Complete Guide to Growing Cosmos

Updated/Fact-Chacked on April 7, 2022 by John

Growing cosmos is a rewarding endeavour, they have a long flowering season from may to October, proving lots of great flowers. Cosmos is not too difficult to grow and requires only minimal attention once the plants are established. 

If you are interested in growing the cosmos, here in this blog post, we will go over the basics of growing the cosmos in your home or business garden. You’ll learn how to plant them, what soil is best for them, and how to take care of these plants once they’re grown.


Things You Should Know Before Growing Cosmos


  • Cosmos are mainly annual plants, they flower set seed, then die – you can collect the seeds for next year. You can also buy some varieties as tubers, these will be tender perennials and need to be overwintered.
  • Keep the flowers away from small children and pets because cosmos leaves have sharp points on some of them. If ingested by either of these two groups of people, they can cause irritation to the digestive system.
  • Cosmos flowers are pollinated by bees, butterflies and other pollinating insects. If you plant cosmos in your garden or planter box, it is possible that these pollinators will visit them every year as well.
  • You can get many free plants if you collect your cosmos seeds and sow them the next year; Only works with open pollinated varieties and not cosmos hybrids

Best Varieties of Cosmos to Grow

There are many different varieties of cosmos to choose from, but some work better for certain climates or regions than others. Listed below are some of the most popular varieties, along with information on how well they grow in different conditions.

Cosmos bipinnatus

This variety is ideal for cooler climates. It has long stems that are perfect for cutting, and they attract butterflies into the garden. However, some people have had trouble getting it to grow in warmer regions of the country.

Cosmos sulphureus

These yellow flowers are often seen growing along roadsides, and they are resistant to deer. They can grow up to five feet tall, but gardeners should plant them near the back of their flowerbeds because they don’t attract butterflies as other varieties do.

Cosmos sulphureus Superbus

This variety is similar to Cosmos sulphureus except it has more vibrant colours. It’s also a bit shorter, which makes it more ideal for growing in containers.

Cosmos Sea Shells

This variety is ideal for coastal regions because it does not require much water and can survive in difficult conditions.

Cosmos Gazebo Red

This variety has a smaller height, which makes it easier to grow in pots. However, they attract very few butterflies and are not ideal for colder climates

How to Grow Cosmos

Here is a guide to growing the cosmos. It’s pretty simple and straightforward, but you’ll get the best results if you follow these steps carefully.

Planning to grow your cosmos

If you are planting cosmos in a border, find a sunny location in the centre of a border; an area that is likely to lose colour after summer – cosmos will inject colour into that area until the start of November. Cosmos are one of the best autumn flowering plants to consider for continual colour through the season.

Cosmos can easily be grown from seed, without having to start them off in a greenhouse; simply plant them in your border where you wish them to grow.  Alternatively, buy cosmos plants from your local nursery during the summer months and plant. See details below for planting in containers.

Planting Cosmos in Planters or Containers

First, you need to figure out how big of a planter or container your cosmos will fit in. Cosmos can grow up to 36 inches tall with flowers reaching 24 inches across, so they do best in containers at least two feet deep and wide. You’ll also want something large enough for the roots to have plenty of space to spread out.

Before you plant your cosmos into a container, make sure it has drainage holes in the bottom. If there aren’t any, drill some yourself, being careful not to damage the planter with either too big of a hole or by making an unbalanced hole that will let the soil fall out when it’s time to plant. 

Cosmos like their roots well-watered, so be sure there is a way for excess water to flow out of your planter or container and away from your cosmos’ root system.

growing cosmos in a container will add long flowering colour to a patio

The best potting soil for cosmos is one that contains a high amount of organic material, but it should also be lightweight enough to easily drain. 

If you’re starting with an existing container or planter, make sure the potting mix will allow water to flow through it freely while still holding together well when pressed into place by your hand.

Once you have your planter or container prepared, it’s time to plant your cosmos seeds. Press them about a half-inch into the soil and cover with the potting mix before gently misting everything with a spray bottle. 

You can also water directly from the top of your planter or container if you want to use less water, but it’s still important that excess moisture be able to drain easily from the soil as well as any containers with drainage holes in them.

Watering Your Cosmos

Cosmos really only need watering once or twice a week, but they do best when given consistent amounts of water. If you live in an area that gets very hot and dry during the summer months, you may need to increase your watering schedule slightly so the cosmos don’t die from lack of moisture. 

Make sure not to over-water either as this can cause roots to rot which will kill your plant.

Trimming Back Your Cosmos

Cosmos need trimming when they are about 6 inches tall. Take a sharp pair of scissors and cut off the bottom portion of the stem. 

This will encourage it to grow more roots which in turn means stronger plants that can better bear weight from big blooms without tipping over or being damaged by wind. You’ll also want to trim off any leaves that are low enough to touch the soil so they don’t rot and attract bugs.

deadheading cosmos flowers to produce more flowers

Fertilizing Your Cosmos

The best time to fertilize cosmos is after you’ve already pinched back or cut off the tips of their stems when they are about six inches tall. A balanced fertilizer (like one with 5-5-5 in it) can be used, but make sure it doesn’t contain any weed killers or other chemicals that could harm your plants.

Pinching Back Your Cosmos Again

Cosmos need to be pinched back again when they’re about 12 inches tall and then every time after that until their flowers reach full size at 24-36 inches across. Each time you pinch off the tips of their stems, new ones will grow and your cosmos will become bushier. 

Once you’ve reached the end of this step in growing cosmos, your flowers should be at full size and you can start removing any leaves that are touching the soil to prevent bug infestations or plant rot.

Deadheading Your Cosmos

When it comes time to deadhead your cosmos, you’ll want to pluck off any of the old flowers that are past their prime. 

If you don’t remove these they will attract bugs like stink beetles or fruit flies which can cause problems for your plants later on even though it’s not necessary in terms of fertilizing them or keeping them healthy overall.

Enjoying Your Cosmos

Cosmos are wonderful flowers that can be used in many different ways. Once you have them planted and watered, they don’t require much work or upkeep which means if you forget about your garden for a week here or there it won’t mess things up too badly. 

You’ll also get to enjoy the gorgeous flowers they put out in the summer and fall when other plants have died back or gone dormant.

What Plants Go Well with Cosmos: Cosmos Companions

Cosmos grow well within a wild meadow style border using calendula or zinnia. For great colour combinations also look at dahlias and nepeta – to see how these look together, check out GW’s cosmos companion guide.

Growing Cosmos FAQs

What Soil Is Best For Growing Cosmos?

It should also be lightweight enough to easily drain. If you’re starting with an existing container or planter, make sure the potting mix will allow water to flow through it freely while still holding together well when pressed into place by your hand. You can also choose a mix of half garden soil and half compost to fill in larger spaces.

How Long Does it Take for Cosmos Seeds to Germinate?

Cosmos seeds should germinate within ten to fourteen days, but they can take much longer if the conditions aren’t right.

If you don’t see sprouts after this amount of time has passed, it’s best not to bother planting them as more than half of your seed packets will likely fail and die out without ever-growing into them.

When Will My Cosmos Flower?

Cosmos will start blooming about 70-80 days after they are planted, which is usually around mid-summer. If you’re growing your flowers from seed, it’s best to wait until the weather has begun warming up before planting them outdoors so that they have plenty of time to grow strong and healthy. 

Once your cosmos begin blooming, they can continue for up to three months or more before dying out.

What Is The Best Time of Year To Plant Cosmos?

It’s best to plant cosmos in the early spring or late fall when there are no signs of frost, but you can also try growing them indoors if this is not an option. 

Cosmos grow especially well in climates that have long summers because their blooms continue for so much longer than they do elsewhere.

What Is The Best Time of Day To Water Cosmos?

It’s best to water your cosmos in the early morning or at night so that they have plenty of time to dry out before the sun starts beating down on them. 

If you can’t remember when you last watered your plants, it might be a good idea to set up a watering schedule for yourself so that you’re never forgetting about them and your cosmos end up dying without ever opening.

How Long Will My Cosmos Flower Last?

Cosmos will typically last for around three months if they’re kept alive by deadheading them regularly. If you don’t cut off the old flowers, however, your plants may stop blooming altogether or become home to pests that can ruin their appearance and spread disease.

How Do I Know If My Cosmos Is Healthy?

Cosmos plants that are healthy will always be green and not brown or wilting. If you see any black spots on your leaves, it means they have a fungus that is difficult to treat but can usually be stopped by keeping the plant cool and moist.

Cosmos can be a beautiful addition to any garden, but they require special care. If you want yours to grow as tall and strong as possible, follow the steps outlined in this guide carefully so that you end up with gorgeous flowers rather than dead plants!