth. Second, if it annoys your neighbor, it is not guaranteed that he/she will not spray it with Round Up "by mistake"...

It will take some time to re-establish at its new location. Try to keep the soil on the rootball if it is possible. Plant the crown 3-4" below the surface.

It will be fine.

I know from prior experience that you can move them now but fall is better. Our suddenly dry summer is really hard on them.

I protect mine from rabbits and overly-energetic weed-whacking husbands with a cage of wire mesh around the bottom 18" or so. Unless the neighbor really hates the clem and wants to harm it, that's what I'd do. It would be easier for you and the plant not to have to move it. And everyone is right, cutting it back shouldn't hurt it at all - it will probably respond by putting out many more stems and blooming lower down than in the past.

I had a very similar experience but it was a tornado that took Sweet Autumn down, arbor & all. I don't know much about Henryi but SAC is a vigorous climber & it was 2 year after, when it was suggested I should cut her down to 20" in the Fall, so as not to catch the winds of winter. Sure enough a late tornado came thru!

I dug her up & parked her in a huge pot ~ created a 6' tall-wire-fencing around ~ her vines/blossoms poured out of the circular fencing like bubbly champagne! I just now set her in a more permanent home in mid May & she's vigorously climbing up her new garden-put-together wire arbor!

I'm sure your Henryi will do well ~ I fear the thought of *round up mistake* by Alina!

I'm not sure there is any need to move it. The OP clearly stated the cutting back was done by mistake, not intentionally, and it is only an assumption the neighbors are unhappy or annoyed with the location of the vine. And a well-established clematis vine growing in close enough proximity to a tree to grow up that high into it will not be easy to move regardless of the season.

Yes, it will regrow.....it is nearly impossible to kill a clematis by too heavy or inopportune pruning. The PNW climate and weather is generally very encouraging for clematis of any kind. You may not see a huge amount of regrowth this season, but continue to care for the vine as you would had it not been cut back. The unintentional pruning now may just result in a stronger, fuller plant in the future.

Years ago, 44 to be exact, My neighbor was burning off his bermuda and caught my Mme Baron Veillard and Mme Eduard Andre on fire(early March, before I had cut them back) They burned to the ground. I was sure they were dead. They came back better than ever. I still have Baron Veillard, blooming now and she is still my most vigorous clem. They can take a lot.

Thanks for all the good advice and encouragement. I am inclined to move it because I foresee more clean-up work and other uncertainties in its current location, including possible replacement of the old wire fence it used to climb on. My plan is to relocate it to where it can beautify my ancient and not terribly attractive apple tree.

My neighbor felt horrible about having cut it. It had been camouflaged by the blackberry brambles they were clearing and she didn't recognize it or realize it was mine. They are actually the best of neighbors and they love my garden. When they eventually get theirs going, it will include a number of divisions and offspring from my collection.

Just a thought about the putative id. Does anyone else think it looks like C montana rather than C henryi? In which case a radical haircut every few years is just what it likes.

I think you may be right flora. Hard to tell the size of the blooms but I don't think they're large enough to be Henryi.