10-20' is conservative for Sweet Autumn....

All Clematis take several years to reach maturity. To speed them along as much as possible dig big well amended holes, plant deep and cut back hard the first three years.

I came to a conclusion that planting Clematis in the shade is not a good idea. I bought several so called shade tolerant/loving Clematis and planted them along the west side of the home - dappled shade with some direct sun after 2 pm.
All of them bloom sparsely and only once.

So, I have:

Some sun + dappled shade
Shade tolerant varieties
Much warmer zone than yours

and still have sparse blooming.
I think you should opt for some other plants.

I have 3 that have been growing along a tall kennel cyclone fence for years that I basically do nothing with. I cut them all down and off the fence in the fall. (Is this wrong?)

When you say " Cut them back hard" What do you mean and when?
Good info on the Sweet Autumn, thanks I'll keep researching.

Hi Deb, My Alabast is blooming now(early like everything else), has huge flowers. Located on north (front) side of house, always in shade, never gets any direct sun but has light. Bought it years ago at a large nursery in Cedar Rapids, instructions stated "plant in shade only - Type 1." It blooms on old wood. Blooms better here than at my previous house...not enough light there, too shady. I like it now.
Marlene

Cutting back hard means to cut all the foliage off to a 4-6 inches above the soil. I cut back in the late fall after frost has killed off the leaves and there is little chance there will be regrowth before spring. In other zones where there could still be growth people wait until very early spring.

Newly planted clematis are cut back hard for three years to help rooth growth. After that they are not cut back (type 1), cut back lightly (type 2) or cut back hard ( type 3). Some choose to cut back hard type 2 as well as it is easier or less unsightly. Here in my z:4 type 2s may freeze back to the ground depending on the winter. Cutting back type 2s hard means losing some of the early bloom which are often doubles.

Thanks, mnwsgal, that really helps. I have several and although I never knew it, they must be type 3. I never cut them back the first couple of years and they have done well. I always cut them back in the fall.
Thank all for your time! D

Deb, if you know what varieties you have you can look up the pruning info on Clematis on the Web. Some of the type 2 have interesting double blooms but only single blooms if cut back like a type 3. I cut some of the stems of my type 2s to the ground and the other stems are left and only trimmed back in the spring when I can see where the new growth stops. I want to see those double blooms.