ack...also...when are you supposed to trim these? fall or spring? how far down should I cut?

hmmm..... grew it once but treated it like an annual
It flowers well late summer/fall. Only trick I can think of is getting enough "hardwood" to allow flowering.
That means good growth . Mine were in very rich soil
hope this helps

Hmmm, it is pretty sandy overall, but considering the rate and size to which it grew, it couldn't have been TOO bad of soil... :( oh well, we'll see how it does next year... anyone else know whether to prune them or when to?

I lived in MN and these grew like crazy. It does take a year or two for them to really get going and then you can expect tons of blossoms. I figured the first year was just for the roots so sounds like you are already ahead of the game. I would wait until Spring to make any pruning decisions - the plant will make it clear what needs to be cut away.

Gaoyuqing, how much Sun is your SLV getting?
They need Full Sun more than good soil.
I planted mine in what turned out 2B mostly shade.
It didn't do much so I moved it after 2 or 3 years.
Moved it to a Full Sun location and it bloomed profusely.
It was really Beautiful!
My soil here is heavy clay and I didn't ever fertilize it.
But it got too big so I gave it away after 3 or 4 years.
Seems like it died back every winter.
It bloomed so late that I waited until Spring (and warmer weather) to cut it back - heavily. I only left enuf of the old vines for new vines to climb up on.

If you are trying to grow these in semi-shade and want white flowers in fall - try growing Sweet Autumn Clematis.
Best Wishes, Barbara.

Ok great thanks :) both vines, especially considering where they've climbed and spread to should be getting plenty of sun. One could get more, but it turns out that when laid across the shingles of my little garden gate, the sun cooks them :P will try some different things next year and wait it out a bit to see if I'll get more of a blooming next year. Thanks for the tips and info, that was just the sort of info I was looking for :)