Giant Sunflower
Helianthus annuus
Also called mammoth sunflower
Giant sunflowers (Helianthus annuus) are very tall annuals grown from seed after the last frost in full sun. Species plants can reach about 10 feet (305 cm), and some giant cultivars grow taller.
Quick facts
- Germination time
- 7–14 days
- Germination temperature
- 21–24 °C70–75 °F
- Sowing depth
- Not yet verified
- Spacing between plants
- 61–76 cm2–2½ ft
- Days to maturity
- Not yet verified
- Sun
- Full sun
Safety for cats and dogs: not yet verified.
By SeedBattle EditorsUpdated
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01 · Overview
About giant sunflower
Giant sunflower is the very tall form of the common sunflower, Helianthus annuus, also called mammoth sunflower. It is a warm-season annual that completes its life cycle in one growing season. Plants of the species reach about 10 feet (305 cm) tall. Some giant cultivars grow taller: ‘Mongolian Giant’ reaches 12 to 14 feet (366 to 427 cm) and has 16 to 18 inch (41 to 46 cm) flower heads. Giant cultivars can reach 8 feet (244 cm) or more and often need staking. The seeds, petals and young flower buds are edible.
Growing details
- Start indoors
- Not yet verified
- Sow outdoors
- At the last frost
- Water
- Not yet verified
- Containers
- Not yet verified
- Height
- 0.5–3 m1½–10 ft
- Spread
- 46–91 cm1½–3 ft
- Frost tolerance
- Not yet verified
- Soil pH
- pH 6.0–7.5
- Life cycle
- Annual
Sowing timeline
When to sow giant sunflower seeds
Counted from your area's average last spring frost date. Find yours from a local weather service or garden center, then count back or forward.
- Germination time
- 7–14 days
- Germination temperature
- 21–24 °C70–75 °F
02 · How to grow
How to grow giant sunflower from seed
Sow seed outdoors after the last frost, once the danger of frost has passed and the soil has warmed above 50 °F (10 °C). Give plants full sun, meaning 6 or more hours of direct sunlight a day. Plant in a sheltered spot, since tall types can be damaged by wind.
Sowing depth. Some growers sow about ½ inch (1.3 cm) deep. Others sow 1 to 2 inches (2.5 to 5 cm) deep.
Germination. Seeds come up in 7 to 14 days at 70 to 75 °F (21 to 24 °C). Another estimate is 1 to 2 weeks.
Spacing. Thin tall varieties to stand 2 to 2½ feet (61 to 76 cm) apart. Larger branching types can be spaced 18 to 24 inches (46 to 61 cm) apart.
03 · Care
Caring for giant sunflower
Water regularly after germination to help plants establish. Once established, plants tolerate drought. Regular watering through flowering and seed set gives the best results.
Use a balanced fertilizer and avoid high-nitrogen types, which push leafy growth over flowers. Tall plants may need stakes to stay upright in wind.
At a glance
How giant sunflower compares with other flowers
Worked out from the growing facts of the 79 seeds in Flowers on SeedBattle.
- Giant Sunflower
- Each of the other flowers
- Their average
- Germination timeSprouts sooner than average
7–14 days
Average for flowers: 12 days, from 42 other seeds
- Germination temperatureWarmer than average
21–24 °C70–75 °F
Average for flowers: 21 °C70 °F, from 40 other seeds
- Spacing between plantsWider than average
61–76 cm2–2½ ft
Average for flowers: 34 cm13 in, from 13 other seeds
- HeightTaller than average
0.5–3 m1½–10 ft
Average for flowers: 69 cm2½ ft, from 68 other seeds
04 · Problems
Common giant sunflower problems
Caterpillars, slugs, snails and beetles can chew the leaves. Plants may be affected by powdery mildew, leaf spots and rust. Slugs and snails also feed on young plants. Birds and squirrels can dig up seeds and eat seedlings. Sclerotinia rot can cause brown rot at the base of the stem and wilting. Because the plant self-seeds, it can become weedy.
05 · Harvest
Harvesting giant sunflower
Seeds reach maturity about 70 to 100 days after planting. Harvest when the seed heads face downward, the inner petals rub off easily, and the back of the head is lemon-yellow. Dry the heads in paper bags or cheesecloth in a dark, dry, airy place, and the seeds are ready in a couple of weeks.
Giant Sunflower questions
How long do giant sunflower seeds take to germinate?
Seeds usually come up in 7 to 14 days at 70 to 75 °F (21 to 24 °C).
When should I plant giant sunflowers?
Sow seed outdoors after the last frost, once the danger of frost has passed.
How tall do giant sunflowers grow?
The species can reach about 10 feet (305 cm). ‘Mongolian Giant’ grows 12 to 14 feet (366 to 427 cm) tall.
Can I grow giant sunflowers in a pot?
Containers are recommended for shorter or dwarf types that top out at 3 feet (about 91 cm), not for giant types.
Quick answers
More giant sunflower questions
Short answers taken from the growing facts on this page.
How far apart should I plant giant sunflower?
Space the plants 61–76 cm2–2½ ft apart.
What temperature do giant sunflower seeds need to sprout?
Giant sunflower seeds germinate at 21–24 °C70–75 °F.
Is giant sunflower an annual or a perennial?
Giant sunflower is an annual.
What soil pH does giant sunflower need?
Aim for soil at pH 6.0–7.5.
Side by side
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