Ask Mr. Smarty Plants

The Lady Bird Johnson Wildflower Center Native Plant Database description of the Clematis texensis (Scarlet clematis) gives instructions for propagation of the plant.   Gather seeds when they are no longer green but before the cluster of achenes completely dries and drops the seeds to the ground.

For a detailed description of seed treatment read this recent answer to a previous Mr. Smarty Plants question.

"There are several ways to do moist, cold stratification and you can find any number of sites explaining them if you Google 'cold stratification'.  The whole idea is to mimic the environmental conditions the seeds would experience as they overwinter on the ground where they fell from the parent plant.  Seeds from many plants won't germinate unless they have this 'cold shock'.  Here are several simple methods:

1.  Use a clean jar with a lid, a resealable plastic bag, or some sort of non-rusting container that will fit in your refrigerator.  Mix your seeds with a small amount (about 3 parts media to 1 part seeds) of moist sterile potting medium (e.g., sphagnum moss, fine sand, vermiculite) and put the mixture into your container in your refrigerator.  Leave a small opening (punch holes in the jar lid or leave the plastic bag partially open) to allow gas exchange—seeds do need oxygen to survive and germinate—and be sure that the mixture isn't too damp or it will grow mold before the seeds germinate.  After a couple of weeks check to see if your seeds have begun to germinate.  You should then check them at least once a week until they do begin to germinate.  You may need to mist the mixture occasionally to be sure that it doesn't dry out.  When they are beginning to germinate, you can then plant them in small pots until it is time to put them into the soil.

2.  Alternatively, you can thoroughly dampen a couple of paper towels, wring out excess water, lay the seeds on one side and fold the other part of the moist paper towel over them.  Store the paper towels in a plastic bag or container with a small opening in the refrigerator.  Be sure the damp towel is making good contact with the seeds.  Check often to be sure the paper towels don't dry out.

3.  Sean Watson, the Wildflower Center's nursery manager says if you have enough refrigerator space you can even sow the seeds directly into 4-inch pots or seed flats, water them and then put them into a large ziploc bag to keep their environment moist while germination occurs (but be sure to leave a small opening to allow oxygen to get in). After the recommended time of stratification (misting the pot/flat in between), he takes the flat/pot out, puts it in the greenhouse and sees if anything starts to germinate (that way he's not disturbing the seeds at all). If he doesn't see any germination after a week or two, he puts the pot/flat back in the fridge for a couple of weeks, and then back to the greenhouse—repeating if necessary.

Sean also says that he usually collects clematis seeds and sows them in a container outside to overwinter. Some may germinate the following Spring after mother nature has naturally stratified them (more may pop throughout the summer/fall if kept moist, and even more the next Spring). This is just a more passive method for the busy grower!"