Fortunately for home gardeners with giant sunflower aspirations, Tom has also focused on breeding the ultimate tall competition varieties ("Giant Edible Sunzilla" and "Heirloom Titan"). Each year, Tom saves his longest stalk and largest seed head of this variety in his garage to measure against the following years. He currently prizes a 17-foot stalk and a 24 in. seed head! In this article, Tom has generously agreed to share his method of growing giants like these in your home garden.
First – Choose the Right Variety
Despite the growing popularity of new color introductions, the sunflowers that grow largest are those most of us still imagine first when thinking of sunflowers. These have tall single stalks with big flower faces of golden yellow petals and chocolate brown centers that ripen into heavy heads filled with seeds that birds will love.
I asked Tom why serious competitors should grow his variety instead of traditional varieties of large sunflowers like Mammoth, Russian Mammoth or California Greystripe. He explained that the seeds in Sunzilla are hybrids, developed to be uniform and consistently reliable - so that when you thin seedlings, you needn't worry that you may have just uprooted your most potentially vigorous specimen. While older varieties often become top-heavy and fall over in wet or windy weather, Sunzilla has been selected for a strong, thick stalk to support its heavy head. In ideal growing conditions, it reaches heights of 16 feet or more and produces huge seed heads.
Site and Soil Preparation Are Critical
Sunflowers need full sun; see 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.
Work in a slow release granular fertilizer – one that also contains trace minerals – about 8 in. deep into your soil. Tom uses Osmocote, but since I'm an organic gardener and raise rabbits, I plan to mix in a bucketful of composted rabbit manure plus a balanced slow-release granular fish fertilizer. Depending on your soil, you may wish to add, in addition to composted manure and an organic slow-release balanced fertilizer, an organic amendment containing trace minerals such as greensand or dried seaweed.
The Right Way to Sow and Thin For Success
To grow the largest sunflowers, it is essential to direct sow seed directly into the garden, rather than start them in pots of any kind. This is because sunflowers have long taproots that grow quickly and become stunted if confined. Peat pots in particular often dry out and block off root growth. Despite many gardener's best intentions, transplanting often gets postponed – so beat the odds and plan to sow seeds in the ground.
Since sunflowers that are planted in midsummer 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. In the shorter-season, cold winter areas of the U.S., this means late May to early June. The ideal spacing in rows for giant sunflowers with large seed heads is 20 in. apart. If you plant closer, you might get taller stalks but smaller heads. If you plant farther apart, the seed head may be larger, but possibly too heavy for the stalk to bear. If you have limited space, Tom recommends sowing in a small clump that will eventually be thinned to one plant.
To sow seeds, water your soil, and press seeds 1 inch deep in clumps of 5-6 seeds about 6-8 in. apart. Put snail bait in a circle around the clump (I like the product Sluggo because it is non-toxic to humans, pets and wildlife), 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 inches, 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.
Feeding and Care of Your Growing Giant
Feed often and water regularly. While the plant is small, water around the root zone, about 3-4 in. 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. Tom uses Miracle Grow or Schulz's growing solution, but as an organic gardener, you can use properly diluted fish emulsion or other liquid organic fertilizers.
Tom recommends that you care for giant sunflowers as members of your family. 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 in extremely windy areas or if they must be grown in conditions that are too crowded or in too much shade.
Harvesting and Enjoying Giant Seed Heads
As the petals fall off, the center florets dry up and the seed kernels begin to swell in the disks, carefully climb a stepladder and cover your flower head with a mesh onion bag or loose burlap or paper bag. This keeps marauding birds from robbing your seeds so that the heads look perfect and complete when you are ready to show them off to friends or proudly display them on their long stalks at your local county fair. Cut the stalks at the base when the ripened seeds develop a hard shell. If you plan to preserve them for your bird feeder, wait until the seeds are completely dry; then remove them by hand or by rubbing them over wire mesh into a basket. Store in tightly closed containers to keep rodents away.
Giant Fun for All Ages
Growing giant Sunzilla sunflowers is a great way for getting children inspired to enjoy the garden. The seeds, which make tasty snacks for songbirds, are large and easy to handle. Like Jack's beanstalk, the plants grow quickly skyward, and the flower heads look cheerful and lion-like. Tall sunflowers can be planted in formation: as forests, playhouses or teepees. You can even grow colorful vines like scarlet runner beans up the stalks if you give the sunflowers several weeks head start. It's easy to get children involved in the excitement of watching how tall a family sunflower grows each day, or let them compete on their own! I plan to give seeds to several neighbors, and compare stalks at the end of the season!
Want more sunflower tips? How To Grow Summer Long Sunflowers