11 Tips for Growing Cosmos in Pots

Cosmos is a beloved, whimsical flower from Mexico’s hot, dry regions. They’re easy to grow from seed, and their daisy-like blooms unfurl in shades of white, pink, purple, yellow, and orange with a cheerful yellow center. If you have limited space, growing cosmos in pots is the perfect solution.

Cosmos are members of the same family as dandelions and sunflowers—the Asteraceae family. They aren’t picky about growing conditions, similar to many of their relatives. Cosmos won’t bat an eye at poor, dry soils and heat, making them a true summer garden champion.

Cosmos add quirk and whimsy to a flower arrangement with their delicate ferny foliage and bending and arching stems. Although useful for the cut flower garden and pollinator patch, they also provide character to your container plantings or patio pots. 

Select Compact or Short Varieties of Cosmos

As with any garden decision, starting with the right varieties is key. If you grow cosmos in pots, select shorter varieties and cultivars.

Aim for varieties that reach 24 inches tall or less when mature. If you’re growing in an extra large pot, you can opt for a taller variety, but be aware that cosmos stems flop over in the wind if they get too tall.

Some popular short varieties (or varieties that are less prone to flopping) include:

Pick the Right Container

Cosmos have tender green stems and leaves prone to wilting when they do not have adequate water.

Choosing the right pot can make all the difference.

Plastic containers will hold in moisture for far longer than clay pots. The reason clay pots dry out quicker is that the clay has natural moisture-wicking tendencies. Clay pots are not solid like plastic. Instead, they have very tiny pore spaces that allow water and air to pass through.

On a hot sunny day, water evaporates from the potting mix through the small pores in the clay. Sometimes, this isn’t bad, but if you know you’re not good at watering your pots regularly, you probably need to opt for a plastic pot.

All pots outside need to have drainage holes. You don’t want to hold in too much moisture!

Plant Your Cosmos Seedlings After Your Expected Last Frost

While sometimes cosmos plants can withstand a light frost, it is ideal to wait until after your last expected frost date to plant cosmos in a container outside.

A much larger, established plant can withstand freezing temperatures. If a few leaf tips shrivel up and die from the frost, it’s no big deal for an older plant. This might be the case in the fall when your cosmos have had time to mature throughout the summer.

However, small seedlings or plant starts don’t have the biomass or energy (i.e., “manpower”) to bounce back if they succumb to frost damage. Do your cosmos plants a favor and wait until after your last expected frost to plant cosmos outside.

Provide Your Cosmos With Adequate Space

Space cosmos seedlings at least 4 inches apart from other plants. It’s a common practice to fill a pot up to make it look fuller, but know that your plants will grow throughout the season. You want to allow your plants room to grow and fill out naturally.

If you don’t have adequate spacing, you can end up with rootbound plants that have quickly outgrown their original container. Sometimes, this isn’t detrimental if you want a short-term display and know you’ll be watering often. However, if you want a summer-long display, give your plants extra space when planting them in spring.

Giving your plants enough space will increase airflow between the plants (which reduces disease), reducing competition between companion plants in the pot.

Choose the Right Location for Your Pot

Cosmos prefer full sun (six to eight hours of direct sunlight per day) to grow and flower properly. 

The great thing about growing in pots is that you can move them around depending on the plants’ needs. 

That said, don’t plant cosmos with other plants that require shade. Make sure everything in the pot with your cosmos can handle at least full sun to partial shade. This will ensure your container looks the best and flowers the most.

Plants that thrive in full sun will flower less in shade. While they tolerate shade for some time, you may not see any blooms. If they do flower, the flower quantity will be less than if it were placed in full sun.

Manage Your Moisture

A good soaking at least twice a week is ideal for most container-grown plants. Cosmos prefer soils on the drier side, so if the weather is mild, you can likely get away with watering once a week. Do not over-water your cosmos.

During the heat of summer, you may need to water 2 to 3 times a week, depending on your climate and the local weather.

Plants growing directly in the ground don’t have high water requirements because the soil naturally acts as an insulator, keeping the roots cooler. 

When you grow plants in pots, the pots are also exposed to the sun and can heat up, so the soil inside will heat quickly. Heat causes rapid evaporation, so monitor your pots when the sun beats down in mid-summer.

Sometimes, the top of the potting mix will dry out, but that doesn’t mean it is time to water. Stick your finger about 2 inches into the potting soil and see if you can feel any moisture. If you can’t, it’s likely time to water. 

Choose the Right Companion Plants for Cosmos Flowers

Cosmos prefer full sun, so try to choose companion plants that also prefer full sun when growing a mixed container.

Mixing up the type of plants you’re growing is also a great idea. For example, cosmos are members of the Asteraceae family. This family includes zinnias, sunflowers, and dahlias. Try growing plants in other families to mix up the shape of the flower, growth habit, and even the color. 

Many people use the “thriller-filler-spiller” method when planting their containers. They choose a tall or focal flower for the “thriller.” The “filler” is a shorter flower that spreads nicely and is usually fairly floriferous (cosmos may be a great fit as a filler). The “spiller” is a plant with a cascading, spreading growth habit.

Pinch Your Cosmos Plants

Pinching your cosmos plants will induce branching and, thus, more flowers. To pinch, snip off the stem above a set of leaves. 

Pinch your cosmos plants when they are about 4 to 6 inches tall. Often, the cosmos puts out a tiny flower on a tiny transplant. Just pinch this little flower off the second you see it. This will push the plant into branching and putting on more vegetative (green) growth.

You may not need to pinch if you choose very compact varieties. However, if you’re growing a variety that matures at 24 inches or taller, make sure you pinch!

Use Low to No Fertilizer

Cosmos are excellent beginner plants because they have very few requirements to thrive. In fact, cosmos will perform better in soils with lower fertility, especially lower nitrogen.

When placed in a high fertility situation, cosmos plants focus on vegetative (leaf and stem) growth rather than reproductive (flower and seed) growth. The lesson? Lay off the high rates of fertilizer with this plant.

Instead, make sure your potting mix includes some slow-release fertilizer. Potting mix is naturally devoid of most nutrients, so if you never fertilize throughout the season, your plants will eventually start looking horrible—even cosmos. Many companies add in slow-release fertilizers. Check your bag of potting mix or add a small amount of compost to ensure your plants have adequate nutrients to grow and bloom. 

You may need to fertilize once every month throughout the summer with a low-nutrient fertilizer to maintain adequate nutrient levels for growth. The main point is that if you skip the fertilizer, your cosmos will likely not suffer.

Watch for Pests

Cosmos have a few issues to watch for. They are not immune to pests, but they are excellent pollinator plants! The most common pests you may encounter on your cosmos plants are thrips and aphids. 

Thrips are extremely tiny (1/16th inch) brown cigar-shaped insects that like to feed on cosmos blooms. Dark-petaled varieties will often show signs of streaking on the petals. Looking closely, you may see these teensy insects crawling inside the flower. Severe infestations may cause the flower not to open properly.

Aphids are small insects that can be green, gray, brown, or black. They are most often present when the plants have lush, green growth in spring, but may appear at almost any time of year. Aphids feed on the sap in your plant’s leaves and stems. They secrete a sugary waste called “honeydew,” which will eventually turn into black spots on the leaves, stems, and petals because of mold growth on the honeydew. Severe infestations will cause plant stunting, yellowing, and deformed flowers.

Thankfully, you can mitigate these pests by spraying forceful streams of water daily until you notice reduced numbers. Horticultural oils are also effective against aphids, but since thrips like to burrow into the flower, they can be harder to kill with oils or sprays.

If the infestation is really bad, remove severely infested plant tissues and spray the rest of the plant with neem oil or insecticidal soap, which are effective against both types of pests. Aphids and thrips can transmit plant viruses, so look for symptoms of viruses if you notice a severe infestation. 

Deadhead Faded Blooms

It is important to deadhead the faded flowers to keep your plant flowering.

All plants have a goal of reproduction. When they flower, plants entice pollinators to fertilize them so that they can produce seeds.

As a gardener only interested in flowers, we want to focus the plant’s energy on additional flower production, not on seed development. We do this by deadheading the flowers. Removing the spent flower head removes the seeds it’s trying to mature, and the plant will start producing a new flower to restart the process.

Once the flowers die back, snip the flower stem off just above a set of leaves. The plant will try to reproduce again, and it does this by producing another flower stem!