Helianthus giganteus aka Giant Sunflower

Helianthus giganteus, or giant sunflower, belongs to the aster family, Asteraceae. This perennial comes from eastern Canada and the central and eastern United States. It grows in a temperate climate. Its wild sites include damp thickets, wet woods, and clearings. It is not native to Britain or Ireland.

The plant forms clumps with branched, spreading growth. RHS gives a garden height range of 8–13 feet [2.5–4 metres]. Its shorter account describes plants reaching 8 feet [2.5 metres]. Mature clumps spread about 3–5 feet [1–1.5 metres]. Plants reach full size in about one to two years. The narrow green leaves have a coarse feel. They fall away at the end of the growing season. Stems range from green to dark red.

Small flower heads appear in late summer and early autumn. Yellow ray florets surround a deeper yellow centre. The flowers suit cutting and bring late colour to beds. This tall plant fits cottage gardens and informal borders. RHS also suggests it for wildlife gardens and prairie planting. Other settings include coastal gardens, wildflower meadows, and borders beside walls. PFAF lists sunny woodland edges as another place to grow it.

Give it full sun for the growing conditions RHS recommends. The same source also lists partial shade as suitable. Choose moist soil that drains well and contains plenty of humus. Moderate soil fertility and a neutral to alkaline pH suit it. It can grow in clay, loam, sand, or chalk soils. A sheltered site suits its tall stems, which may need support. RHS rates it H4 for UK hardiness. PFAF gives USDA zones 4–8. Remove spent flowers to help extend the flowering season. Raise new plants from seed or divide established clumps. Division every two to four years also helps keep clumps strong.

Slugs and snails may damage the plant. Powdery mildew and sclerotinia diseases can also affect it. PFAF lists the seeds as food, either raw or cooked. Their small size makes them slow to handle. Dried seeds can be ground and mixed with cornmeal for bread. PFAF reports no known hazards, though this is not a detailed safety assessment. It lists no known medicinal uses or other uses beyond food. It also reports no weed potential.