Growing Cosmos From Seed – Planting Guide

Introduction to Growing Cosmos From Seed: Cosmos are annual flowers with single and double varieties. They are prized for their abundant, silky, daisy-like flowers and can be grown simply from seeds and they require little care. Its vibrant color helps in attracting butterflies, bees, and birds to the garden. Plant them in a container else in the garden beds or simply use them as cut flowers.

A Step-by-Step Guide to Growing Cosmos From Seed

How to Grow Cosmos
Cosmos Flowers (Image source: pixabay)

Popular Types of Cosmos Plants

  • Bright Lights Mix – an attractive combination of yellows, oranges, and reds.
  • Cosmic Orange – drought tolerant orange color flower.
  • Peppermint Candy – lovely petals display in magenta and white.
  • Seashells Series – features different tubular petals and stunning colors.
  • Chocolate Cosmos (Cosmos atrosanguineus) – these are dark red color flowers that smell like chocolate.
  • Tall Cosmos is best for the back of the garden and filling in mixed borders among constant such as cowslip, phlox, and daisies, as well as poppies, cleome, and asters.
  • Dwarf Cosmos, like the Ladybird series, make great companions for zinnias, Dark-blue morning glory, pansies, and alchemillas. 

Some Facts about Cosmos Plant                

  • Flowers will bloom from 50 to 55 days.
  • Flowers are available in multiple colors like white, pink, red, orange, yellow, and chocolate, magenta.
  • Plant leaves are deeply lobed, pinnate, or feathery.
  • The plant can tolerate poor soil and doesn’t require any fertilizer.
  • Flower blooming can depend on the variety of Cosmos we choose.
  • Cosmos plants are ideal for direct sunlight.
  • Consider well-drained soil with a pH value of 6.0 to 6.8

Choosing and Preparing a Planting Site for Growing Cosmos

  • Cosmos don’t need any special soil preparation. They such as soil that is not too rich, as rich soil will encourage leaves at the levy of blooms.
  • Soil should be well-draining.
  • Cosmos can be grown in most soil types, but prefer neutral to alkaline soils. Cosmos can grow in warm, dry weather very well. They are even drought-tolerant.
  • Based on the selected variety, Cosmos can grow anywhere between 18 – 60 inches tall, so plan properly.

Different Types of Soil Preparation for Cosmos Seeding

  1. Add 70% off Coco peat and 30% Compost
  2. 50% Garden soil and 30% compost and 20% river sand.

The Complete Process of Seeding the Cosmos

  • Take a small pot with drainage holes at the bottom and fill it with the soil mix. Fill 70-75% of the pot with the soil mix.
  • Moisten the soil mix before you sow seeds in it. Space the seeds 1-2 inches apart; in a 7-inch diameter pot, we can easily sow 10-15 seeds. (Don’t place them too close to the edge of the container).
  • Now cover the seeds with a half-inch layer of the same soil mix. Make good soil contact when pressing the soil, as seed-to-soil contact is vital for germination Water the soil lightly with a light shower, being careful not to displace the seeds. Tag it and keep the pot in full sun.
  • Make sure to keep the soil a little moist during the germination phase. Normally after 6-7 days seeds will start to germinate.
  • Add water as the soil starts to dry up.

Planting the Cosmos after Germination                           

Soil Preparation: Cosmos plants prefer neutral to slightly acidic soils and they will grow in poor soil where many other flowering plants languish. Cosmos perform in medium moisture conditions, but they will perform adequately even in dry soils. Avoid soils that are too rich, which can plant to get too tall and flop over you should grow them close enough to other plants for them to survive once seedlings turn 5-6 inches tall, we can transplant them at their final locations.

  • Make sure that the soil mix is moist before we can transplant them.
  • Take out the seeding by keeping the root ball intact and unharmed to avoid transplant shock.
  • After every 25- 30 days spray them with a balanced NPK (nitrogen, phosphorus, and potassium) fertilizer water mix on the leaves for vigorous and flowering. Make sure you don’t overwater your Cosmos, as overwatering and over-fertilizing can result in fewer flowers. Flowers will start blooming after 50 to 55 days. It may take more time based on a selected variety of Cosmos.
  • At this stage, plants start to grow fuller and you can see at least 4-6 buds on each plant.
  • For healthy constant growth keep the soils a little moist at all times and avoid dry wet conditions.
  • Remove dead flowers and dying flowers to encourage stronger more vigorous growth and this is called deadheading.
  • After you cut the dead flowers off, more buds will form beneath the cut flowers stem giving more flowers.
  • Water the plant regularly, whenever topsoil turns dry. Overwatering can cause rot and fungal diseases in the soil. So make sure the soil is moist but not soggy. It is important to deadhead blooms in the Cosmos to keep them flowering. In this way, a plant stops putting its energy into producing seeds and instead concentrates its resources on producing flowers.
  • Some taller varieties grow up to 5-6 feet tall. They are the perfect choice if you wish to add some height to your garden borders else to a cut flower patch. Finally, the seeds are ready; you can easily pull them off the flower. Grasp the base of the flower head with a light touch and fill outward to brush the seeds into your hand.

Do Cosmos Plants Need Full Sun?

For an optimum flower, growth chooses a location that receives full sun. Cosmos attracts beneficial insects like bees and butterflies to a butterfly garden. Pollination will be helped by such plants in vegetable gardens along with marigolds. They will provide you with a beautiful flower if you give them enough light and water.

How to Take Care of Cosmos Flower Plants

In case if you miss this: Potato Seed Germination.

Cosmos Care
Cosmos Care (pic credit: pixabay)
  • Deadhead the plants (prune the faded/dead flowers) to prolong blooming. This will speed up flower development and promote branching.
  • Because some Cosmos plants can grow tall, staking can be necessary. Stake them if necessary to provide them with ample protection from strong winds. Cut back (or prune) the stem tips to encourage balanced branching. Fencing can serve as a support.
  • Regularly water plants until they are established or if the weather is unusually dry. You should not overwater Cosmos plants, as overwatering; over-fertilization can result in fewer blooms on the plants. Cosmos tolerate dry soil, even in hot, arid spots that get direct sunlight.
  •  In the spring, thin seedlings or remove blossoms before they go to seed to avoid weeds growing in your grass beds. Since self-seeding marsh beds will become weedy.

Harvesting Seeds and Cut Flowers from Cosmos Plants      

  • To harvest more seeds, remember to leave a few flowers on the plant since they will self-seed and produce more seeds.
  •  After the flowers have bloomed, you can cut them off at any time, but it’s best to pick a few rights as the petals open.
  • Cut the blossoms when they first open and you can keep them in the water longer than a week. The lower leaves can be stripped off and placed in a vase.


Non-organic Cosmos seeds are sprayed with harmful fungicides and other types of chemicals. So, Cosmos seedlings purchased from nurseries have neonicotinoids, which are toxic to bees and other insects.

Cosmos flower uses in multiple ways

You may also check this: How To Grow Plants In Hydroponics.

Cosmos Uses
Cosmos Uses (pic credit: pixabay)
  • Cutting flowers, as a border or backdrop, and as cut flowers are some of the uses of Cosmos. There is nothing more delightful than the smell of Cosmos and the butterflies that it attracts. The seeds are attractive to small finches. Cosmos can be used to attract beneficial insects to your garden such as lacewings, tachinid flies, hoverflies, and parasitic mini wasps, all of which feed on destructive insects.
  • Aside from treating jaundice, fever, and other disorders, Cosmos is used medically. Cosmos does not seem to have medicinal uses, but some believe that it is an antioxidant and that it protects against DNA oxidation.
  • In addition to their visual charm, Cosmos sulphureus flower petals (C. bipinnatus, C. caudatus or Mexican Aster) are edible and brighten salads if grown from organic seeds.
  • Choosing open-pollinated Cosmos varieties over hybrids, and ideally organic, will help you save seeds. The reason for this is that otherwise, they won’t grow true-to-type, so it is a lottery what plant variety you’ll get.

Common Pests/Diseases in Cosmos Plants

Stem canker:                                                                                                      

The first signs of stem canker are noticeable during the second half of the growing season. A reddish-brown color develops on the stems of plants at the beginning of their reproductive phase. In most cases, stem node lesions are found around the nodes.  Usually, green stem tissue will appear both above and below individual stem cankers, making stem cankers easy to diagnose. The canker subsequently enlarges longitudinally, changes color from dark brown to become slightly sunken, and eventually completely girdles the stem. As a result, the free flow of nutrients and water in the plant is disrupted. It is often mistaken for stem discoloration caused by Phytophthora that coalesces into cankers. As opposed to Phytophthora, stem cankers usually develop on higher parts of the plant. An advanced stem canker may cause the plant to die prematurely and often suddenly. A diseased plant’s foliage initially shows yellowing between the veins. Following this, tissue death occurs between the veins. Eventually, leaves die, but they often remain attached to leaf stems (petioles).

Powdery mildew:

Cosmos are commonly affected by this type of problem. The conditions such as high humidity and leaf dryness favor fungus growth. It is a highly specialized pathogen that forms an intimate connection with the host. Some conditions are favorable to both the host and the pathogen. A majority of fungi grow outside infected plant parts, using roots-like structures to sink into roots and take up nutrients. A white growth visible is composed of both mycelium and spores.

Symptoms of powdery mildew: Grayish-white, powdery spots develop on leaves. A certain amount of yellowing and premature defoliation may be observed. Keeping plants in good shape for proper air circulation is a cultural control measure.

Gray mould:

Dark brown to black blight appears on flowers, buds, leaves, and stems of plants caused by gray mold. A black or brown pattern appears on the petals and buds of flowers infected with the pathogen. Brown spots may appear on the petals of plants with large petals. Leaf spots occur when an infected petal falls on the leaf or where there is a wound on the leaf. Leaf spots appear as round brown blotches surrounded by darker rings of brown. Leaf spots can turn the whole leaf brown when it is wet outside. The infection can progress into the stems of flowers and leaves. The stems of infected plants are brown and often have darker brown rings around them.

The growth of gray mold is influenced by wet weather during the growing season. Plants should be spaced correctly so air can flow freely between them. Dividing or thinning overgrown perennials. Maintain a dry environment for your plants. Overhead watering should be avoided. Make sure that your soaker hose or drip irrigation system is set up.

Don’t water late in the day. After you have watered your plants, give them time to dry.

Water Requirement for Growing the Cosmos Plants 

If you establish your Cosmos plants correctly, you won’t have to water them at all unless the drought lasts for a while. Last, of all, these plants require watering.

Temperature and Humidity Levels for Cosmos Plants

Cosmos are happy in hot weather, and almost any degree of humidity is conducive to their growth. The Cosmos grows best in temperatures above 60 degrees Fahrenheit (16 degrees Celsius). The plants will not grow or flower below 55°F (13°C). If the weather is pleasant, keep the plants outside during the day any time the temperature is 40 degrees or higher. Within a couple of days, they should be able to adapt to cooler temperatures.

Cosmos Plants Dimensions

Cosmos can grow height maximum of 3 to 8 feet, minimum 1 to 3 feet. Width of the Cosmos 1 to 2 feet wide.

Avoiding Common Mistakes when Growing Cosmos Plants

Observed by their size and beauty, Cosmos flowers are named for the ability to make an observer think of the majesty of the universe. Hot and dry conditions are beneficial to both annual and perennial plants, as they both grow rapidly and flower profusely. Consequently, the Cosmos is one of the easiest flower varieties to grow in the garden; however, you must take care not to make mistakes.

Burying the Cosmos seeds – Adding fresh seeds each year is the best way to grow annual varieties of the Cosmos flower. Occasionally, a previous year’s seed might germinate in your garden, but supplying fresh seeds for your garden will ensure a healthy harvest. It is important not to bury seeds too deeply in your soil when spreading them out. Spread a thin layer of soil and mulch rather than sprinkling seeds on top to make sure the seeds become entrapped in the soil.

Over Fertilizing in Cosmos plantation – Cosmos grow best in poor to moderate soil. Deserts in Mexico and the southwest of the United States attract them despite seemingly infertile conditions. In especially fertile soil or if you overfeed your Cosmos, then the plant will grow more greenery than anything else, becoming tall and spindly with few of the flowers that make the plant so attractive. However, this does not mean you should not provide any fertilizer. Make sure you fertilize your Cosmos once or twice a year with a general-purpose fertilizer.

Over Watering during Cosmos plantation These flowers originated in the deserts of Mexico and the American Southwest. Since these are dry areas, the Cosmos is adapted to survive in low-water conditions. Watering these flowers effectively requires watching them closely. If you see the tips of the greenery beginning to wilt, make sure to water them. You shouldn’t do it otherwise. Furthermore, you should plant Cosmos in soil that drains well. When you have clay soil, try adding sand or planting moss in a raised flowerbed to facilitate proper drainage.

Keep plants from being affected by Sunlight – The best conditions for growing Cosmos are full sun or partial shade. These flowers should not be planted in shady areas of the garden. If you don’t want fewer flowers and spindly growth, make sure you place them someplace with plenty of sunlight, at least eight to ten hours.

Keep plants from being affected by wind Most types of Cosmos grow to a height of 7 feet or more. Many gardeners suggest staking the taller varieties, especially in windy areas since the wind will knock them over. Therefore, if you live in an area where the wind blows a lot, you may not be able to grow these flowers.

Commonly Asked Questions about Growing Cosmos from Seed

1. Are there any tricks to getting Cosmos to bloom?

Usually, some fertilizers like “More Bloom” or “Bloom Booster” are made with much less nitrogen and more phosphorus to support healthy blooms in flowering plants. Also, bone meal is a good way to encourage flowering. Also, it can be wise to add fertilizer only at the time of planting.

2. Are the Cosmos able to grow in containers?

Container gardening is a successful method of growing Cosmos flowers. For container gardening, you’ll want to find dwarf or compact varieties of species plants, since they can grow up to 6 feet tall (2 m.).

3. Why are Cosmos seedlings so leggy?

At their most basic level, leggy seedlings are caused by inadequate lighting. It could be that the window you are growing your seedlings in does not provide enough light or that the lights you are using as grow lights are not close enough to the seedlings. Both scenarios will result in leggy seedlings.

4. Why are my Cosmos seeds not germinating?           

Too much moisture or too little moisture in the soil caused Cosmos seeds not germinating. Water will damage them if too much is applied they will rot. You should wet & drain the compost thoroughly before adding seeds to ensure they remain evenly moist.

5. Should I soak Cosmos seeds before planting?

For most seeds, it is recommended that you soak them for 12 to 24 hours as opposed to overnight. Once the seeds have been soaked, they can be planted as directed. Soaking your seeds will reduce germination time, which means you will have happier, growing plants sooner.

6. What is the growth rate of the Cosmos from seed?

 For a Cosmo to germinate, it takes between five and seven days. Within 4-6 weeks after germination, the seedlings reach the size required for transplantation.

7. What conditions are required for Cosmos to grow?

Cosmos like full sun conditions, except when it is extremely hot, where they can tolerate part shade. Prepare soil that is loose and weed-free before planting the garden. Arid soil is preferable to wet soil for Cosmos. A soil that is too moist may cause disease.

8. Do Cosmos return year after year?

Cosmos plants annual plants that grow only once a year. To have bloomed every year, you will need to reshow your seeds the following spring.


Please enter your comment!
Please enter your name here