My first was a Jackmanii, which flowered hugely~twice~the first year planted and each year thereafter, seeming to love my poor soil the best. That was 13 yrs ago and at another home I owned....when I moved, I took it with me and gave it to my mother. It now grows at the end of her porch here, still flowering hugely and covering the entire end of the porch each year.
She cuts it back to a foot tall each fall and it grows back to the roof again each summer. She and I have both had other clematis since then, but none like this Jackmanii and none have survived well in our poor soils like this one. It gets no fertilizer, it's in heavy clay, ridge top soil with it's feet exposed to the sun and chickens digging around it all the time.
LOVE the Jackmanii!!! Would love to try that Montana clematis if I could fine one here locally...I've seen those covering whole carports and they are just as lovely and hardy as our Jackmanii is.