When Is a Flower an Inflorescence?

Learn how to tell the difference between various flower structures, from catkins to corymbs.

By Oscar H. Will III
Fall 2017

When identifying flowering plants, we often examine the showy flower of those plants to know what they are. I mean, everyone knows a sunflower by its flower, right? But wait: The sunflower’s flower is actually an "inflorescence" because it’s made up of many individual flowers, arranged in such a way as to look like a traditional flower with a radial array of petals.

Take a close look at the sunflower’s inflorescence (see photo, above) and you’ll find hundreds of individual flowers, or florets — both ray flowers (those that comprise the edge and have petal-like corollas) and disc flowers (those that form the center and have very small corollas). The entire structure is borne on a receptacle, which is supported by the flower stalk (peduncle). Folks tend to call the entire structure a flower, but technically it’s a class of inflorescence known as a "capitulum."

Next, look closely at the yarrow inflorescence. It, too, will be a collection of tiny flowers arranged in an inflorescence structure known as a "corymb," which organizes a number of small clusters of ray and disc flowers in a flat-topped fashion. The yarrow is particularly interesting because its inflorescence effectively organizes a cluster of smaller inflorescences, individually known as a phyllary.

Photo by Kyra Haas

Keying out inflorescence types can be puzzling, vexing, and enjoyable all at the same time, especially if you enjoy solving logic riddles! Pictured on the pages that follow are some types of inflorescences that you encounter routinely, but this is really the tip of the iceberg.

Keep in mind that some inflorescences are distinguished by the order in which the individual flowers bloom. In many cases, the individual type of inflorescence can be further modified with regard to symmetry and overall shape. And this makes classifying them an interesting puzzle. For example, the tomato may deliver simple individual flowers or inflorescences generally of the raceme or cyme types. If we take a closer look at the cyme type, there can be simple cymes and dichotymously-branched cymes where the apex of the peduncle branches more or less equally into two. This is especially evident in some of the cherry or grape tomato cultivars where the fruits are borne in clusters. Next time you harvest fruit in your garden, or pick flowers for the table, take a moment to determine if that “flower” is really an inflorescence. If it is, see if you can identify it by type.

Capitulum

Inflorescence with tightly packed flowers that lack pedicels (individual stalks) born on the enlarged tip of the peduncle (main stalk of the inflorescence).

Examples: marigold, sunflower

Catkin

Inflorescence composed of flowers without pedicels borne laterally along the peduncle. This is a modified spike inflorescence. The flowers closest to the main branch form first.

Examples: willow, birch, walnut

Corymb

Inflorescence composed of clustered flowers or phyllaries with unequal pedicel length attached to an unbranched peduncle. This structure most often looks flat-topped or slightly domed. The outside flowers form first.

Examples: yarrow, dogwood

Cyme

Inflorescence composed of flowers with pedicels borne on the ends of the central peduncle and its lateral branches. It usually appears as a flat-topped structure. The flowers closest to the peduncle form first.

Examples: baby’s breath, chickweed

Spike

Inflorescence composed of flowers that have very short pedicels or lack them altogether, attached laterally to the peduncle, which is typically unbranched. The bottom flowers form first.

Examples: spearmint, plantain

Umbel

Inflorescence composed of flowers with pedicels that are all attached at the tip of the peduncle. Umbels often appear spherical or flat-topped. The outside flowers form first.

Examples: onion, parsley, geranium