How to grow cosmos – expert tips on when and where to plant these flowers (2023)

Garden favorites for summer color, learn how to grow cosmos and you can enjoy clouds of these daisy-like flowers dancing in the breeze and filling garden beds and borders.

Ranging from pale pastels and whites through to bright pinks, crimson, orange and other zingy colors, these annuals are enduring favorites for flower bed ideas. They will happily self seed for several years, filling your yards with their open-faced blooms that can go on for months and months, adding to your palette of late summer flowers and still displaying their cheery blooms until the first frosts of fall.

Cosmos comes from the Greek ‘kosmos’, meaning beautiful, and growing tall with their fine, feathery deciduous foliage, cosmos are definitely one to include if you're planning a cut flower garden as the more flowers you pick, the more you'll get.

There are many cultivars available, including doubles, others with tubular rays and some bi-colors, in different tall and compact varieties.

Once you know how to grow cosmos you'll be using them to brighten up all corners of your garden.

How to grow cosmos from seed

How to grow cosmos – expert tips on when and where to plant these flowers (1)

(Image credit: Getty Images)

It is easy to get to grips with how to grow cosmos from seed and they can be sown any time throughout spring depending on the hardiness zone where you live. The seeds are large, long and thin, so easy to handle and germinate quickly.

'Cosmos are the very best, low-maintenance and floweriest plants in the world. With little effort, they give you buckets of cut flowers and they have a good vase life, too. Easy to grow, long-flowering and brilliant for picking, it’s hard to fault them,' says expert plantswoman, Sarah Raven (opens in new tab).

These pollen-rich flowers are also an excellent lure for bees and other pollinators to attract them into a wildlife garden. 'They are stacked with nectar and pollen, so bees and butterflies love them,' agrees Sarah. too.

Cosmos bipinnatus, the most commonly grown, also known as Mexican aster, originates from the Americas.

When it comes to growing cosmos from seed you have two options. You can grow them in modules or seed trays indoors, or plant them directly out in the garden. There are benefits to both.

How to grow cosmos – expert tips on when and where to plant these flowers (2)

(Video) Flower Gardening : How to Grow Cosmos Flowers

(Image credit: Future/Leigh Clapp)

How do you successfully grow cosmos?

Growing cosmos in modules or seed trays indoors to start them off, gives you more control over the blooms. Keeping them off the ground and under cover will also protect the delicate seedlings from slugs and harsh weather. Plus, since you are starting your plants off indoors, you are able to start planting earlier – in early spring – so will have well established flowers that attract bees by early summer.

Sow indoors in module or seed trays, covered with about 2mm of good, fresh compost. Water from below, allow excess water to drain away and position in a warm place, ideally 60 to 70 0°F (16 to 21°C), to germinate, which takes around 30 days.

Move seedlings to a cold frame or light, sheltered spot for a few weeks before planting out to harden them off. This will result in 'much faster growing and longer-lasting flowers,' says gardening guru Monty Don on hisblog (opens in new tab). He also recommends doing the same for young cosmos that are bought in the garden center, too.

Direct sowing cosmos outdoors

(Image credit: Leigh Clapp)

If you choose to direct sow cosmos outdoors, this is a low-maintenance choice that is beautiful for a cottage garden idea. 'I place them in groups so they make drifts and clumps rather than straight lines,' says Monty.

For how to grow cosmos seeds outdoors, wait until the soil has warmed up, then:

  • Rake your seedbed area to remove any clumps of soil and achieve a crumbly texture;
  • Cosmos don’t need any special soil preparation – in fact, a too rich soil will encourage foliage rather than flowers;
  • Sow seeds lightly, spaced about 2-3in (5-8 cm) apart;
  • Thin out seedlings;
  • Water until established but don’t over water as that can lead to less flowers.

How to grow cosmos – expert tips on when and where to plant these flowers (4)

(Image credit: Getty Images)

Where do cosmos grow best?

Cosmos grow best in a sunny spot, protected from wind, with well-drained, light soil. It is advisable to mulch the ground to conserve moisture, and if you learn how to make leaf mulch you can use your own organic garden material.

If you live in a very warm zone that can suffer extreme heat, cosmos will tolerate part shade.

Cosmos tolerate most pH levels, but do best in neutral to alkaline soils and are quite drought tolerant, so might be worth adding to your planting palette if you're planning a dry garden. Long periods or wet and cold are detrimental and can delay flowering.

Plant your cosmos in a group to make a real focal point statement for late summer into fall until the first frost. This will also attract more bees than if they are dotted through the garden.

Cosmos are useful for cheery color in any area of a backyard and suit a range of styles – from planting a cottage garden border, creating a prairie style naturalistic planting design, or planting a wildflower meadow.

(Video) Flower Gardening Tips : How to Grow Cosmos

‘I love to grow lots of different varieties of cosmos, putting together different heights and colors in pots and for borders for wonderful layers of intense colour,’ says Sarah Raven.

They are also excellent for companion planting alongside vegetable and fruit crops in a kitchen garden, as they entice pest predators and valuable pollinators.'We plant cosmos with our vegetables, in rows to help support each other,' explain Henrietta Courtauld and Bridget Elworthy, the Land Gardeners (opens in new tab).

When to plant cosmos seeds

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(Image credit: Leigh Clapp)

Early spring is the best time to sow cosmos seeds under cover indoors, which will then produce flowers that bloom earlier in summer.

Alternatively you can direct sow cosmos seeds in their flowering position once the soil has warmed up. This will differ depending on the area where you live, but will be from about late May in cooler regions, and earlier in warmer zones.

Bought seedlings can be planted in late May or June.

Is cosmos easy to grow?

Yes cosmos are easy to grow. 'Cosmos is one of the easiest flowers to grow from seed that is directly sown in the garden bed,' advise the experts at American Meadows (opens in new tab).

‘As they grow, stake cosmos if necessary, and water them regularly,' says Sarah Raven.

Cosmos suffer from few pest problems. Watch out for slugs and snails, though, especially when the plants are young and tender, so use slug barriers. To avoid powdery mildew and fungal diseases ensure your plants have space and the soil isn’t soggy. 'Cosmos prefer dry, arid soil over wet conditions. Soil that is too moist may lead to disease,' advise the American Meadows experts.

How to grow cosmos – expert tips on when and where to plant these flowers (6)

(Image credit: sarahraven.com / Jonathan Buckley)

Can you put cosmos in pots?

Cosmos, particularly the shorter varieties, can be grown in pots, and make attractive patio and container plants. As they are good at attracting beneficial insects to veg and fruit crops, why not include them in your vegetable garden container ideas?

Try a mix of shorter dwarf cosmos varieties, such as the Sensation and Sonata mixes, grown from seed, or buy as seedlings and plant out from May, spaced about 11in (30cm) apart.Cosmos work particularly well in pots with zinnias.

Use a light potting mix, in pots with good drainage and place in a sunny spot. Water regularly and feed with a liquid fertilizer every few weeks during summer.

(Video) Growing Flowering Plants : Growing Cosmos Plants

How to grow cosmos – expert tips on when and where to plant these flowers (7)

(Image credit: Leigh Clapp)

How to keep cosmos flowering

If your cosmos aren't flowering, you could be making one crucial mistake. An important step in how to grow cosmos and keep them flowering is to deadhead the blooms. This stops the plant putting its energy into creating seeds and instead puts its efforts into creating more flowers.

Chances are you will be busy in the garden deadheading roses and petunias, and other blooms, so add cosmos to the list.

'As long as you don’t cut the plants right to the ground, but above a pair of leaves, more buds will form to fill next week’s vases and more for the week after that. The lower you go in the plant, the more delay between the flower you've just picked and the next flower,' explains Sarah Raven.

'We love cosmos because it is light and airy and has the joy of endless giving – every time you deadhead another flower appears,' add the Land Gardeners.

How to grow cosmos – expert tips on when and where to plant these flowers (8)

(Image credit: Thompson & Morgan)

Do cosmos come back every year?

Nearly all cosmos are annuals meaning they do not come back every year. In order to have blooms every year, you will need to resow the seeds the following spring.

The only exception to this rule is chocolate cosmos,cosmosatrosanguineus, which is grown like a dahlia from a tuber and is a perennial. Chocolate cosmos is loved for its delicious vanillary-chocolate scent and velvety brown flowers, and since it is a perennial, will come back year after year.

Annual cosmos can also self seed. If you let some of your cosmos flowers die naturally and fall to the ground they will germinate seeds by themselves. Allowing plants to self seed is a step on the way to creating an eco-friendly garden.

You may also like to try collecting seeds to save from the flowers to sow next spring. If you want to save seeds be aware that hybrids will not grow true-to-type, so select varieties that have been open pollinated, as opposed to hybrids – which can vary widely in the next generation.

You can collect cosmos seeds at the same time as you are collecting zinnia seeds and those of other flowers that you love. This is a cost-effective way to populate your garden with flowers, year on year.

Scatter the collected seeds in your garden or save in labelled envelopes or paper bags for sowing the next year.

How to grow cosmos – expert tips on when and where to plant these flowers (9)

(Video) Cosmos | Cut Flower Facts

(Image credit: Leigh Clapp)

Do you pinch cosmos?

You should pinch out the growing tips of cosmos to encourage branching and flowering, and in around 12 weeks you should see your first blooms.

If you then want to enjoy those flowers in the house as well as in the garden, cosmos make excellent cut flowers. To harvest for cut flowers, cut the cosmos blooms when they are beginning to unfurl in the morning, as this is when there will be the most moisture making them less likely to wilt.

Plunge the blooms into a bucket of warm water, stripping off lower leaves to avoid them in the water. There are many tricks for how to keep flowers fresh in a vase. Re-cut the stems regularly and refresh the water and cosmos should last up to 10 days in the vase.

How to grow cosmos – expert tips on when and where to plant these flowers (10)

(Image credit: Pippa Blenkinsop)

Do cosmos need staking?

The taller varieties of cosmos will need staking to prevent them flopping over.

They can grow up to 8 feet (2.5m) and flowers can reach 3 inches (8cm) across.

‘It’s worth taking the time to stake them properly as they benefit greatly from growing straight early on. If they collapse, they’ll never grow or flower as well as when vertically supported,’ explains Sarah Raven.

How often should I water cosmos?

Water cosmos regularly until they are established, or during dry spells. It is important, however, that you do not over-water cosmos as this can result in plants with fewer flowers.

Cosmos can tolerate dry soil, even in a hot, sunbakedspot. In fact, they prefer poor soil and seem to thrive on a bit of neglect!

How to grow cosmos – expert tips on when and where to plant these flowers (11)

(Image credit: Leigh Clapp)

Do cosmos like sun or shade?

Cosmos are sun loving flowers, although they will tolerate a little shade in warmer climates.

(Video) Sowing Cosmos Flower Seeds in Unheated Greenhouse - Cut Flower Garden

Cosmos and zinnias are good companions and mix them also with other sun-loving plants in borders, beds, meadow planting and containers, such as Verbena bonariensis, alstroemeria, calendula, eryngium, dahlias and nepeta.

'Cosmos can be combined with almost all full sun border plants to give that "wow" factor mid to late season. Infilled in a shrub border or alongside flowering plants, such as monarda, salvias and gaura works well,' says garden designer Charlie Bloom.

The range of colors allows for diverse combinations, whether bright and breezy or more subtle, so there’s lots of scope.

FAQs

Where is the best place to grow cosmos? ›

It is best to plant them in a sunny spot and into soil which has has some organic material, such as Farmyard Manure, dug into it. This will help them to retain water. They should be planted 30 to 45 cm (12-15 inches) away from each other to allow for bushy growth.

What is the best month to plant cosmos? ›

The shorter varieties of cosmos are perfect for pots and seedlings can be planted out from May. You can start by growing them from seed undercover in March-April, or you can buy cosmos seedlings. Space the seedlings about 30cm (1ft) apart in good compost. Keep them well watered and they'll be in flower by early July.

When should you plant cosmos seeds? ›

To give your cosmos as long a flowering season as possible, sow the seeds early, indoors, in March or April. Once they've sprouted and have two pairs of leaves, transplant into individual 7cm pots and grow on under cover.

When should cosmos flower? ›

Early spring is the best time to sow cosmos seeds under cover indoors, which will then produce flowers that bloom earlier in summer. Alternatively you can direct sow cosmos seeds in their flowering position once the soil has warmed up.

Do cosmos do well in pots? ›

Cosmos grow in both beds and containers—and they also make great cut flowers! The soil should be well-draining, but other than that, cosmos don't need any special soil preparation. In fact, they like soil that is not too rich, as rich soil will encourage foliage at the expense of blooms.

Do cosmos need big pots? ›

Which Container Is Best? Because cosmos are very drought tolerant and heat loving, their roots tend to reach rather deeply. A vessel of appropriate size should allow at least 12 inches of depth and be equally wide, or wider.

How often do you water cosmos? ›

Keep the surface of the soil moist, watering daily with your spray bottle if needed. Ideally, keep the seeds warm while they're germinating, at a temperature of at least 60°F.

How do I start cosmos? ›

Starting Cosmos Seeds Indoors

Cosmos should be seeded indoors in cell trays or 2-inch soil blocks about four weeks before your last frost. Cover the seeds lightly with soil, and place the tray on a heat mat with a humidity dome. Bottom water or mist to keep the soil moist until cotyledons emerge.

How long do cosmos take to grow? ›

Cosmos are incredibly easy to grow, making them perfect for beginning gardeners. Seeds can be started indoors to get a jump-start on the season or sown directly into garden beds once the weather warms. Either way, cosmos will bloom in just under 3 months from the date you sow them.

Can I just throw cosmos seeds on the ground? ›

Time: Sow seeds directly in the ground once all danger of frost has passed. Spacing: I scattered my seeds on the soil and lightly covered them with dirt. Once the seeds begin to sprout, I will thin the seedlings out to 10-12 inches apart.

Can you plant cosmos straight into the ground? ›

How to plant cosmos. Sow seed in early spring directly into the soil where you want your cosmos to grow, or into small pots or modules filled with free-draining seed compost. If growing in pots, pot on seedlings when they are large enough to handle. Plant out in late April/May after the danger of frost has passed.

How do you care for cosmos? ›

Light: Cosmos prefer full sun conditions, except in extreme heat where they can tolerate part shade. Soil: Prepare the garden with loose, weed-free soil. Cosmos prefer dry, arid soil over wet conditions. Soil that is too moist may lead to disease.

Do cosmos flowers come back every year? ›

Cosmos (Cosmos spp.) is a moderate reseeder, which means that it drops plenty of seeds to bring it back year after year without becoming an uncontrollable nuisance. For cosmos to reseed itself, you have to leave the faded flowers in place long enough for seeds to form.

Do cosmos come back after cutting? ›

Cosmos produce a delightful array of colored blooms from pink and white to yellow, red, and even brown. These pretty annuals produce prolifically, especially if you know how and when to cut back and pinch. Cutting cosmos will give you new flowers for the entire growing season as well as blooms for floral arrangements.

How do I keep my cosmos blooming? ›

While it's a good idea to deadhead your cosmos to keep them blooming, be sure to let a few go to seed toward the end of the season so you'll have more plants the following year. Let them grow wherever they land or save seeds so you can choose where to grow them.

How many cosmos should I plant? ›

Plan on growing one cosmos plant per gallon of your container. If growing in pots, do not enrich the soil, it makes the plants grow tall, leggy, and droopy. Also, tall varieties will need staking in containers.

Does cosmos need fertilizer? ›

Fertilization. Do not fertilize cosmos. Nutrient rich soil produces plants that are weak-stemmed (floppy), late blooming and flower sparsely. Avoid soil that have been heavily amended with fertilizer and compost.

What flowers look good with cosmos? ›

Cosmos are versatile companions, thanks to their (usually) simple flower shape, upright habit and feathery foliage. They match well with dahlias, zinnias and marigolds—all of which also trace their roots to Mexico and whose flowers can have a similar shape and size to cosmos (depending on type).

Can cosmos be grown in small pots? ›

Yes, You Can Grow Cosmos in a Container.

How deep should cosmos be planted? ›

Plant seeds in prepared soil about 2 inches (5 cm) apart and one-half inch (1 cm) deep. Thin to 8 inches (20 cm) apart in all directions for dwarf varieties; allow 12 inches (30 cm) between very tall varieties.

How many hours of sun do cosmos need? ›

Cosmos is a short-day plant, which means it will bloom best when day length is 14 hours or less. With long days (over 14 hours of light per day), the plants are slower to bloom and the overall percentage of plants that produce flowers is decreased. The effect of photoperiod, however, is greatest with young plants.

How tall do cosmos flowers grow? ›

Typical plant height for cosmos is 1 to 5 feet.

Can you take cuttings of cosmos? ›

Cosmos atrosanguineus, or chocolate cosmos, is a popular perennial that is typically propagated from root cuttings. Grow annuals from seed or nursery starts. Start annuals from seed indoors 4 to 6 weeks before your last spring frost or direct sow in spring after last chance of frost is past.

How long do cosmos flower for in the garden? ›

A gorgeous cottage garden favourite. Cosmos are half-hardy annuals- that means they grow, bloom and set seed and die all in one year, meaning that you need to sow fresh seed each year.

Why do my cosmos have no flowers? ›

If your cosmos plant will not flower but has produced a lot of healthy looking leaves, it may be due to overfertilization. If you are presently using a 20-20-20 fertilizer, with 20% nitrogen, phosphorous, and potassium, try switching to a type with less nitrogen.

Why are my cosmos dying? ›

Bacterial wilt is one of the classic cosmos flower diseases. As it might seem, it is a bacterial disease that causes stems to wilt at the base. The entire stem and flower will become infected and finally the root system. You must dig up the plant and destroy it, as there is no cure.

What colors do cosmos come in? ›

Bright green fernlike foliage is the perfect complement to the daisylike flowers of cosmos, which come in shades of white, pink, yellow, or orange.

Does cosmos need good soil? ›

Cosmos plants originated in Mexico and South America, hence they flourish in sheltered and full sun positions. They can thrive in relatively poor soil, but they do need watering and flower best in good quality well-drained soil.

Do cosmos flowers spread? ›

Plants get very bushy and prefer a little extra room to spread out, so space plants 12 to 18 inches (30.5 to 46 cm) apart. Once in the ground, cosmos will grow rapidly, so be sure to stake them early, while they are still young.

Is cosmos full sun or shade? ›

Light: Cosmos prefer full sun conditions, except in extreme heat where they can tolerate part shade. Soil: Prepare the garden with loose, weed-free soil. Cosmos prefer dry, arid soil over wet conditions. Soil that is too moist may lead to disease.

Is cosmos hard to grow? ›

Cosmos are freely flowering annuals that are easy to grow by sprinkling some seeds in the garden after any danger of frost has passed. These quintessential cottage garden flowers reach full maturity in about two months.

Does cosmos need much water? ›

Cosmos is simple and easy to grow from seed sown in the spring, readily reseeds itself, and tolerates a wide range of soil types. Once established, Cosmos needs little water, no fertilizer and not much care, which makes it well suited to Utah's climate.

How tall do cosmos plants get? ›

Typical plant height for cosmos is 1 to 5 feet.

What grows well with cosmos? ›

Cosmos are versatile companions, thanks to their (usually) simple flower shape, upright habit and feathery foliage. They match well with dahlias, zinnias and marigolds—all of which also trace their roots to Mexico and whose flowers can have a similar shape and size to cosmos (depending on type).

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