Cosmos is one of the most popular and heaviest producing flowers in the cutting garden. They are also one of the taller plants you might grow. How tall a plant grows can determine where in the garden you plant it so that it gets enough sun and doesn’t shade out other plants.
Knowing how tall cosmos grow will help you decide which varieties to grow and where to place them in your garden.
Cosmos grow to heights of 20-54 inches tall, with a range of 40-48 inches being the most common. Factors such as the variety chosen, sun exposure, and irrigation will have a role in the final height of a cosmos plant.
The cosmo variety you choose to grow will be the most significant indication of how tall your cosmos will grow. But even then, the growing conditions will influence plant height to varying degrees.
How tall cosmos grow depends mostly on the variety
The average height for cosmos is right around four feet tall. Dwarf varieties will top out at two feet tall, while the tallest plants may get to almost five feet tall. Choose a variety that will fit your requirements depending on the space available in your garden and how you want to use the blooms.
|Cosmos variety||Days to maturity||Flower color||Plant height (inches)|
|Versailles Mix||90-110||pink, white, red||40-48|
|Cupcakes Mix||90-100||pink, white||40-50|
|Double Click Cranberry||75-90||deep cranberry red||42-46|
|Double Click Bicolor Violet||75-90||cream, pink||42-46|
|Japanese Kiiro||75-90||cream, yellow, pink||42-48|
|Sensation Mix||75-90||pink, white, red||48-54|
Dwarf cosmos are varieties that have been bred to remain short. Even if grown in optimal conditions, they’ll never be the tallest plants in the garden. Dwarf varieties are excellent choices for smaller gardens or if you’re looking for a variety suitable for growing in containers.
Xanthos and Xsenia are two options if you want shorter plants. I have a packet of Xanthos seeds in my stash, and I’m looking forward to growing this new-to-me variety this spring. Plus, those buttery yellow petals are a nice change from the typical pink and white shades that cosmos are known for.
Cosmos in the middle height range of 42-46 inches have the most varieties to choose from. No matter what color scheme you are designing in your garden, you’ll find a variety that works for you, from cranberry red to snow-white, and single blooms to pouffy double blooms.
Cupcakes Mix is particularly interesting. The petals are crinkled to look like cupcake papers with slight ridges. Charming colors of pink and cream and yellow centers make this an excellent variety for romantic displays. Fizzy White has similarly fluted petals but in white and with tufted centers.
For bolder colors, Double Click Cranberry is a great choice. At the height of 42-46 inches, the blooms develop as both semi-doubles and doubles. This means that instead of a single layer of petals on the bloom, the flowers have a double row of petals, making them look fluffy and rounder. Both types of blooms are deep cranberry red with yellow centers.
Here are Cupcake Mix, Fizzy White, and Double Click Cranberry from Johnny’s Selected Seeds. All the texture and color you could want!
At the tallest end of the spectrum, the classic variety Sensation Mix will tower over your other plants at almost five feet tall, or 54 inches. This variety is known as “garden cosmos” and is ubiquitous in cottage gardens. Long-lasting blooms in pink, white, and red make this variety a great choice if you have room for it.
As you can see, the combinations of height and color are (almost!) limitless.
Cosmos height is affected by growing conditions such as sun exposure and water
No matter which variety of cosmos you choose to grow, it won’t reach its full potential if the right growing conditions aren’t met. Cosmos are a pretty easy-going plant in terms of how much water and nutrition they need. We’ll get to that in a moment.
But cosmos are sticklers when it comes to the amount of sunlight they get. Native to the Southwest United States and Mexico, cosmos can handle the heat and sunlight. They thrive in at least 8 hours of full sun per day, and this amount will affect how tall the cosmos grow.
If grown in partial shade, which is only 4-6 hours of sunlight per day, the plants might not reach their full height or be as robust. The stems are also likely to be shorter, giving you few options for arranging cut stems in bouquets and vases.
Water is another factor that can affect how tall cosmos grow. Cosmos do not like to be overwatered or have “wet feet.” This means they need well-draining soil that won’t retain water around their roots. If cosmos are overwatered it will stress the plants and they may not reach their full height potential.
Overwatering can also lead to root rot and fungal disease, which will almost certainly negatively affect how well the plant is able to grow.
A stressed plant is also more susceptible to pests and disease, which will affect the growth of the plant as well as the quality of the blooms. If a plant is fighting off aphids or beetles it won’t thrive and achieve its full height. Even if it does grow tall, pests like beetles eat the leaves, making them unusable and weakening the plant.
Take your pick of cosmos for your garden, short or tall
From dwarf two-foot tall plants to tall five-footers, you can plant a cosmos that will reach the right height for your garden. Keep the shorter varieties toward the front of your flower bed to help them get the sun exposure they need. They’ll also be easy to reach there.
On the other hand, if you plant tall Sensation Mix then keep it to the back of the bed or, even better, plant the seeds in a separate bed so they have room to stretch out.
No matter how tall cosmos grow, enjoy those blooms all growing season long.