I am no expert, but I do grow several varieties of butterflyweed. I think you have been too nice to it. I would go out & get some plain garden soil, clay if you have it, & repot them. You could probably plant outside directly, but I am not familiar with your temps. Butterfly weed does not like fertilizer, & I agree, the sawdust may have the culprit. Maybe not so much the ph, but in making the soil too nice.

I would also check how wet the soil is. I have had this happen from the roots drowning in wet soil.
Good luck with youe seedlings.

I cover my newly transplanted plants for out doors with top of plastic water bottle for rain or too much wind. Use 2 liter coke for larger plants. Just cut off right below the neck of bottle .Olay! mini "greenhouse"
Nancy from KY

did you start them indoors or outside? when you repotted did you move them? i start asclepias outdoors (a la wintersowing). i never repot--just plop them in the ground when they have a pair of new loeaves. that doesn't exactly help you, but if you started them indoors i would move them outside with some protection, wean them over a week from the protection and get them in the ground. they dont like being moved (so i would not repot them in the future), and they have a taproot; be careful of breaking the roots.

Thank you, those of you who replied to my post. I have concluded that my problem was probably just in repotting them as "inthegarden" suggested. I knew they were fussy that way, but have done it successfully with others so thought I might get away with it. I started them in the greenhouse in soilless (sp?) mix and moved them into individual cells when they had 4 leaves. That was when I began to see some failing. At that point I put them into my standard paper pots in my wildflower mix,(and outdoors) but they never really recovered. Many have dried up, but about half are hanging on, about three inches tall and discolored. Some have tried to branch but they are pretty sad looking plants. I plan to plant them out now, hoping that at least some will eventually recover, but next time I will definitely plant them directly in soil. I am trying to grow them for sale, so can't just plant them in the ground.