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The Ultimate Guide to Growing Mammoth Sunflowers

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Are you looking to add some serious wow-factor to your garden this season? Look no further than mammoth sunflowers from the family of sunflowers also known as helianthus annuus. These towering plants can reach up to 12 feet tall and boast blooms that can measure up to 2 feet in diameter. 

But growing these huge sunflower plants takes more than just planting a seed and hoping for the best. I’ll take you through everything you need to know, from choosing the right seeds to harvesting your giant blooms, so that even if you’re a beginner gardener you can cultivate the biggest and most beautiful sunflower heads in the neighborhood. Are you ready to get started? Let’s get planting!

What are mammoth sunflowers

To begin our journey to growing mammoth sunflowers, it’s important to understand what exactly these plants are. Mammoth sunflowers, also known as giant sunflowers, are sunflowers that have been specifically bred for their large size. They can grow to be up to 12 feet tall, with monstrous blooms that can measure up to 2 feet in diameter. These towering annual plants are a true showstopper in any garden, and are sure to impress anyone who sees them.

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While mammoth sunflowers may look similar to other types of sunflowers, they are bred specifically to be larger and more impressive. Just like any other sunflower, they require plenty of sunshine and water to thrive. However, mammoth sunflowers can be a bit more finicky than their smaller counterparts, and require careful attention to detail during the growing process.

Let’s discuss how to choose the right seeds for your mammoth sunflowers, as well as where to purchase them and what to look for when selecting them.

Choosing the Right Sunflower Seeds

When it comes to selecting the right mammoth sunflower seeds, there are a few things to keep in mind. First, make sure to choose a seed packet that is specifically marked as “mammoth” or “giant.” These sunflower varieties are bred to produce larger sunflowers with thicker stalks that can support the weight of the heavy seed head.

It’s also important to consider the soil and climate conditions of your area. Some varieties of mammoth sunflowers are better suited to certain climates or soil types. For example, if you live in an area with a short growing season, you may want to choose a variety that has a shorter maturity time.

When it comes to purchasing your seeds, there are a few options to consider. You can find them at your local garden center or nursery, or you can order them online from a trusted seed supplier. Here are some great options from True Leaf Market, a reputable seed supplier that specializes in heirloom and non-gmo seeds:

  • ​Organic Mammoth Grey Stripe Sunflowers- This heirloom mammoth variety reaches maturity in 90 to 110 days and has a distinctive grey stripe down each seed. Fantastic sunflower to grow for harvesting seeds.
  • Black Russian Sunflower– An heirloom variety also known as Russian Mammoth Sunflowers, this variety is ornamental and reaches maturity in 65-70 days. The dried sunflower stems can also be used as firewood!

Preparing the Soil for Sunflowers

To grow mammoth sunflowers you want the right conditions and preparing the soil for these tall plants is crucial. Sunflowers love well-drained soil that’s nutrient-rich. Follow these steps to prepare to plant your sunflower seeds or seedlings:

  • Start by removing any rocks, debris, or weeds from the area where you plan to plant your sunflowers. 
  • Dig the soil to a depth of at least 12 inches, then mix in compost (organic matter) or well-rotted manure to help improve soil fertility. 
  • Make sure the soil is moist before you plant your sunflower seeds. Sunflower seeds need consistent moisture to germinate properly. 
  • Test the soil moisture level by squeezing a handful of soil. If it sticks together in a ball, it’s too wet. If it crumbles apart, it’s too dry. Ideally, the soil should hold its shape but crumble when lightly touched. 

Once the soil is properly prepared, it’s time to move on to planting your sunflowers. With the right seeds and soil, you’ll be well on your way to growing mammoth sunflowers that will make a great addition to your vegetable garden or flower beds. 

Planting Your Sunflowers

Now that you have prepared the soil, it is time to plant your sunflowers. The seed package for all giant varieties will have valuable information about plant depth and spacing. The heavy heads on mammoth sunflowers mean that planting depth is important. Follow these easy steps:

  • Start by digging small holes about 1 inch deep. 
  • Space the holes about 6 inches apart. 
  • Place 2 to 3 sunflower seeds in each hole, pushing them down to cover them with soil.
  • Water your sunflowers immediately after planting them. This will help the soil settle around the seeds and keep them moist. Use a watering can or gentle hose spray to avoid washing the seeds away.

Once your sunflowers start to grow, thin them out to leave one plant per hole. If you feel that your seedlings are too close to each other, gently remove the weaker ones from the group. This will give the stronger ones more room to grow and produce bigger blooms. See my full post on thinning out seedlings if you need extra help. 

Caring for Tall Sunflowers

Now that you have successfully planted and thinned out your sunflowers, it’s time to learn how to care for them. Sunflowers are relatively low maintenance, but they require some attention to ensure that they grow strong and healthy.

First and foremost, sunflowers need plenty of sunlight. Make sure that they are planted in a location that receives at least six hours of direct sunlight per day. Additionally, be sure to water your sunflowers regularly, especially during periods of drought. Aim to keep the soil consistently moist but not waterlogged.

Another crucial aspect of caring for sunflowers is fertilization. Sunflowers are heavy feeders and require regular fertilization throughout the growing season. It’s a good idea to use a balanced fertilizer once a month or use a slow-release fertilizer at planting time. (I love Espoma’s Organic Flower Tone Fertilizer in my garden.)

As your sunflowers grow taller, they may need some support to prevent them from toppling over, and support will also help if you live in an area with strong winds or the occasional storm. Use stakes or trellises to prop them up and keep them upright. For best results, an 8-foot pole firmly staked into the ground will provide additional support for the large flowers that will form.

Harvesting Mammoth Sunflowers

At the end of the growing season, it will be time to harvest the seeds. The seeds will be ready when the backs of the flower heads turn brown and start to dry out. You can also tell that the seeds are ready by gently pressing on the backs of the flower heads; if they are firm, the seeds are ready to harvest.

To harvest the seeds, you have a few options:

  1. Cut the flower heads off with a sharp pair of scissors or garden shears. Make sure to leave a few inches of stem attached to the flower head. Then, hang the flower heads upside down in a cool, dry place, such as a shed or porch, to dry out completely for a few weeks. Once they are dry, the seeds will easily fall out of the flower head when you gently rub them.
  2. Allow the seed heads to stay on the plant until you can clearly see the seeds are forming a stripe (for Mammoth Gray Stripe Sunflower) and then cut the head off and harvest immediately. 

If any birds get to your sunflowers before you do, don’t worry. You can still harvest the seeds that they missed by using a mesh bag or cheesecloth to cover the flower head after it blooms, which will allow the seeds to mature and ripen, but keep birds from eating them.

Place sheet of newspaper or a large towel down on the ground and begin to harvest the seeds by rubbing them. They should fall out of the flower head easily and fall onto the towel you’ve laid down. See my video of harvesting sunflower seeds below, or see my full post for How to Harvest Sunflower Seeds. Then check out my recipe for Easy Roasted Sunflower Seeds

​Growing Mammoth Sunflowers FAQ

Here are some frequently asked questions about growing this large variety of annual plant. 

How long do mammoth sunflowers take to grow? 

That depends on the variety of sunflowers you have chosen. Different varieties of mammoth sunflowers can range from 65 to 120 days to reach maturity. Be sure begin your sunflower seeds after the last frost date to give your plants as much time as possible to grow.

Can sunflower seeds be started indoors?

Yes! Many types of sunflower seeds, including the taller varieties, can get a head start by beginning indoors. Sow sunflowers in a seed starting cup or even in a simple paper cup with holes poked in the bottom. Use nutrient-rich soil or a seed starting mix and plant 1-2 seeds per cup. Place on a sunny window sill and watch them sprout! Plant outdoors once danger of frost has passed and be sure to harden off for best results. 

Can sunflower seeds be saved to plant the following year?

Yes! You can always plan to save seeds from your sunflowers to plant for the following year. As with all seed saving techniques, be sure that the seeds have dried out completely before storing them for growing the next year. This is a wonderful way to always have a supply of seeds on hand, and it saves you money. 

Growing mammoth sunflowers can be an enjoyable and fulfilling experience with the right knowledge and tools. Remember to choose the right seeds, prepare the soil, plant correctly, care for them, and harvest at the right time. By following these steps, you can create a garden that is the envy of your neighbors. So, start planting today and watch your sunflowers grow taller than everything else in your garden!