Sunflowers are stunning with their bright petals, but growing mammoth sunflowers takes it to a new level—literally! They're truly the opposite of growing dwarf sunflowers!
When grown in the ideal conditions, mammoth sunflowers can tower over your entire garden and even grow taller than your house.
To grow Mammoth Sunflowers you need to find the correct balance of location, exposure to sunlight, quality soil as well as enough water to keep them growing but not too much that you flood the plant. It is a delicate balance but one that you must learn.
Let’s look at how to grow these giant beauties so you can wow your neighbors with the tallest flowers on the block.
What Is A Mammoth Sunflower?
Mammoth sunflowers are a nickname for the sunflower varieties that can grow at least 12 feet (3.7 meters) or taller. It’s also a name for the cultivar of Helianthus annuus, called the Russian mammoth.
This sunflower is prized for its extravagant height and delicious seeds. Sunflowers are native to North America, but they were brought to Europe during the 18th century and soon became popular in eastern Europe.
Today, sunflowers are Ukraine’s national flower, and they’re the top producer of sunflower seeds in the world.
The most popular giant varieties include the Russian mammoth and the American giant. Other varieties are harder to find, including Paul Bunyan, sunzilla, and Mongolian giant.
Almost every mammoth sunflower will have solid yellow petals, a yellow or brown center, and plump seeds. They’re annual sunflowers, which means they’ll complete their entire lifecycle in one year and won’t come back next year.
They can only come back if you save seeds or if they drop seeds and they sprout in the spring. Birds, squirrels, and deer love sunflower seeds, so you’re better off saving them yourself!
Mammoth Sunflower Care Requirements
Mammoth sunflowers are usually easy to grow. They grow quickly because of their giant size, so they’re a great flower to keep in your garden if instant gratification helps you stay positive!
Success depends on the conditions they’re grown in, so let’s make sure you’ll be able to grow these.
Location
Mammoth sunflowers need plenty of unobstructed space above and below them since they grow up to tall heights and develop a deep taproot.
The long root keeps them anchored to the ground and allows them to find enough water below the surface. Sometimes, mammoth sunflowers require some extra support.
Plant them next to a building or a fence to give them something to lean on. It will also protect them from gusts of wind to prevent them from falling over.
You can try to grow mammoth sunflowers in a container, but the container will need to be huge. It should be no less than 5 gallons.
Like many other plants, sunflowers won’t grow as big in confined spaces, so don’t expect potted mammoth sunflowers to grow as big as they would if they were planted in the ground.
Sunflowers don’t transplant well, so you should plant the seeds in their final location if you can. You may succeed with transplanting, but you’ll need to be super careful if you do!
Sunlight and Temperature
Mammoth sunflowers grow best in USDA hardiness zones 2-11. You can’t grow them until all danger of frost has passed, so cooler climates will have shorter growing seasons and will most likely result in shorter flowers.
The temperatures should consistently be 50°F (10°C) or warmer when it’s time to plant.
Sunflowers require as much sun as possible. They need at least six hours of direct sunlight. Planting them next to a fence or house could put them in danger of not receiving enough sun.
Make sure obstructions are to the north or east of your flowers so they can get plenty of south and west-facing sunlight.
If your sunflowers are exposed to cold temperatures or shady conditions for too long, they may not grow to their maximum size.
Soil
Some sunflower varieties are forgiving with soil quality, but mammoth sunflowers are a bit picky! It’s reasonable, though, because they require plenty of nutrients due to their quick growth.
When a plant grows to a large size in a short amount of time, they’re going to need a steady supply of nutrients.
The soil you plant your sunflowers in should have a neutral or slightly alkaline pH, ideally between 6.0 and 7.5. If you can’t be neutral, it’s better to lean toward alkaline rather than acidic.
Choose a soil that’s a good mix between fertile humus and loose loam. The soil should be well-draining to prevent standing water.
Fertilizer
While fertilizing isn’t necessary for your sunflowers to grow, you’ll find that it helps them grow much taller. Nitrogen is key to achieving soaring heights, but it can limit the growth of blooms.
Opt for nitrogen-heavy fertilizers in the early stages of growth, and decrease the amount you give it toward the end of the growing season.
Liquid fertilizer is easiest because you can measure it as needed and pour it around your sunflowers. Follow the directions on the label because each kind may have different instructions.
If you don’t want to use fertilizer, you almost can’t go wrong with compost! Compost is full of naturally-derived nutrients, and you don’t have to measure or hope you’re doing it right.
Add a fresh layer of compost around your sunflowers once or twice each month to continually feed them.
Water
Mammoth sunflowers need a lot of water when they’re young, but they don’t like wet feet, so never allow water to puddle around your sunflowers.
If you’re not sure whether a particular part of your yard drains easily, test it out before planting sunflowers by giving the area heavy watering.
If it doesn’t immediately seep into the ground, it may not be suitable for growing sunflowers. Young sunflowers will need to be watered almost every day to be kept in moist—but not wet—soil.
Once they’ve grown to be a few feet tall and are established, they become more drought tolerant and will only need about one inch of water per week. If you get soaking rain, you can probably skip watering for the week!
The amount of water your sunflowers need will depend on your location. Sunflowers grown in the arid southwestern part of the United States will likely need more water than in the northern states with mild summers.
The best thing you can do to check is stick your finger two inches into the ground next to a sunflower. If the soil is dry all the way down, it’s time to water!
How To Plant Mammoth Sunflowers
- Plant seeds about 1 inch deep and 6 inches apart. Cover the seeds with soil. Rows should be 30 inches apart. You’ll thin out most of the seedlings, but initially planting a lot will guarantee you have some to grow!
- Water seedlings every day. Water requirements are discussed in detail above.
- Thin seedlings to 12 inches apart once they are 3-4 inches tall. Remove the weakest seedlings so that there is plenty of space between the others.
- Thin seedlings to 20 inches apart once they are 12 inches tall. 20 inches is the ideal space between flowers in a row, so you won’t need to thin anymore after this.
- Keep the soil moist until the flowers are a few feet tall. You can start watering less frequently as they grow taller.
- Fertilize as needed throughout the growing season. A fresh layer of compost every 2-4 weeks will be fine, or you can use a liquid fertilizer. You can read more about this above.
How To Harvest Sunflower Seeds
Sunflower seeds are a healthy snack, so you’ll want to harvest them after your beautiful blooms start dying. If you don’t want to eat them, you can at least save the seeds to have more flowers for next year!
Backyard wildlife love these seeds, too, so you’ll need to move quickly, or they’ll beat you to it. There are a couple of different methods you can use to harvest seeds.
Harvest On The Stalk
This method isn’t the most visually appealing option, and it may require a ladder, but it’s the simplest way to do it!
Place a paper bag or cheesecloth around the flower head once the petals start to die. Tie the bottom of the bag or cloth securely around the stalk with string. You can also use duct tape or twist ties if you don’t have any string.
Allow the flower to dry out completely before cutting the stalk. As the seeds mature and dry out, they’ll fall out of the flower head. If you secured your bag or cloth correctly, the seeds won’t be able to fall out.
Once the sunflower is dead (brown stalks and dead leaves), you can cut the top off and remove the seed catcher. If some seeds haven’t fallen out yet, you can remove them by hand.
Harvest Off The Stalk
If you don’t want a garden of paper bags in your yard, this option will work better for you.
Remove the flower head with about a foot of the stalk attached to it. Place a paper bag or cheesecloth around the flower head and secure it just as described in the previous method.
Hang the flower head upside down in a cool, dry place. A garage, shed, or closet will work well as long as these areas are temperature controlled.
Once the stalk looks completely wilted, which will be brown and dry, you can remove the seed catcher to collect the seeds.
Before You Store Seeds
Make sure the seeds are dried out completely before storing them for later! Any moisture can cause mold and ruin the seeds, whether you want to eat or plant them. Seeds should also be thoroughly dried before you try to roast them.
Store seeds in an air-tight container, whether they’re cooked or uncooked.
Common Problems
Even though mammoth sunflowers are pretty easy to care for, they’re not without their problems. They’re susceptible to pests and diseases just like any other cultivar of Helianthus annuus.
Young seedlings will be susceptible to cutworms that live in the soil and like to eat roots and tender seedlings. Bacillus thuringenesis, or BT, is an effective, organic way to get rid of these critters.
Banded sunflower moths, along with other kinds of moths, are attracted to sunflowers. They’ll want to lay eggs on the flower heads, and the larvae will eat up the seeds!
The sunflowers won’t look as pretty, and you won’t have any seeds to harvest in the fall. BT and insecticides will be an easy way to get rid of these pests.
Fungal diseases such as Alternaria leaf spot and downy mildew can spread and infect your entire sunflower crop.
You can try to prevent these diseases by applying fungicides early in the season before the diseases show up. These diseases can’t be cured, so you’ll need to remove infected plants immediately.