Cosmos Flower Growing Tips (2023)

By: Sherri Ribbey
Looking for a flower that’s easy to grow and looks great? Give cosmos a try — it flowers from late spring to frost and won’t disappoint.

Cosmos Flower Growing Tips (2)

Grow lots of cosmos flowers

Cosmos has been a staple in cottage gardens for generations, but this lovely heirloom annual is a first rate addition to just about any casual garden you have right now. Cosmos flowers sit on long stems that gently nod in the breeze — a great addition to meadows and cutting gardens, too.

Two types of cosmos

The two most common cosmos are tall cosmos and sulphur cosmos. You’re probably most familiar with tall cosmos. This is the one you see in the grocery store seed rack every year. Plants grow 1 to 4 feet tall and have slender, ferny leaves. The 2- to 4-inch blooms come in shades of pink, red, white or violet and bicolors. Flowers are most often single but there are also doubles and other interesting variations — meet a few of my favorites in the gallery below.

Sulphur cosmos has 2- to 3-inch orange, red or yellow single blooms and grows 1 to 6 feet tall. Its foliage is coarser than that of tall cosmos and it tends to grow taller. ‘Bright Lights’ is a dwarf variety in a mix of yellow and shades of orange. But if you prefer a particular color, look for ‘Cosmic Yellow’ or ‘Sunset Orange’.

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Cosmos flowers are great in bouquets

Besides looking great in the garden, cosmos is a wonderful cut flower. To get the most from your summer bouquets check out these tips:

  • Cut your cosmos as the flower just opens and the center is still tight.
  • Use sharp scissors to cut the slender stems at a leaf node — that’s the spot where the leaf joins the stem. The plant will branch from there and form even more flowering stems.
  • Strip off the leaves that will sit below the water before putting your stem in a vase. Otherwise it will rot and shorten the life of the bloom. Do leave some of the frilly foliage at the top of the stem near the flower — its fine texture will be a nice contrast to larger leaves in your bouquet.

Deadheading cosmos flowers

Cosmos usually starts blooming in early summer and continues until frost if you deadhead. While you don't have to deadhead, doing so keeps the planty looking tidy and encourages a quick rebloom. Here’s how to do it: Cosmos produces multiple flowering stems near the top of the plant. The center one opens first. As that one fades, clip it out and the side stems will take off more quickly. When they’re all done cut the whole group off above a leaf node to encourage more growth and more blooms.

Cosmos Flower Growing Tips (3)

Shearing back

If you have a large planting or some back-of the-border beauties that are hard to reach, you may or may not want to be as detailed. The photo above shows a less fussy method. You can even go so far as to cut your cosmos back to within 12 to 18 inches of the ground, and the plants will rebloom again in a few weeks.

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Save cosmos seed

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. The video above shows you what to look for and how to harvest cosmos seed. They may not come true, or look like the parent plant, though. Check out the simple steps below on how to save cosmos seeds.

Cosmos Flower Growing Tips (4)

How to save cosmos flower seeds

  1. You’ll know it’s time to harvest when the spiky seeds are dark brown. Carefully grab a handful — they fall off quite easily at this point — to add to your stash.
  2. Once you get them indoors, sort out the seed from any debris and put them in a shallow bowl or on a tray out of direct sunlight for about a week to dry. This helps induce dormancy and harden the seedcoat.
  3. Store the seed in a plastic bag, jar or other handy container and keep it in a cool, dry place that stays 32 to 41 degrees F (the refrigerator is fine.)
  4. And don’t forget to add a label with the date so you know what you have next year.

Cosmos flower varieties to try

Looking for some cosmos to brighten your borders and liven up bouquets? Besides the traditional single flowers that bloom in pink, magenta, white or bright orange, there are plenty of exciting new looks for cosmos. Now let’s take a look at a few gorgeous cosmos varieties that you’re going to want to try.

Cosmos sources

Cosmos Flower Growing Tips (5)

‘Sensation Mix’ tall cosmos (Cosmos sulphureus )

Type Annual Blooms 3 to 4 in. single blooms in lavender, pink, magenta and white Light Full sun Soil Poor, well-drained Size 48 to 54 in. tall and 12 to 14 in. wide

(Video) How to Grow Cosmos From Seed | ALL SEASON FLOWER
Cosmos Flower Growing Tips (6)

'Seashells’ tall cosmos (Cosmos bipinnatus)

Type Annual Blooms An heirloom with fluted petals, this classic has stood the test of time. Flowers come in pink, red and white Light Full sun Soil Poor, well-drained Size 36 to 48 in. tall and 16 to 20 in. wide

Cosmos Flower Growing Tips (7)

‘Bright Lights’ sulphur cosmos (Cosmos sulphureus)

Type Annual Blooms This variety is a flower powerhouse — it blooms nonstop with 2 ½-in. blooms in shades of bright orange, yellow and red Light Full sun Soil Poor, well-drained Size 36 to 48 in. tall and 15 to 18 in. wide

Cosmos Flower Growing Tips (8)

‘Xanthos’ tall cosmos (Cosmos bipinnatus)

Type Annual Blooms Soft yellow 2 ½-in. open-pollinated blooms with a white center ring will come true if it reseeds Light Full sun Soil poor, well-drained Size 20 to 24 in. tall and 14 to 16 in. wide

Cosmos Flower Growing Tips (9)

Sonata White tall cosmos (Cosmos bipinnatus)

Type Annual Blooms Part of the Sonata series 2 to 3 in. flowers come in white, pink, magenta, purple and a mix. Sometimes you’ll find this series in multipacks at the garden center Light Full sun Soil Poor, well-drained Size 18 to 24 in. tall and 12 to 14 in. wide

(Video) Cosmos Flower Growing & Care | How to Grow Cosmos plant Easily | কসমস ফুল | कोसमोस |
Cosmos Flower Growing Tips (10)

‘Double Click Rose Bonbon’ tall cosmos (Cosmos bipinnatus)

Type Annual Blooms Tall enough for the back of the border, the frilly pink semidouble to double 3-in. blooms add a romantic look to the garden Light Full sun Soil Poor, well-drained Size 36 to 48 in. tall and 18 to 24 in. wide

‘Sensation Mix’ tall cosmos (Cosmos sulphureus )

Type Annual Blooms 3 to 4 in. single blooms in lavender, pink, magenta and white Light Full sun Soil Poor, well-drained Size 48 to 54 in. tall and 12 to 14 in. wide

Cosmos Flower Growing Tips (12)

‘Xanthos’ tall cosmos (Cosmos bipinnatus)

Type Annual Blooms Soft yellow 2 ½-in. open-pollinated blooms with a white center ring will come true if it reseeds Light Full sun Soil poor, well-drained Size 20 to 24 in. tall and 14 to 16 in. wide

Cosmos Flower Growing Tips (13)

'Seashells’ tall cosmos (Cosmos bipinnatus)

Type Annual Blooms An heirloom with fluted petals, this classic has stood the test of time. Flowers come in pink, red and white Light Full sun Soil Poor, well-drained Size 36 to 48 in. tall and 16 to 20 in. wide

(Video) How to Pinch Cosmos for More Flowers
Cosmos Flower Growing Tips (14)

Sonata White tall cosmos (Cosmos bipinnatus)

Type Annual Blooms Part of the Sonata series 2 to 3 in. flowers come in white, pink, magenta, purple and a mix. Sometimes you’ll find this series in multipacks at the garden center Light Full sun Soil Poor, well-drained Size 18 to 24 in. tall and 12 to 14 in. wide

Cosmos Flower Growing Tips (15)

‘Bright Lights’ sulphur cosmos (Cosmos sulphureus)

Type Annual Blooms This variety is a flower powerhouse — it blooms nonstop with 2 ½-in. blooms in shades of bright orange, yellow and red Light Full sun Soil Poor, well-drained Size 36 to 48 in. tall and 15 to 18 in. wide

Cosmos Flower Growing Tips (16)

‘Double Click Rose Bonbon’ tall cosmos (Cosmos bipinnatus)

Type Annual Blooms Tall enough for the back of the border, the frilly pink semidouble to double 3-in. blooms add a romantic look to the garden Light Full sun Soil Poor, well-drained Size 36 to 48 in. tall and 18 to 24 in. wide

Published: May 28, 2021

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(Video) How to Grow Cosmos From Seed - How to Prune For More Flowers and General Care

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FAQs

How do you successfully 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.

How can I make my cosmos grow faster? ›

Cosmos plants need full sun to bloom. Even the hint of shade, can restrict flowering. Also, to encourage more blooms, you need to deadhead the old blooms. For faster blooms, prune between the main stem and a leaf.

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 is my cosmos plant 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.

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.

What conditions do cosmos need to grow? ›

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.

Why are my cosmos growing so slowly? ›

Bad weather, seeds that have been stored for more then a year, too much fertilizer and slow draining soil can all affect cosmos blooms. Cosmos is unusual in that they require more hours of uninterrupted darkness then day light to flower (short day plants) so it may flower later in the season.

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.

What is the best fertilizer for cosmos? ›

Osmocote is one of the most popular and well-respected fertilizer brands and when it comes to cosmos fertilizers you can't go wrong with this plant food. What you'll love about this product is that it is easy for beginner cosmos gardeners. All you need to do is apply it once every 6 months.

Do cosmos flowers need a lot of 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 do you fertilize cosmos? ›

Sprinkle a half teaspoon of blooming plant fertilizer on the soil around each plant and water it into the soil. Repeat this feeding once every three to four weeks until the end of the flowering season.

How long does it take for cosmos to flower? ›

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.

Will cosmos grow in pots? ›

Cosmos flowers can be successfully grown in containers. Species plants can grow as much as 6 feet (2 m.) tall, so look for dwarf or compact cultivars for containers.

What is killing my cosmos? ›

While many insects may nibble on cosmos now and again, like grasshoppers, the most common pests that set up their cafeterias in your plants are aphids, thrips, and Lygus plant bugs.

How long do cosmos plants last? ›

We take you through the easy process of sowing cosmos seeds. Cosmos are half-hardy annuals that grow, flower, set seed and die all in one year, but unlike hardy annuals, they can't withstand low temperatures.

Do cosmos like sun or shade? ›

Grow cosmos in moist but well-drained soil in full sun. Deadhead to prolong flowering and lift in autumn, after the first frosts.

What season do cosmos grow? ›

Sow cosmos in early spring so the plants can become established before hot weather comes. Cosmos also can be started indoors in containers and set out when the seedlings have at least five leaves, or purchased as bedding plants. Plant seeds in prepared soil about 2 inches (5 cm) apart and one-half inch (1 cm) deep.

Can cosmos survive heat? ›

Cosmos is a heat-loving plant and does best above 60°F(16°C). Below 55°F(13°C), growth and flowering are inhibited. To direct seed: after last frost, once soil temperature is above 60°F (16°C), sow seed thinly in rows, barely covering the seeds (planting seeds too deeply will hinder germination).

Why are my cosmos so short? ›

Excess Water. Since cosmos are native to Mexico, they are used to dry, humid soil conditions. In fact, they thrive in drought-like conditions. So overwatering is often a big reason for a reduction in blooms.

Should you feed cosmos? ›

Ensure you deadhead Cosmos throughout the season. By taking off the spent flowers this will encourage more flowers to bloom. You can also feed the Cosmos plants with tomato feed weekly or fortnightly for even more flowers.

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.

What kind of soil do cosmos like? ›

Cosmos need light soil with average to poor fertility that has a neutral or slightly alkaline pH and is well-drained. Soil that is too rich yields weak-stemmed, sparsely flowered plants that bloom late and flop over, so avoid soil that has been heavily amended, and don't feed the plants.

Should you pinch cosmos? ›

Pinching back in some plants also stimulate additional flower production. Not all plants respond this way, but cosmos do. Start cutting back cosmos early in the growing season and continue to do it regularly, and they will reward you with abundant flowers.

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.

Can cosmos be overwatered? ›

Water regularly until plants are established or if it is unusually dry. Make sure you don't over-water cosmos; over-watering and over-fertilization can lead to plants with fewer flowers. Cosmos can tolerate dry soil, even in a hot, arid, sunbaked spot.

Why are my cosmos so leggy? ›

Cosmos seedlings become leggy because of a lack of sunlight. When you transplant them, make sure the legs are covered. If, like me, you've waited too long to re-plant your seedlings and they're reaching toward the sky, simply bury a lot of the stem in the soil.

Are Cosmo easy to grow? ›

Cosmos are half-hardy annuals that grow, flower, set seed and die all in one year, but unlike hardy annuals, they can't withstand low temperatures. They're quick and easy to grow from seed, flowering in as little as 12 weeks, and can be added to borders and pots for a burst of showy colour.

How do you keep cosmos blooming? ›

Deadheading. One of the most common reasons why your cosmos might not be flowering strong is the lack of pruning and deadheading. Be sure to deadhead any spent blooms in order to promote continual flowering. When plants are around 18 inches tall, prune off all stems until they are around a foot tall.

How long does it take for a Cosmo 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 cosmos grow in shade? ›

While bedding plants are sold in spring, cosmos are simple and inexpensive to grow from seeds. Plant them in full sun (in very hot regions, cosmos can take afternoon shade) and give them protection from strong winds. Cosmos tolerates a wide range of soil types, including poor soil.

How much sunlight does a Cosmo 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.

What month does 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.

How many times do you pinch out cosmos? ›

Pinch out the growing tip of each stem when 3 pairs of true leaves have grown to encourage stems to branch and produce more flowers. Then plant out in late May or June once the risk of frosts has passed.

What do you do with cosmos when finished flowering? ›

Place them in pots until they've finished flowering, then shelter over winter in a frost-free place until spring.

How tall should cosmos be before pinching out? ›

Start with cosmos seedlings that are at least 6-8 inches in height. Use scissors or your nails to pinch cosmos after the top three inches or so. Make sure to leave branches on both sides if you're trying to double your flowers.

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