Hummm.....that sounds so strange. They may be forming little rootballs because of the inability to expand into your heavy clay soil but if you've got an amended hole about the size of a 5-10 gallon pot, there is no reason for them to do this.

Since you already grow other clems, you know that the first year they (the roots) sleep; (so expect nothing much as everything is happening under ground); the second year they (a few stems) creep; and the third year they leap (reaching in all directions, with plenty of blooms).

Is the soil location of this clematis somehow contaminated? The roots are obviously resisting growth upward for some unknown reason - have things grown in this spot before? Is it too close to a downspout from your house - thinking that perhaps too much water pooling into the amended soil where your rootballs are and not leeching out of that area due to the heavy clay beyond the amended hole?

Your sweet autumn sounds like it is stuck in a time warp of year one, over and over again, not able to get beyond the root ball formation.

Any chance you can move it's location as it apparently wants to stay alive, just can't seem to develop in it's current location.

Here's mine from late summer last year - just starting it's flush of flowers - a white morning glory seed found it's way up and thru the sweet autumn so that complemented the flower display in early fall. It outgrew it's 5 foot trellis and became so heavy that I had to improvise and keep it propped upward using an additional ladder - LOL
{{gwi:529078}}
Best wishes for a successful year with your sweet autumn.

Thanks for the answer sydseeds. Actually we do not yet have our house up. It should be up this fall so maybe I'll wait and transplant the SAC to that location. As far as contamination, I don't think that is a problem. The area where it's planted used to be a dog run. When we bought the farm, 7 yrs. ago, we worked up the area so I could start moving my plants up here. The other plants that were in that location did very well. The soil there is actually beetter than where we now have the garden. I just can't figure out why the roots always go back into the ball formation and the top never grows. Maybe if I move it near the house that will solve the problem. Thanks. Marg