Description

Giant sunflower
Helianthus giganteus

Perennial. Also known as tall sunflower. 1.4 to 3 meters high. Needs staking. Intense flowering in August and September. Blooms in the first year. Yellow flowers attractive to bees, bumblebees, hoverflies, birds and butterflies. Native to North America. Hardiness zone 4. (Possibly up to zone 3).

Full sun to part shade. Prefers moist soil, but tolerates dry, sandy soil very well. Direct sowing in June-July or sow indoors 4-8 weeks before transplanting. 1 cm deep, 30 cm between plants. Low germination. (10-20 days at 18-21ºC). We have never succeeded in achieving germination rates of over 50% with this species of helianthus. Stratification is often recommended, but makes little difference. Seeds that require cold treatment.

Germination: 30-40% in December 2023
Ecological seeds produced at Catherine’s Ornamental Garden.

We grow helianthus giganteus in sandy soil in hardiness zone 4b, where it does not re-seed and its rhizomes are not invasive. We don’t know whether or not it can be invasive in warmer areas or more humid terrain.