Ask Mr. Smarty Plants

You may be eligible for the "I stumped Mr. Smarty Plants award!" We found an answer but it was talking about marigolds, but we think it's possible the same answer may be true for your plants. Read this article from eHow Why do Flowers on Marigolds Bend? From that article:

"Overhead watering may cause stem breakage and bending if blossoms become saturated; always water at the base of the plants.

Marigolds are heliotropic, meaning their flowers and leaves turn toward the sunlight whether the plants are grown indoors or out. Cells near the base of the blossoms control the bending, by shrinking or enlarging to move the flowers toward the light source. By keeping marigolds in full sunlight all day, you'll help avoid this survival mechanism and keep flowers from possibly bending away from main focal areas of the garden."

There are four species of the gender Vernonia (ironweed) native to Ontario: Vernonia fasciculata (Prairie ironweed), Vernonia gigantea (Giant ironweed), Vernonia missurica (Missouri ironweed) and Vernonia noveboracensis (New york ironweed). Symphyotrichum laeve var. laeve (Smooth blue aster) is also native to Ontario. You can follow each plant link to our webpage on that plant to see if you get any clues of what you might be doing wrong in terms of soil or watering of those plants. We did some extra research on heliotropism, which obviously occurs in many plants.

We checked all of these ourselves and concluded that all could do well in moist, fertile soils and full sun. About the only possibility that presents itself to us, since we cannot see the plants, is the caution against overhead watering, which can weigh down large heads of flowers, thus bending the stem.