With heights reaching 15 feet or more, mammoth sunflowers are aptly named. Their towering stems and giant flower heads never fail to impress. If you want to add a splash to your garden, growing mammoth sunflowers is surprisingly easy with the right approach. Follow these tips for when, where and how to successfully plant mammoth sunflower seeds or starts.
Choosing Mammoth Sunflower Varieties
Many cultivated sunflower types can grow quite large, but some varieties are bred to reach truly mammoth proportions.
Recommended Large Varieties
Mammoth Russian: Heirloom type, seed hulls striped black & white
Mongolian Giant: Single or multi-headed, seed hulls solid black
Moonwalker: Pollen-free for cutting, 12+ inch blooms
Titan: Hybrid variety producing enormous flower heads
Mini Mammoth Options
If 15+ feet is too towering for your site, opt for “mini” mammoth varieties reaching 8-12 feet high, like Mini Mammoth and Sundollar.
When to Plant Mammoth Sunflowers
Sunflowers require full sun and warm soil to thrive. Plant them according to your area’s last expected spring frost date:
Wait 2-3 weeks after your region’s final frost date
Ideal soil temperature for germination is 55-75°F
Wait 1-2 weeks after the final regional frost
Plant hardened off transplants or seedlings after danger of frost
This ensures seedlings won’t perish from a late cold snap.
Conditions to Plant Mammoth Sunflowers
Site selection is key when growing giant sunflowers. Choose a location meeting these criteria:
- Sun Exposure : Pick a spot receiving full sun for at least 8 hours daily. Morning sun is ideal.
- Soil Conditions : Sunflowers need nutrient-rich, well-draining soil. Amend poor soils with compost.
- Air Flow : Allow ample space between plants for air circulation to prevent fungal diseases.
- Shelter from Wind : Protect young stalks from toppling over with windbreaks as needed.
- Room to Grow : Give mammoth varieties 1-2 feet spacing for roots to spread safely.
How to Plant Mammoth Sunflower Seeds
If starting mammoth sunflowers from seed, follow these steps for robust germination:
- Direct Sow Outdoors – Sow seeds directly in the ground 1 inch deep and 6 inches apart after frost danger passes.
- Indoor Start – Presoak seeds 12-24 hours before sowing indoors in peat pots 2-4 weeks before last frost.
- Optimal Soil – Ensure the planting area or potting mix is loose, crumbly, and nutrient-rich.
- Row Covers – Cover newly planted seeds with row cover fabric to retain warmth and humidity.
- Sunny Window – If starting seeds indoors, provide ample sunlight by a sunny window or grow lights.
- Thin Seedlings – Clip off weaker seedlings with scissors to retain the strongest plant every 6 inches.
Transplanting Mammoth Sunflower Starts
For a head start, plant purchased transplants or overwintered seedlings. Follow these tips when transplanting:
Harden Off – Set plants outdoors in filtered sun 1-2 weeks pre-transplant to toughen them up.
Wait Until After Frost – Ensure nighttime temperatures stay above 50°F before transplanting.
Bury Stems – Plant seedlings slightly deeper than they were originally growing to anchor stems.
Stake Immediately – Put stakes or trellises in place at planting to support mature stalks.
Avoid Root Disturbance – Transplant seedlings carefully using peat pots to minimize root damage.
Caring for Growing Mammoth Sunflowers
Once your mammoth sunflowers are happily established, maintain growth with:
- 1-2 inches of water weekly, supplementing with sprinklers in heat
- Side dressing nitrogen fertilizer monthly to feed fast growth
- Staking larger varieties before toppling occurs
- Pruning lower leaves periodically for airflow
- Inspecting frequently for pests like birds and beetles
- Harvesting mammoth flower heads for cutting when the backs begin to turn yellow
Planting mammoth sunflowers is a fun and rewarding experience. With minimal care, your mammoth sunflowers will rapidly ascend to stunning heights all season long.