When it comes to growing annual sunflowers (Helianthus spp.), many gardeners subscribe to the motto: the bigger, the better. If you agree, then choose the tallest sunflower varieties, such as "Mammoth," "Giganteus" or "Paul Bunyan Hybrid." To help these enormous monsters reach their fullest potential height, usually about 10 feet, adopt a few cultural techniques known to increase sunflower growth.
Grow sunflowers in an area that allows them to receive a minimum of six hours of sunlight a day. Before planting, cultivate the soil about 12 inches deep, incorporating about 4 inches of compost or well-rotted manure to encourage better water drainage and more fertile ground.
Thin your sunflower plants when they reach 2 feet tall so one healthy plant stands every 2 feet. If you are growing more than one row of sunflowers, space rows 2 to 3 feet apart and stagger the plants rather than lining them up with each other. Thinning sunflowers in this manner provides the remaining sunflowers plenty of room for root development, and it increases sunlight exposure within the patch.
Control weeds throughout the growing season so your sunflowers have exclusive access to the nutrients and water in the soil. Hand pull weeds, or carefully hoe or till between rows and plants one or two times a week for effective control. Alternatively, use a broadleaf herbicide that is effective for the weeds invading your garden and use as indicated on the label. Apply 2 or 3 inches of mulch around the sunflowers to discourage weeds from germinating.
Water regularly with 1 inch of water to maintain even moisture throughout the growing season. Even though they are drought tolerant, sunflowers thrive and grow larger with consistent moisture. If you added mulch around your flowers, it also helps conserve moisture within the soil.
Apply an all-purpose, granular fertilizer, such as one labeled 15-30-15, starting when the seedlings first emerge and reapply weekly throughout the growing season. Apply the fertilizer evenly around the base of the plants, watering it in afterward. Do not apply more than the rate recommended on the label. Otherwise, you can stunt growth rather than encourage it.
Support the flowers with stakes once they reach 3 feet tall, especially in high wind areas. Insert a 6- to 8-foot stake and loosely tie the plant with a soft material, such as cloth, every 18 to 24 inches as it grows. Ideally, insert stakes at planting time right behind the plants rather than later when you can disturb the roots.
Things You Will Need
- Tiller or spade
- Compost or well rotted manure
- Broadleaf herbicide
- Garden hose
- Botanical Interests: Sunflowers - Tips for Growing Tall Sunflowers
- Thomas Jefferson Agricultural Institute: Sunflower
- Burpee: All About Sunflowers
- University of Central Arkansas: The Effects of Miracle Gro on Sunflower Growth
- Alabama Cooperative Extension System: Sunflowers Add Big Splash of Color Along Fencerows
- UC IPM Online: Sunflower—Helianthus Spp.
- BananaStock/BananaStock/Getty Images