Grow Giant Sunflowers

Giant sunflower head in sunflower field

The Spruce / K. Dave

Perhaps you want a temporary screen to block the view of a vacant lot or nosy neighbor. Or maybe you want to try to break the Guinness World Record for growing the tallest sunflower, last set in Germany with a 27-footer. It could be that you just want to take up the challenge of growing something as tall as your house from a bean-sized seed in one season. Many sunflowers are genetically programmed to become giants, but with some extra care, you can grow a colossal stand of blossoms that will cause passers-by to stare and wonder.

Choose a Giant Sunflower Variety

Gardeners that rely on open-pollinated seeds and heirloom varieties can choose from ‘Mammoth,’ ‘Titan,’ or ‘California Greystripe’ sunflower seeds. If you’re looking for organic seeds, you are more likely to find these old-fashioned choices for sale. Although you probably won’t break any records with the heirloom types, they produce delicious seeds for snacking and bird feeders, and you can coax at least 12 feet of height from one of these plants.

Some gardeners prefer growing newer hybrid sunflower varieties. Hybrids have been carefully developed to produce consistently tall plants with strong stems. If you decide to grow hybrids, you will not be able to count on growing identical plants from the seeds you saved the following season. Giant-sized hybrid sunflower varieties include ‘Kong,’ ‘Sunzilla,’ and ‘American Giant.’

How to Plant Giant Sunflower Seeds

Although it seems like a good idea to give sunflowers an early start indoors, this strategy can backfire. The act of transplanting sunflowers can stunt their growth, denying you a couple of feet in height at the end of the growth cycle. Planting sunflowers outdoors just after your area’s last frost allows the plants to develop their deep taproots in situ.

Take this flower’s name to heart and plant your sunflowers where they will receive full sun. Six hours a day is required for general vigor, but 8-10 hours is even better, providing the energy needed to produce strong stems. If your property has a sunny area close to a windbreak, choose this site to prevent stem breakage on mature plants during storms.

Sunflowers are touted as low maintenance plants that thrive in poor soil, but these growing conditions will not yield the sturdy giants you desire. The secret side of sunflowers is that they are greedy feeders and drinkers that crave deeply worked soil with excellent drainage. If you don’t like double digging, consider planting them in raised beds heavily amended with manure and compost. Plant your seeds at least six inches apart in rows three feet apart, knowing that you will thin them after they germinate.

Giant Sunflower Care

When your sunflowers are about three inches tall, pluck out the smallest specimens, leaving about 18 inches between the tallest plants. This is necessary for adequate air circulation, sunlight, and nutrients for each plant. Keep the sunflower bed weeded, as weeds not only compete for nutrients but also introduce insect pests that may bring fungal diseases to the flower garden. Combine your fertilizing and watering chores by feeding your sunflowers twice a week with diluted fish emulsion or another balanced flower fertilizer. Small plants only need a couple of gallons of this nourishing drink per week, but as the plants surpass you they will triple their thirst.

When the petals of your sunflowers begin to shrivel, the plant has reached its peak height. If you are growing an heirloom type, save seeds from the largest several plants, and plant the largest seeds (with the most life-sustaining endosperm) next season to increase your giant sunflower growing success. In this way, you will improve each generation by selecting the largest and most vigorous flowers to replant.

Giant sunflower head closeup

The Spruce / K. Dave

What to Do With Giant Sunflowers

The heirloom giant sunflower varieties are treasured for their oil-rich seeds, both for animal food and as snacks. When the petals wither and the bloom's beauty has faded, you must protect the developing seeds from hungry scavengers with some fine netting or cheesecloth fastened with a rubber band. When the back of the bloom turns brown the seeds are ready to eat or to store in a cool, dry place for winter bird feeding.

Sunflowers make a quick and cheap temporary hedge or child's play area in the landscape. Growing a successful hedge may lead to more ambitious projects: If you are the lucky owner of a large property, you may consider growing sunflowers for profit. Small sunflower fields are in high demand for weddings or as family-friendly attractions for portrait backgrounds. When the visitors have gone and the flowers have lost their luster, you will still have the seeds as a cash crop, making the sunflower a double-duty money maker.