That happened to mine last year. I later found out the neighbor on the other side of the fence had sprayed roundup to get rid of his bindweed. It almost killed my mature Jackmanii. Could it have been exposed to an herbicide? I also noticed black aphids on my Jackmanii this year that was damaging the flower stems. You might check it for insects. Do you have clay soil? I would keep it well watered and no more fertilizer this year and hopefully it will rebound next year.

I would trim it back at least by 50%-; forget todays blooms-water it, spray it for ds and insects-- fertilize it--Argoflash by Wayside and let it set stronger roots for next year. Just leave it be and by next year it will knock your soxs off-it needs a rest, and nourishment. (Another opinion)--dazi

Thank you both for your suggestions. I also thought of another possibility....next year when 20 stems come up, cut out 10 of them, making the roots put all the energy in half the stems....(think it will work?) I know it isn't herbicide, we live in a rural area and neighbors are quite a distance away, and other plants haven't been affected...and I don't use Roundup. I don't see any insects. I will keep watering well, give it some fertilizer and see what happens next year!

I loved your picture of your garden with the Jackmanni spilling over the white picket fence :-) Is that what Argoflash does?

Sometimes it seems to me, a plant needs something different to give it a jolt or as I say; "a kick in the pants to get moving". I gave my beauties a dose of Argoflash this spring but as you see this one is the only one that leaped out of its skin. I think yours might benefit from a change from its regular feeding routine.Something totally different.
Are the stems on your Jackmani so thick that they are preventing your plant to thrive? I don't know if that happens. I've never heard that they need to be thinned out. Do clems last forever? or do we just expect that they will. 10 or 20yrs? hmmm. I don't know...Also when you get down to the base of him, check out if any voles, moles,whatever could have tunneled underneath your clem and destroyed some of the roots. In any case you should have enough of a good sized vine that it will come back again. Just my thoughts and I'm sure not an expert. Good luck!

Jackmanii plants have been known to live over 100 years.