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How to Plant Cosmos Flower? (Complete Growing & Care Tips)

How to Plant Cosmos Flower? (Complete Growing & Care Tips)

Cosmos (Mexican aster) is an annual flower originating in Mexico. Spanish explorers sent it in Spain in the 16th century, and British ambassador’s wife Marchioness of Bute brought first Cosmos to England in 1789. The circle was finished fifty years after that when this beautiful plant came to the US.

Until these days, this plant hasn’t a real name. Actually, we still use the name of its genus meaning ‘beautiful’ in Greek. It is quite easy growing this plant from seeds and enjoying its cup-shaped, single or double petaled flowers surrounded by the airy foliage. You can choose between white, golden-yellow, orange, pink, chocolate, and magenta blooms, which will beautify your garden.

Facts about Cosmos Plant

Facts About Cosmos Plant

For the first time, Cosmos was discovered in Mexico in 1860, and the last time it was seen in the wild in 1878.

Gardeners were growing Cosmos atrosanguineus (chocolate Cosmos) for years. That half-hardy perennial became popular in the 1880s because of the slender tubers, tall flower stems, and fantastic chocolate coloring and fragrance. Unfortunately, this variety disappeared from most gardens in 1942.

Since experts from the Royal Botanic Gardens at Kew continued to grow Cosmos, there was an initiative in the 90s to reintroduce this plant to Mexico. However, all the Kew plants were sterile. Without seeds, the only possibility to propagate this species is in the laboratory.

Nowadays, you can find these flowers in nurseries and gardens, but you can’t propagate them by yourself.

Recently, the Japanese produced a new hybrid ‘New Choco’ and Thompson & Morgan developed bushy plant ‘Chocamocha’ with cute reddish flowers. Also, there are three new varieties, which come from New Zealand:

  • Dark Secret – It has small, well-shaped, lightly scented crimson blooms
  • Spellbound – It has large, highly fragrant flowers
  • Eclipse – It has huge, rich-colored, fragrant blooms

In the end, Germans produced ‘Black Magic’ from seeds. You can find red or chocolate varieties with sweet scents from vanilla to chocolate. If you prefer growing plants from seeds, there is a chance for you.

Modern Varieties of Cosmos Plant

Cosmos bipinnatus

Cosmos bipinnatus

This 1.5 – 4 feet (46 cm – 1.2 m) tall flower is the most familiar variety available these days. You will adore its large, white, pink, red, or burgundy flowers with yellow eyes. The foliage is delicate with slender, thread-like leaflets.

Even though they are quite floppy, you can use the taller types as border plants. On the other hand, shorter types are an ideal choice for a container.

There are a few really intriguing varieties including:

  • Cupcakes – The edges of its petals form an attractive cup shape
  • Pop socks – Its blooms have double pom-poms, but can also be single or anemone-centered
  • Rubenza – I love these unusually abundant, deep-ruby-red blooming
  • Xanthos – It is the first variety with yellow flowers

Cosmos sulphureus

Cosmos sulphureus

Their blooms are smaller, but you will enjoy extraordinary scarlet, yellow, or orange shades surrounded by the broad, dark foliage. You can grow this variety in a container and even cut its flowers for a vase.

How to Plant Cosmos Flower in Your Garden

Propagating by seedlings

Cosmos Flower Propagating by seedlings

You can find seedlings or even young plants of many Cosmos varieties in garden centers. If you purchase young plants, the only thing you need to do is to put them directly into the garden and water abundantly after planting.

On the other hand, you should take care of delicate seedlings. The best option is to plant them indoors in the very beginning to give them enough time to establish well. After the danger of frost passes, you can transplant them into your garden.

Propagation by seeds

Cosmos Flower Propagation by seeds

It is quite easy growing fertile types of Cosmos from seeds. Basically, you should sow them directly in the garden and wait for new plants to grow. The only thing you should do is to decide if you want to start sowing in spring or autumn.

Planting in spring

Cosmos Flower Planting In Spring

Since Cosmos seeds will germinate quickly, you shouldn’t sow them before all danger of frost passes. Do it in April, but you can also sow seeds indoors in a pot about a month before the last frost.

As soon as the frost passes, you should transplant seedlings into the garden to give them enough time to establish before hot summer days come. Find the spot where your young plants will be protected from too low temperatures and wind until set.

Planting in fall

Cosmos Flower Planting in fall

In many regions, it is more convenient sowing Cosmos seeds in autumn, after the period of hardest frost passes. That way, seeds won’t germinate until the following spring.

The procedure of Cosmos sowing

Seeds of this plant are relatively large, and you won’t have any problem to handle them. You can expect your plants to develop quite quickly, even if the temperatures are low.

In the ground

Cosmos Flower In the ground

If you decide to sow Cosmos directly in your garden, place them approximately 0.25 inches (6 mm) deep into the ground. Give them enough space for spreading. On average, you need to put these plants about 12 to 18 inches (30.5 – 46 cm) apart.

Your Cosmos doesn’t need special preparation before sowing seeds. They actually like when the soil is not too high in nutrients. If it is highly fertile, you will get the lush foliage without much blooming.

If the conditions are desirable, you can expect to see first flowers approximately seven weeks after sowing seeds. After that, you will enjoy continuous flowering until the first frost in fall.

Given that Cosmos is a self-seeding plant, you can get new plants in spring if you let the seed heads mature and blow seeds away in autumn. Alternatively, you can collect dried seeds from the ground, save them for the next season, and plant them by your own.

Keep in mind that seeds from hybrid varieties probably won’t give the same plant the next year. They will develop the appearance of their parent species.

In a pot

Cosmos Flower In a pot

Starting Cosmos indoors is practical because you can sow seeds much early. Pick out an at least 3 inches (7.6 cm) deep pot with enough drainage holes. After filling it with starting medium, you should scatter the seeds over the surface of the ground.

Avoid planting seeds too deep, and leave about 1 inch (2.5 cm) of space between them. It will be enough to cover seeds with 0.25 inches (6 mm) of the medium and tap the soil lightly.

Water seeds regularly and keep the temperatures of 60 to 75 F (15.5 – 24 C) constant by covering the pot with polythene or glass. Cosmos seeds need about a week to germinate. Once seedlings sprout, thin them if necessary.

You can also consider the possibility of sowing seeds in the pot every two weeks. That way, you will have enough young plants if you need to fill space in your garden after other early-flowering species fade.

How to Care Cosmos


Cosmos Flower Space

When you decide to plant a single plant, you should provide at least 7 inches (18 cm) of space each way. However, most gardeners prefer growing a bunch of these lovely flowers.

If you decide the same, try to plant Cosmos sulphureus 10 to 12 inches (25.5 – 30.5 cm) apart and provide 24 inches (61 cm) for Cosmos bipinatus. The gap between rows should be at least 7 inches (18 cm).


Prepare the loose, dry, well-drained, and weed-free soil for your plant. Choose the part of the garden with clay, loam, or sand soil types balanced in ingredients.

Medium moisture or dry ground is the best option for this plant. It is better to avoid growing it in too fertile soil because you won’t be satisfied seeing leggy and floppy growth.


Cosmos Flower Light

Cosmos prefers growing in full sun unless you live in a region with hot weather in summer. In that case, you should provide part shade for your plant during afternoons.

If you want to help your flowers grow healthy and vigorous, take care to provide at least six hours of sunlight a day for them.

If you grow this plant in a container, place seedlings in a windowsill faced to south or west. Turn them occasionally to expose all their sides to direct sun.


Cosmos Flower Temperatures

Like any other desert plant, your Cosmos will thrive in high temperatures regardless of the humidity level.


Cosmos Flower Watering

When it is young, your Cosmos will need regular watering with a garden hose to get started the proper growth. Once established, this flower can stand dry conditions without problems.

That means that you don’t need to water this plant except in periods of a long drought. Take care to water the roots without splashing leaves.


Cosmos is not a heavy feeder, but it requires some compost or a balanced organic fertilizer when planting. Be careful with a level of nitrogen in fertilizer since it encourages the lush growth of leaves instead of string stems and abundant blooming.


Cosmos Flower Mulching

You should use a garden hand rake and add some mulch around the base of your flower in early spring. That is the way to retain moisture and keep the surroundings weed-free.


When you plant a group of Cosmos plants, you should keep them upright, especially if you live in a windy region. Proper supporting will prevent the plants from breaking in heavy rain or wind as well.

Pruning and deadheading

Cosmos Flower Pruning and deadheading

Deadhead faded flowers to prolong the period of flourishing, encourage new growth, and support branching. In mid-summer, cut the branches to one-third their length and wait for a late-season re-blooming.

If you want to get a branched plant, you should prune it adequately. For example, if you know that Cosmos grows 1 to 5 feet (30.5 cm – 1.5 m) in height, you should pinch its tops when they reach 2 feet (61 cm) height.

That way, you will get lush flower instead of single tall stems. Once new branches form, keep pinching their tops in the same way.

Cosmos Pests and Diseases

Unlike most flowers you can grow in your garden, Cosmos is quite resistant to common pests and diseases. That makes it an easy to grow plant indeed. However, I will list you some possible issues you may face while growing this lovely flower.

Aster yellows

Cosmos Flower Aster yellows

It is a chronic bacterial infection spread by winged leafhoppers, insects feeding on plants’ sap. You will notice the greenish leafy growth on infected buds and yellow shoots and mottling on infected stems.

Prevent this disease by planting healthy and certificated flowers. Once you see insects on your plant, spraying insecticidal soap to get rid of them before transmitting the infection. In the case that your Cosmos catches the disease, you should burn the whole plant.


Cosmos Flower Aphids

They will appear when your plant is under stress. The common causes are over-crowded planting and long-lasting drought. Solve the problem by spraying insecticidal soap over leaves.

Powdery mildew

Cosmos Flower Powdery mildew

Spores attaching to the host plant causes this fungal disease. You will spot a white powder over the foliage, which will become yellow. If left untreated, you can expect leaves drop off. Treat the affected plant with a fungicide.

Gray mold

It usually appears when planting these flowers too close to each other or during long-lasting rain periods. Fungi infect damaged plants and spread quickly to others.

Prevention is not too complicated. Follow directions about spacing when planting Cosmos and provide excellent airflow for your plant. That will be enough.


Sometimes, viruses can slow the growth of your flower and cause lesions on the foliage and blooms. Don’t try to cure plants infected with viruses, but destroy them and start with new ones the following season.

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  1. K says:

    I overcrowded planting and had to transplant my cosmos when they were about 3” and barely had true leaves yet. I had 9 seedlings in a 4” cell. Live and learn, right?!

    They shriveled the first day after transplant, took in tons of bottom water in adjusting, and went right back in my dome of 75*F, 18:6 LED. Honestly, I trimmed the crispy part of their cotyledons off once they’d fully perked up. They have true leaves. I managed to save 6 of them and now their legginess is tucked away, so they’re stronger than before.

    I so appreciate the cool history and spacing info of different types of cosmos in your writing. Wish I had read it sooner. Thank you for this full list of info on cosmos. I would add that cosmos are intolerant to heavy wind and don’t like drastic temperature changes. One cold night and I’m sure they’d dampen.