Mammoth sunflower

Mammoth sunflower (Helianthus annuus ‘Mammoth’) is an heirloom giant sunflower. One thick, unbranched stem grows 9 to 12 feet tall and carries a single yellow flower head about a foot across. The heads hold large striped seeds for eating, roasting, or feeding birds. Sow seeds outdoors in full sun after the last frost, about 1 inch deep and 2 feet apart. Plants bloom in about 90 days. The seeds are ready to harvest when the back of the head turns brown.

Mammoth sunflower basics

Grow Mammoth if you want the classic giant sunflower and a big harvest of snacking seeds. Pick a shorter, branching variety if you want many blooms for cutting or have a windy, exposed yard.

The Mammoth sunflower is an heirloom cultivar known for its height and its large seeds. These drought-tolerant plants are easy to grow from seeds, even for beginner gardeners. In late autumn, the heavy heads droop over and feed hungry birds.

While all sunflower plants originated in North America, this giant variety was developed in Eastern Europe. As such, it is sometimes referred to as the “Russian Mammoth” sunflower or the “Russian Giant” sunflower.

The flowers

Mammoth sunflowers have golden yellow petals around a brown center. They have the classic sunflower look.

Each plant makes one flower. The stem does not branch, and the single head at the top grows about 12 inches across.

Plant height

Mammoth sunflowers reach 9 to 12 feet tall in good conditions. This is taller than most sunflower varieties. ‘Titan’ and ‘American Giant’ are two of the few common varieties that compete with it.

A plant this tall can topple from its own weight or a strong wind. University of Missouri Extension advises: “If you plant a variety that gets taller than 10 feet, stake or brace it.” A heavy-duty tree stake works well.

Sunflower history

Sunflowers are native to North America and were grown by Native Americans. Eastern European farmers made the plant popular in the nineteenth century, when demand for sunflower oil rose. Today, Mammoth is grown for its large seeds. Oilseed types with small black seeds hold more oil.

How to plant the Mammoth sunflower

Mammoth sunflowers are easy to plant. Start with good seeds, sow them about an inch deep in rich soil, and keep the ground moist while the seedlings grow. Here are the details:

Mammoth sunflower seeds

Look for seeds labeled “Mammoth sunflower,” “Striped Mammoth sunflower,” or “Heirloom Mammoth sunflower.” Mammoth Sunflower seeds can be ordered any time of year and generally ship in the winter or early spring in time for mid-spring planting.

When to plant Mammoth sunflower seeds

Mammoth sunflower seeds grow best when planted soon after the last frost date in your local area. Seeds sown before the last frost date can lose tender seedlings to cold. You can also plant in early summer, into mid-July in many areas. Late plantings may not reach full height before fall cools down.

Soil for planting Mammoth sunflower seeds

Mammoth sunflowers grow in many soil types. They prefer loam, but they also grow in sandy soil and tolerate clay.

Loam is a mix of sand, silt, and clay with plenty of organic matter. It drains well and still holds water.

Sow the seeds in the ground instead of in seed trays. The seedlings grow fast and build the strongest roots when they grow in place. Avoid transplanting them.

Planting Mammoth sunflower seeds

Mammoth Sunflower seeds are best planted in a full-sun location after the danger of frost has passed. Clear plant debris and weeds first. Then add an inch or two of organic compost to the top of the soil. Rake the surface smooth.

Sow seeds about 1 inch deep. For giant varieties, University of Missouri Extension says to “plant 24 inches apart” and notes that seeds “will germinate in about 7 to 10 days.” If you have plenty of seeds, sow 2 or 3 per hole. When the seedlings have two sets of leaves, keep the one with the thickest, straightest stem. Snip the others off at the soil line instead of pulling them, so you don’t damage the roots of the one you keep.

Sunlight requirements for giant Mammoth sunflowers

Mammoth sunflowers need full sun to grow to their full height. They need at least six hours of direct sunlight a day to thrive. They grow taller with closer to eight hours. In shade, the plants stay stunted and the flowers stay small.

A sheltered spot protects the tall stalks from wind. Stake any plant that starts to lean.

Watering Mammoth sunflower plants

Steady watering matters most in the 6 weeks after planting. Keep the soil around the seed moist. Watch for water pooling on the surface, which points to poor drainage or frozen soil. Water should soak in past the seed without pooling.

Once the seedlings are established, Mammoth sunflowers need less water. Some gardeners never water them and rely on rain. To reach giant heights, though, most plants need extra water.

They tolerate drought, but a lack of water stunts their height, most of all in the first few months. They need about one inch of water per week. If you get plenty of rain, you may not need to water at all.

I usually water my sunflowers once weekly if temperatures are mild and twice weekly if the weather is quite hot. I water at the soil line to avoid wasting water and to avoid moisture getting on the foliage of the plants and potentially spreading disease. I also like to water first thing in the morning, while it is cool.

Fertilizing Mammoth sunflower plants

Most Mammoth sunflowers grow well without fertilizer. Some growers add nutrients for the best shot at the tallest flower. Fertilizer helps these heavy feeders grow tall, sturdy stalks in poor soil.

Fertilizers range from homemade compost to aged manure or even bagged fertilizer products. Follow the label directions on any bagged product. Liquid flower fertilizers are popular with giant sunflower growers competing to grow the tallest sunflower around. They require frequent application as the nutrients are in solution. For an easier option, use an organic slow-release fertilizer.

Harvesting Mammoth sunflower seeds

Mammoth seeds are popular for their size and flavor. The plants take about 90 days to bloom and up to 120 days for the seeds to mature. To harvest, either cut the head before all the seeds mature and dry it indoors, or let the whole head dry on the stem.

To dry the heads indoors, wait until the sunflower head is maturing, but the back is still yellow. Cut the stalk off about 6 inches below the head of the flower. The seeds on the outside should be ready to harvest, while the inner seeds should not quite be ready yet. Hang each head upside down in a warm, well-ventilated area to let the seeds dry and mature.

To dry the heads on the stalks, beware that hungry critters (birds, squirrels, deer, rabbits) may get to the flower heads before you do unless you protect them with netting, cheesecloth, et cetera. This can be a bit tricky on these tall plants. Once the heads dry, cut them off.

The sunflower seeds can be harvested once the head is completely dry. Put a bucket under the head to catch the seeds. Wear gardening gloves, since the seed head can be sticky. Then rub the sunflower head so the seeds pop out and fall into the bucket. The seeds can be rinsed with fresh, clean water and set out to dry or coated in a bit of oil and roasted.

Mammoth sunflower FAQs

How tall do Mammoth sunflowers get?

Mammoth sunflowers reach 9 to 12 feet tall in full sun with steady water. The single flower head grows about a foot across.

How far apart should you plant Mammoth sunflowers?

University of Missouri Extension recommends 24 inches apart for giant sunflower varieties. Sow seeds about 1 inch deep.

How long do Mammoth sunflowers take to bloom?

About 90 days from sowing. The seeds need up to 120 days to mature.

Do Mammoth sunflowers need staking?

Often, yes. Missouri Extension advises staking or bracing any variety taller than 10 feet.