Sensations Rainbow Boucle Problem

A friend picked out this yarn for me to knit a lengthwise stole/shawl in garter stitch. I used 10.5 needles to cast on 250 sts using long-tail and have done about two inches using #9 needles. The cast-on edge has very little elasticity while the body has a lot of stretch. I've tried several different types of cast-on and all have the same result. I'm concerned that the edge will not hold up in use. Any suggestions????