Milkweed, All Grown Up

The pretty Asclepias (aka Milkweed) that I planted in April has been a wonderful little burst of color by the back door. But, somehow I'm not sure if it's growing correctly. It keeps getting taller, but it's very spindley. It doesn't seem like it would be sturdy enough for a butterfly cocoon--which is part of why I chose to plant it. I wonder if maybe I need to plant more of it to make it a bit thicker and more protective?