In a garden, many prairie plants and many wildflowers in general tend to get too tall, grow too quickly, and fall over. The soft soil in gardens doesn't help. plus the large size of the plants, soft stems, and young root systems common in gardens all add to the problem. The main problem is that too much fertilizer and too much water and too little competition allow the plants to grow too much.

To avoid this I would never fertilize prairie plants in a garden and try to keep the soil very lean. However, if the garden has been used for gardening or as a lawn you may already have too much fertilizer and the problem may continue for years. Avoid watering even if your plants wilt, unless you have young plants that might not yet be able to survive a hard drought. If you can't avoid excessively fast growth, you can cut most plants back without seriously delaying flowering if you cut a couple of months before flowering. This means it may be too late to cut back plants this year, but maybe next year it will work. I cut a few of my oversized plants back to about 2 feet tall once they have grown to about four feet tall, typically in June.

frequent light watering.
and
small amount of native 'thatch'.

froggy

Do you have grasses? I noticed how much more my plants are supported by the maturing grasses this year.

Vera

Thank you for the replies -- they all make sense. I planted everything last fall and at the time thought that a bit of fertilizer would help the roots get established. I haven't watered them at all -- letting nature take care of that and we've had some decent precipitation in Chicagoland at the start of the season. I also mulched the entire area to help retain moisture throughout the season. I did install grasses; however, I primarily have them situated behind the perennials as a backdrop, so they have support on one side but not on the other three. I started to assume that perhaps I need to install more grasses or other like-sized perennials to add support and the needed competition for the nutrient-rich soil. A follow-up question: In light of the situation, I recently staked the compass plants hoping that would help them retain a turgid (sp?) structure, but one that had been flowering now has its flower wilting. Was I wrong to stake the plant?

The stem may have been damaged when it fell over, which could cause the problem you describe. You were not wrong to stake. I don't think staking would hurt them.