I don't think it's ligusticifolia. That usually just grows east of the cascade in the PNW. It's probably clematis virginiana, sweet autumn clematis or vitalba. Here is a website address that describes the difference between virginiana an SAC. The leaf shape is different. The vitalba should be done blooming now and it can get huge.

Here is a link that might be useful: clematis comparison

I am sure it is not C. virginiana or C. vitalba.

Clematis ligusticifolia is native to three neighboring states, Nebraska, Kansas and Oklahoma. It is sold in Nurseries and two varieties in addition to the native have been developed.

Since I posted, I have talked to a nurseryman. He believes Clematis ternifolia aka sweet autumn or Japanese Clematis is the local noxious weed.

I still wish I could find description of the leaves of these two very similar Clematis. Mine has five, smooth leaflets. It grew to the top of my 20 foot tall crepe myrtle and covered it. I made a choice. The clematis died.

I have some seedlings coming up on the other side of the yard. They are about tree feet long and are blooming. I will be keeping one of them. I will move it to another location where I need a large ground cover.

The leaves on SAC are smooth and rounded. They don't have prominent veins or serrations on the edges. They have 5 leaflets. That web site mentioned that. I can take a picture on mine tomorrow when the sun's out if that would be helpfull.

Hi here is a photo of SAC leaves. They are very smooth and rounded. They remind me a lot of lilac leaves. There are no serrations along the edge of the leaves. Mine is just started to put out buds and should be blooming in a week or two.

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If the leafets are rounded and smooth, I'd agree with flowerfan that it is most likely C. terniflora, SAC. The leaflets on ligusticifolia are pointed and with edge serrations and a much more defined pattern of venation. In my area there is also a difference in bloom time - ligusticifolia blooms earlier in the season than SAC and is now covered in seedheads. SAC is still almost entirely in bud with only a few scattered opened blooms.

SAC does have a reputation for profligate self-seeding and has become a nuisance plant in many areas. I've never had a seedling emerge from my monster plant but that may just be my location. OTOH, ligusticifolia has "appeared" in my garden spontaneously and can be quite bothersome if I don't catch it before it takes off. In my densely planted (OK, overly planted) garden, noticing these interlopers is not always very prompt :-))

btw, C. ligusticifolia is widely naturalized throughout the western US, from Canada to northern Mexico and from the Dakotas to the Pacific. Since it is so adaptable to climates and hardiness zones, I'd not be at all surprised to find it in Missouri, as well.