If it has been a while since your high-school botany lessons, you may be unfamiliar with a lesser-known flower part -- the calyx. Overshadowed by more recognizable structures, such as flower petals and stems, the calyx is still an important part of most flowers. It wraps an unopened flower bud like a gift and unwraps itself to allow the bud to burst forth into flower.
You're probably familiar with the green leaflike structures that bind the flower buds of a rose (Rosa spp.). These structures are individually called sepals, and they collectively form the calyx. The calyx forms at the base of the flower bud, around the receptacle, which is a round structure that sits atop the flower stem. Each sepal in the calyx is generally green, resembling a tiny leaf, but sepals may be brightly colored and resemble the flower petals. Roses grow in U.S. Department of Agriculture plant hardiness zones 4 through 10, depending on the cultivar.
The calyx protects the developing flower bud by providing a barrier against weather, insect pests and diseases. The sepals that make up the calyx contain chlorophyll, like the leaves, so they can photosynthesize. Brightly colored sepals, which are called tepals, attract pollinators that fertilize a plant’s flowers. After the flower buds open and the flowers are fully formed, some sepals wither and die because the calyx has fulfilled its function.
After a flower bud has matured to the opening stage, the calyx responds to environmental cues, such as temperature and light, to know precisely when to trigger its release from the flower bud. The corolla, which is the collective term for the flower petals, may open rapidly as the calyx unzips itself all at once, or the calyx may gradually peel down as the flower opens in stages. Almost simultaneously, the calyx of the California poppy (Eschscholzia californica), which grows in USDA zones 6 through 10, splits open as the flower petals unfurl. A day lily flower (Hemerocallis spp.), which grows in USDA zones 3 through 9, opens at midnight because of the way its calyx responds to light and dark.
A day lily has a somewhat indistinguishable calyx because it’s disguised to look like flower petals. The additional flash of color from a day lily's showy sepals attracts pollinators to its flowers. Likewise, Lenten rose, also called hellebore (Helleborus orientalis), which grows in USDA zones 4 through 9, is another flower whose calyx is disguised as lookalike flower petals. Because Lenten rose flowers have a nodding habit, the color on the underside of the calyx is visible, which helps attract pollinators during the cool late winter and early spring weather when the blooms open.