The Differences in a Black-Eyed Susan & a Sunflower

About Black-Eyed Susan

• Black-eyed Susans are a group of about 30 native wildflower species belonging to the genus Rudbeckia. Sometimes also called coneflowers because of the flower’s prominent central dome, they prefer open meadows in full sun, and as they are tolerant of longer periods of drought, they are quite forgiving of neglect in the garden. Some species are short-lived, growing as annuals or biennials, such as the popular Gloriosa daisy (Rudbeckia hirta) and the brown-eyed Susan (Rudbeckia triloba). Of perennial black-eyed Susans, the most widely available type is the cultivar “Goldsturm” (Rudbeckia fulgida “Goldsturm”), which is hardy from U.S. Department of Agriculture zones 3 through 9.

About Perennial Sunflower

• Most of us probably call to mind a plate-sized flower with large seeds when imagining a sunflower. Perennial sunflowers lack the huge flowers of their annual cousins, but make good meadow-garden plants because of their vigorous growth, impressive heights and ability to withstand abuse. Like black-eyed Susans, many of the 38 perennial species of this native plant feature bright yellow flowers 2 to 3 inches across atop strong, hairy stems. Most grow well between USDA hardiness zones 6 through 9, though some are hardy down through zone 4. They require full sun but will grow well on a range of soils from damp to dry. Two notable selections include “Lemon Queen” (Helianthus “Lemon Queen”), a popular cultivar with light yellow flowers, or Jerusalem artichoke (Helianthus tuberosus), which has edible tubers.

Primary Differences

• The primary distinguisher between black-eyed Susans and perennial sunflowers is in their overall height. “Goldsturm” is compact, growing from 2 to 3 feet tall, and even wild types typically grow to only 5 feet or less. Exceptions are “Herbstsonne” (Rudbeckia nitida “Herbstsonne”), which can reach 8 feet tall and is hardy from USDA zones 2 through 9, and giant coneflower (Rudbeckia maxima), which reaches 8 feet and has an unusual, pointed central flower cone. Most types of perennial sunflower, by contrast, grow from 5 to 8 feet in height, with some reaching 10 feet or more. Most perennial sunflowers require division every two to three years, while black-eyed Susans rarely require division except to keep the plant from spreading too far.

Subtle Identifiers

• As indicated by their common name, most types of black-eyed Susans have a dark central flower eye, while most types of perennial sunflower have pale or green flower centers. Many types of black-eyed Susans have reflexed, or downward-facing, petals, while the petals on sunflowers are often daisylike, held perpendicular to the ground. Because they are both members of the Aster family of plants and share many key characteristics, distinguishing the two can be extremely tricky and can sometimes only be accomplished by using a plant identifier key or the help of a trained botanist.