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Cosmos flowers – Ultimate Australian Guide

Cosmos flowers are a real treat for the eyes – each variety has its own brilliant palette, from pastels to deep shades. The flowers are whimsical as they seem to dance in the wind.

In our ultimate guide about cosmos flowers, we’ll go through how to grow and care for these beauties, propagation advice for cuttings and seed, and also sharing what to look out for in terms of pests and diseases.

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Introducing Cosmos Flowers

Cosmos flowers are Mexican natives that are easy to grow and love bright, sunny areas

There are over 25 species of cosmos or Mexican aster as it’s more commonly known. Cosmos is part of the Aster family of plants. Cosmos reseeds itself and can handle dry and fertile soils. It’s a hardy plant and has very few needs.

Cosmos plants are Mexican natives that are easy to grow and love bright, sunny areas. The flowers are colourful and look very similar to daisies.

They’re in flower throughout summer, and attract some beautiful visitors in the form of birds, bees, and butterflies. The good news is that cosmos flowers grow easily from seeds and they can even handle poor soil conditions.

Cosmos produce daisy-like flowers in shades of pink, orange, red and yellow, white, and maroon. The flowers can be bowl or cup shaped and can reach a height of almost 2 metres.

Cosmos flowers can grow in the garden and pots and also look amazing in a vase as freshly cut jewels.

Plant Name:

Cosmos Flowers 

Genus:

Cosmos

Common Names: 

Cosmos, Mexican aster, Cut-leaf cosmos

Plant type: 

Annual

Size:  

Up to 1.8 m tall and 90cm wide

Sun:  

Full

Soil type: 

Well-draining

Soil pH: 

Acidic, 6.0–6.8

Flowering time: 

Summer to autumn

Flower colours: 

Golden yellow, white, pink, magenta, orange, yellow, red, chocolate

Native to:  

Northern South America, Central America, and southern North America

Varieties of Cosmos flowers

There are more than 25 species of cosmos but three species are most commonly used in gardens and landscaping. 

Sulfur Cosmos (Cosmos sulphureus)

Cosmos sulphureus has golden yellow flowers, is very drought tolerant and loves hot weather

Cosmos sulphureus is native to Mexico, Central America, and northern South America. It has golden yellow flowers, is very drought tolerant and loves hot weather.

The plant can grow to a height of 1.8 metres and produces double and semi-double flowers. Some of the more recent cultivars of cosmos flowers are shorter, with more orange shades and smaller flowers. 

Garden Cosmos (Cosmos bipinnatus)

Cosmos bipinnatus produces colourful daisy-like flowers in white, pinks, reds, and orange

Cosmos bipinnatus produces colourful daisy-like flowers in white, pinks, reds, and orange. They reach a maximum height of 1.2 metres so they are shorter than C. sulphureus and have many popular hybrids.

They aren’t as heat tolerant as C. sulphureus but C. bipinnatus will grow well in any sunny space. 

Chocolate cosmos (Cosmos atrosanguineus)

Cosmos atrosanguineus or Chocolate cosmos

Chocolate cosmos is a separate species called Cosmos atrosanguineus. The dark red flowers smell like chocolate but it's higher maintenance than other cosmos plants.

Other popular cosmos flower cultivars include: 

  • 'Bright Lights' mix: This variety has a blend of brilliant yellows, oranges, and reds.
  • 'Cosmic Orange': This semi-double orange flower has great drought tolerance.
  • 'Peppermint Candy': The petals are magenta and white.
  • 'Sea Shells' series: A lovely mix of pastel colours with tubular petals.
  • 'Ladybird': This cosmos is a shorter variety with flowers in red, yellow, orange, and gold, and is 45 to 60cm in height.
  • ‘Picotee’: Pretty white flowers with a crimson border or flecks.
  • ‘Purity’: A beautiful and classic white flower.

If you feel inspired to grow other flowering plants in your garden, use our flowering plants category to learn more about what would best suit your needs and your garden. 

How to Grow Cosmos Flowers

You can directly sow seeds outdoors if the danger of any frost has gone. You could also sow seeds indoors in trays or pots with a good seed-starting mix.

Move them into 1.5 metre pots as soon as they’re 90 to 120cm tall. These young plants can then be planted outdoors when all frost has gone.

How to Grow Cosmos Flowers

Cosmos flowers don’t need any fancy soil prep. They are quite happy with a soil that is not too rich, otherwise you’ll end up with great leaves but less than great flowers.

The soil you use should be well-draining and cosmos are tolerant of most soil pH levels. For best growth, go for neutral to alkaline soil. Cosmos will handle warm, dry weather well and are even drought-tolerant. 

These flowers can grow anywhere between 45cm to 1.5 metres tall, depending on the variety so just be sure to plan their growing and living space accordingly. 

You want to sow the seeds lightly, not more than 3.5cm. If you are growing cosmos flowers from seeds, it can take about 7 weeks for the first flower.

If you leave the spiky-brown seed heads to just blow away during autumn, cosmos will probably self-sow in your garden.

Sunlight Needs

If you want your cosmos to have the best flowers, choose somewhere that gets full sun. Cosmos can grow in partial shade but they’ll have less flowers and be less vibrant.

These plants can thrive under continuous full sun in the hottest conditions, which is similar to their native habitat. Cosmos can also thrive in any humidity level.

Best Soil for Cosmos Flowers

Cosmos plants like neutral soil with a pH of 6.0 to 8.0, but poor soil is not a problem for them. These flowers grow best in medium moisture, well-drained soils, but will do alright in dry soils too.

You don’t want to plant cosmos flowers in a rich soil otherwise they will get too tall and flop over. If you’re worried about your cosmos drooping, you can stake the plant or grow it close to other plants that can support it.

Caring for your soil is a make or break for gardening success. Have a read through our soil care gardening tips and articles to make sure you get the recipe just right.

How to Propagate Cosmos Flowers

How to Propagate Cosmos Flowers

Cosmos Flowers Propagation from Cuttings

You need sterile shears or scissors and a pot of sterile, well-draining potting soil. Fill a small 7cm container with moist potting soil and make a shallow hole.

Aim to find a cosmos shoot that has 3 to 5 leaf nodes on the stem and then cut under the last leaf node. You’ll then carefully cut off the leaves at this last leaf node, leaving the node in place for more growth.

Place the cut tip of the stem in the hole, making sure that the last leaf node is above the soil line. Push down the soil around the stem, making it compact enough to keep the stem upright and in place.

Water well and keep the soil moist. You should see new leaf growth in about three weeks. Once you see the new leaf growth, you can gently pull the root ball out of the pot and transplant the root ball to its new spot.

Propagating Cosmos Flowers from Seed

Loosen the soil to a depth of 20 cm, plant the seeds, and cover them with 3.5cm of fine soil. You can do precise spacing at 60cm intervals or you can scatter the seeds.

The plants can then support each other as they grow. You can always thin them out later.

Caring for Cosmos Flowers

Caring for Cosmos Flowers

Fertilising Cosmos 

Cosmos are quite self-sufficient in terms of nutrients and feeding this plant should be a case of less is more. Giving it too much nitrogen will slow down flower production. This is because nitrogen encourages strong greens and discourages flower production.

Most of your balanced fertiliser blends usually contain too much nitrogen for flowering annuals. Many gardeners get trapped in a frustrating cycle. They don’t see any flowers, so they fertilise their plants.

The more fertiliser they add, the fewer flowers appear. If you do find that your cosmos flowers are not blooming, you can add a phosphorus fertiliser like bone meal. When the soil has recovered from the excess nitrogen, your cosmos will be covered in plenty of flowers once again.

When do cosmos flowers need fertiliser? 

Whether you plant your seeds indoors or outdoors, cosmos flowers can use a small amount of fertiliser when they are planted. Choose a fertiliser made for blooming plants, which will have a low nitrogen count.

Mix the minimum amount into the soil when planting cosmos flowers seeds and then you can forget about feeding them for the rest of the season. Fertiliser for cosmos planted in pots is a little more important. 

There is a small amount of soil available for their roots to feed from and so they will need to be fed a bit more frequently. You can sprinkle a half teaspoon of blooming plant fertiliser on the soil around each plant and water it into the soil.

Repeat this process once every three to four weeks until the end of the flowering season. If you notice your cosmos starting to slow down in terms of producing flowers, you can cut back on the fertiliser for a few weeks and then adjust your feeding as necessary.

Cosmos Watering Schedule

Once your cosmos is established, you don’t need to water your plant unless there is a long drought. These are the last plants that need water if there is a water shortage crisis.

Cosmos flowers are quite self-sufficient in terms of nutrients and feeding this plant should be a case of less is more.

Deadheading and Pruning Cosmos Flowers

Pinching back and deadheading your cosmos flowers are two different things but your plant needs both. The main reason to pinch back is to promote new growth as it stimulates growth and a more bushy shape.

Pinching

Pinching is a type of pruning and basically means picking off new growth on a plant. You can use your fingers or shears depending on the stems. For a tougher stem, shears will help get a better cut that minimises infection or disease.

You’re aiming to pinch off a stem just under a pair of leaves. This is where you find two nodes and when you remove the top part of the stem, two new ones will grow from those nodes. Pinching back can also stimulate flower production.

You can start cutting back your cosmos flowers early in the growing season and keep doing this regularly.

Deadheading

Deadheading gives you more flowers throughout the season and this, much like the name suggests, means removing dead flowers. When you see your cosmos flowers start to fade, pinch or cut them back.

The life cycle of a flowering plant ends in setting seeds. When you remove the dead flowers, you trick the plant into creating more flowers so it can get back to seed setting.

When you see a fading cosmos flower, you can cut it back down to the next bud. If there are no buds, you can cut it back to the next set of leaves and for bare stems, cut it back all the way to the base of the plant. 

It’s a good idea to start pinching and deadheading cosmos flowers early but if you wait too long, all is not lost. You can trim back the entire plant by about one-third when most of the flowers have begun fading. 

Pests and Diseases of Cosmos Flowers

Cosmos flowers are not demanding. They are actually quite hardy but there are a few diseases that might cause issues - fungal, bacterial and insect viruses.

Cosmos flowers are not demanding and are actually quite hardy

If you manage to control insects, offer the right water levels, and plant healthy plants you can hopefully minimise the problems. If your cosmos flowers are dying during the growing season, it could be one of the diseases below. 

Two of the most common fungal diseases are Fusarium wilt and powdery mildew. 

Fusarium wilt

Fusarium wilt causes the plant to wilt and discolours the stems and leaves. If you dig up the plant, you will notice a pink mass on the roots. The whole plant will unfortunately die and needs to be destroyed so the fungus doesn’t spread. 

Powdery mildew

Powdery mildew spores float on the wind and attach to any host plant in shade. The fungus forms a powdery white coating over leaves, causing the foliage to become yellow and drop off if it’s not treated.

Make sure your cosmos has good ventilation, bright light, and is watered during the day so the foliage can dry. You can also use a horticultural fungicide.

Bacterial wilt

Bacterial wilt is one of the classic cosmos flower diseases and causes stems to wilt at the base. The entire stem and flower becomes infected and eventually the root system too. You’ll need to dig up and destroy the plant unfortunately.

Aster yellows

Aster yellows is another disease that affects any plant in the Aster family. It is transmitted by leafhoppers and is caused by phytoplasma. You will notice the cosmos flowers dying after becoming distorted and stunted. The foliage will be mottled and yellow. Infected plants will once again need to be destroyed. 

Cosmos plants are a garden buffet for certain insect pests. They don’t do much damage but some of them transmit viruses and disease when they feed.

Thrips 

Thrips transmit tomato spotted virus which has no cure. The flower buds are delayed and distorted and when they do open you’ll notice spotted, ringed, or lined petals. 

Other sucking insects can affect the plant’s health. You can use a good horticultural soap and quick blasts of water during the day to remove many of the pests.

Cosmos Flowers FAQs

Cosmos is an annual that germinates, flowers, and drops seed to prepare for the next growing season

How fast do cosmos flowers grow? 

Cosmos usually take between 7 to 21 days to germinate and will flower within 50 to 60 days of germination.

How long can cosmos flowers live? 

Cosmos is an annual that germinates, flowers, and drops seed to prepare for the next growing season. Cosmos will weaken and eventually die after flowering.

How do you harvest cosmos flowers?

If you want to harvest more seeds, you need to leave a few flowers on the plant because they will self-seed. You can cut the flowers off any time after blooming, but it's best when the petals have opened.

If you cut the flowers when they first open, they should last more than a week in water. 

Jazz Up Your Garden by Growing Beautiful Cosmos Flowers 

If your garden is in need of a bit of brightness and colour, cosmos flowers will jazz it up for sure. You have so many varieties to choose from, whatever shade inspires you most. They are drought tolerant and not too fussy in general.

Once established, you can even forget to water them for a while. They can self-seed and when you remove their dead flowers, they just produce more beautiful ones.

We think cosmos flowers are simply charming and we’re giving them our green thumbs up.

Cosmos flowers – Ultimate Australian Guide

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Author:

Maisie Blevins

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