Held high above the ground on sturdy stalks, the glowing yellow halo of the sunflower is an unmistakable sight. Although annual sunflowers vary greatly in height, "Skyscraper" is one of the tallest seed-producing Helianthus annus in cultivation, reaching heights of more than 10 feet and producing 14-inch flowers. Growing giant sunflowers requires a significant amount of planning if you want them to achieve their potential. In the fall, send a sample of the soil from your sunflower bed to your university extension for testing so you know exactly what nutrients to add when preparing the bed.
Rototill the sunflower bed to a depth of about 2 feet, preparing an area with a 3-foot circumference per sunflower. Work in about 4 inches of compost to create a well-draining site. Incorporate a 5-10-5 slow-release fertilizer into the top 8 inches of the soil. Follow specific guidelines provided by your university extension if your soil requires significant adjustments to achieve a near-neutral pH.
Water the bed thoroughly before planting seeds directly into the prepared bed in early May. Push three to five seeds approximately 1 inch deep into the soil in groups, 20 to 24 inches apart. Apply slug bait around each clump of seeds to prevent slug damage to emerging seedlings.
Water the sunflower bed to keep it moist until the seedlings emerge, five to 10 days later. Thin the seedlings to the best three in each cluster once they've reached about 3 inches tall. At 12 inches, thin again to just two seedlings. Thin to the best seedling per clump when seedlings are about 2 feet tall.
Water "Skyscraper" sunflowers consistently as they grow; deep weekly waterings of about 2 gallons of water per plant mixed with liquid fertilizer is preferred to several shallow waterings. Mulch the plants heavily as they grow to help the soil better retain water.
Things You Will Need
- Slow-release fertilizer
- Slug bait
- Liquid fertilizer
- Sunflowers do best in hot, full-sun locations. Select a suitable site before planting.
- Do not allow sunflowers to stand in water. If after incorporating 4 inches of compost, the soil continues to drain poorly, add more until the drainage improves. Water-logged soil will result in dead plants.
- Learn2Grow: Helianthus Annuus
- WesternGardeners.com: Strike Gold with Sunflowers
- Master Gardeners Santa Clara County: Sunflower Trials 2001
- University of California Cooperative Extension: Sunflowers in Your Garden
- Renee's Garden: How to Grow the Biggest, Tallest Sunflowers: Tips from an Expert
- Master Gardeners of Santa Clara County: Growing Sunflowers in Santa Clara County
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