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Anyone can grow sunflowers.  In fact, sometimes you don't even have to plant them at all .. they sprout up all by themselves as "volunteers."  However, if you want to have the best chance of growing a giant sunflower, here are some gardening tips to help.

Sunflowers need full sun, 6-8 hours of direct sunlight per day—the more the better if you are trying to grow them to their maximum potential.

Choose a well-drained location, and prepare your soil by digging an area of about 2-3 feet in circumference to a depth of about 2 feet. Sunflowers are heavy feeders and deplete the soil more than many other crops—especially if you are growing them to reach a massive height—so the nutrient supply must be replenished each season.

Depending on your soil, you may wish to add, in addition to composted manure a slow release granular fertilizer—one that also contains trace minerals-- about 8” deep into your soil. Woodland plant geneticist Tom Heaton uses a mineral fertilizer such as Osmocote.  If you are an organic gardener, a balanced slow-release granular fish fertilizer can work.

Sowing Seeds - When and How
Since sunflowers that are planted in midsummer (July and August) often flower on shorter stalks, sow your giant sunflowers earlier-as soon as all danger of frost is past and night temperatures are above 50 degrees Fahrenheit both day and night.

To grow the largest sunflowers, it is essential to sow seed directly into the garden, rather than start them in pots. This is because sunflowers have long taproots that grow quickly and become stunted if confined. 

To sow seeds, water your soil, and press seeds 1” deep about 6-8” apart. Put snail bait in a circle around the clump if snails are a problem and cover loosely with netting to protect emerging seedlings from birds. If the soil is kept moist, seedlings will appear within 5-10 days.

When the plants grow to 3”, thin them to the most vigorous 3 or 4. When they are a foot tall, thin them to 2, and when they reach 2 feet high, select the best, most vigorous candidate. The point of this gradual thinning method is to ensure that you’re left with at least one good seedling in the event that predators damage any of the others. Remember, it's critical to thin back to the best single seedling if you're going for giant sunflowers. Leaving even several seedlings growing too close together will keep you from growing a giant in your garden.

Water and Feeding
Feed  often and water regularly. While the plant is small, water around the root zone, about 3-4” from the plant with about 2 gallons of properly diluted liquid fertilizer solution per week. For larger plants, scrape out a small doughnut-shaped moat about 18 inches around the plant and about four inches deep. Pour several gallons of properly diluted fertilizer into the moat every week.  Sunflower roots can grow to 4 feet below the soil surface. Avoid pouring fertilizer directly on the stems, since this can cause them to rot.

Another feeding method for larger plants is to make several holes by driving a steel stake into the ground about 3-4 feet deep and about 1½ feet from the plant. Fill the holes with properly diluted liquid fertilizer. Woodland plant geneticist Tom Heaton uses Miracle Grow or Schulz’s growing solution. Organic gardeners may want to use properly diluted fish emulsion or other liquid organic fertilizers.

Protecting your Giant
Be attentive to weather reports, especially, as your plants become taller and more top-heavy. When heavy winds are predicted, delay watering to reduce their chances of blowing over. Staking isn’t usually necessary for sunflowers, but it can be helpful when it's extremely windy or if they must be grown in conditions that are too crowded or in too much shade.

Reference:  Renee's Garden