Usually in early May we see healthy growth on many spring-blooming perennials like Peonies and Iris and we're anxiously awaiting their glorious display of flowers. This year at the research center our Peony plants are big and strong, but many of the flower buds show signs of freeze damage from the cold temperatures last month.
Most freeze damage occurs long before the buds are visible. It usually happens when the cells are first beginning to form the bud in the newly emerging shoot. It can also happen when the buds are extended and visible, but usually the danger of freeze has passed at this point in the season. Once the buds are damaged, the flowers will not form.
On other perennials like Iris, the flower won't necessarily be a total loss from freeze damage. Instead, the flower spike is often considerably shorter. So if your Iris seem smaller this year, last month's temperatures could be the cause.
We run the annual and perennial flower trials for Kansas State University