The Differences in a Black-Eyed Susan & a Sunflower

Common sunflowers (Helianthus annuus) and black-eyed Susans (Rudbeckia spp.) may easily be mistaken for one another, as both plants bloom in summer with bright yellow flowerheads marked in the center by dark brown disks. There are plenty of differences between the plants, however. Identifying each plant's features allows you to figure out how best to utilize them in the home garden.

Sunflowers tend to grow taller than black-eyed Susans, generally reaching heights between 3 and 10 feet tall, with some varieties growing as high as 16 feet. Black-eyed Susans tend to be low-growing, with a bushy habit. Common black-eyed Susan species such as R. fulgida and R. hirta grow to be about 3 feet tall, with a similar spread. Black-eyed Susans have small, raised discs in the center of flowerheads, while sunflowers have larger, flatter discs. Sunflowers also have wide, large leaves that are rough to the touch and triangular, while black-eyed Susans have narrow, oblong or lance-shaped leaves.

Though there are perennial sunflowers, most ornamental garden sunflowers are annuals and may be cultivated during the growing season anywhere in U.S. Department of Agriculture plant hardiness zones 2 to 11. They will perish once frosts strike in autumn. The black-eyed Susan genus contains both annuals and perennials. Common garden varieties such as R. fulgida and R. hirta both may be grown as perennials in USDA zones 3 to 9.

Sunflowers produce edible seeds that may be harvested at the end of each flowerhead's life and eaten either raw or roasted. These seeds attract birds and squirrels, which are uninterested in black-eyed Susans. Seeds can be harvested by cutting off the flowerheads once the backs have turned from green to yellow and letting them dry out in a dry, warm place. The yellow petal-like florets on sunflowers are also edible and may be eaten raw as a garnish. R. hirta is not only inedible, but it may be poisonous to livestock such as sheep, pigs and cattle.

Black-eyed Susans and sunflowers have similar cultural preferences, with both growing in well-draining, moist soils in full sunlight. Black-eyed Susans are tolerant of strong wind, however, while sunflowers tend to develop tattered foliage in windy situations. Sunflowers over 3 feet tall should be loosely tied to a stake to keep them from falling over. Black-eyed Susans are often massed together rather than grown as specimens and do not have an upright growth habit that mandates staking. Perennial black-eyed Susans may be divided in the spring to increase vigor.