How To Sow Cosmos Seeds To Start & Grow Cosmos Flowers?

Cosmos seeds need no special preparation for sowing them. These are one of the most hassle free flowers that can be grown via seeds.

Grow Cosmos from Seed
Cosmos are eye-catching garden flowers that reach maturity in around two months. Though they can be slow to germinate, they quickly bloom post-germination and continue to bloom across the fall.


Once you come to know how to plant cosmos seeds, you will realize how easy it actually was. Fortunately, the cosmos is one of the easiest flowers to grow. All you have to do is sprinkle cosmos seeds in the garden once the danger of frost has elapsed.


You will see them flowering in as little as twelve weeks. To ensure the cosmos have a long flowering season, you must sow the seeds early, indoors, between March to April.


After they sprout and develop two pairs of leaves, you can transplant them to seven-cm individual pots and grow them under cover. These are half-hardy annuals and grow flowers, set seeds and die all in a year. However, unlike the hardy annuals, the cosmos cannot endure low temperatures.


Cosmos are attractive annual flowers with daisy-like, colorful blossoms that nestle on their slender and long stems. Blooming from summer through fall, these attract bees, butteries, and birds to the garden. With some simple steps, you can learn how to start cosmos from seeds effortlessly.


They are easy to grow from seeds and survive even the harshest soil conditions. The plant yields three to five-inch flowers often confused with daisies. They bloom in different hues of white, yellow, orange, red, pink, and maroon.


Their flowerheads are open or bowl-shaped, and the beautiful plants can reach up to six feet tall. They make lovely cut flowers, and you can grow them in containers and beds. Its leaves grow opposite on stems and are deeply bipinnate, pinnate, lobed, and feathery-looking, depending on your chosen cosmos variety.


You can add them to pots and borders for a burst of showy color. Before we discuss how to grow cosmos from seeds in detail, here’s an overview of cosmos plants.


Cosmos Information Overview


Scientific Name Cosmos bipinnatus, Cosmos sulphureus
Common Names Mexican aster, Cut-leaf Cosmos, Cosmos
Plant Type Annual
Family Asteraceae
Bloom Color White, Yellow, Golden Yellow, Chocolate, Red, Orange, Magenta, and Pink
USDA zones Two through Eleven
Mature Size 1-6 feet tall with 1-3 feet spread
Sun Full Sun
Soil Free-draining Soil with acidic 6-6.8 pH
Bloom Season Summer through Fall
Native Region Southern North America, Central America, and Northern South America
Toxic Nontoxic


Are Cosmos Easy To Grow From Seed?

Cosmos Easy To Grow From Seed
Cosmos is one of the easiest flowers among the different types of flowers you can grow from seeds. However, please watch out for snails and slugs, especially when the plants are tender and young, as they relish the juicy young foliage and can demolish the seedlings.


You must have in mind a general idea of how to plant cosmos seeds, to witness these beauties in time.


You can directly sow them in the garden bed in their flowering position once the soil is warm from May onwards. While sowing, opt for a suitable planting spot that receives full sun, and then prepare the ground by adding organic material like well-rotted manure or garden compost.


Work the seeds using the rake into the seed beds by breaking down any large clumps. Now, thinly sow the seeds in rows, and go on to label every variety and line. When you sow in line, it is easier to spot the weed seedlings, and you can instantly remove them.


When the seedlings are well-developed, you can thin your cosmos. Later, you can shift them to a large pot (three in one pot) to fill in the gaps in the border. It should be easy as the seedlings transplant well and work with taller varieties.
Related: How Long Does It Take Flowers To Bloom?


Should Cosmos Seeds Be Soaked Before Planting?

Yes, it always helps to soak the Cosmos seeds before planting. We recommend soaking them for 12 to 24 hours.


Ideally, for almost all cosmos varieties, you must soak the seeds a day or two before planting as opposed to overnight.


One of the biggest pros of soaking the seeds before planting is that it significantly lowers the germination time. Thus, you will achieve a blooming and happier garden sooner.


What Time Of Year Do You Plant Cosmos Seeds?

You can sow the cosmos between March and April to ensure the best results.


How To Sow Cosmos Seeds?

Sow Cosmos Seeds


To sow cosmos seeds using a germination tray or an old egg carton, follow these steps:


  1. Fill the cells of a germination tray or the cups of an egg carton with seed-starting mix, and moisten them well.
  2. Sow one or two seeds in each cell or cup, pressing them gently into the soil.
  3. Cover the seeds with a thin layer of soil and mist them lightly with water to settle the soil and moisten the seeds.
  4. Place the tray or egg carton in a warm location that receives indirect light, such as a windowsill or under fluorescent lights.
  5. Keep the soil consistently moist but not soggy. You can cover the tray or carton with a plastic lid or plastic wrap to help retain moisture.
  6. Once the seedlings emerge, which usually takes 7-14 days, remove the lid or plastic wrap and place them in a sunny location.
  7. When the seedlings are large enough to handle, carefully transplant them into individual pots or in the garden, spacing them about 12 inches apart.
  8. Water the seedlings regularly and enjoy the beauty of your cosmos as they grow and bloom!


Where and How Deep To Plant Cosmos Seeds?

You must sow the cosmos seeds 3 mm deep in a small seed tray with compost and water them thoroughly. Next, you can cover them with polythene or glass to ensure that the see inside stays warm, as in the greenhouse or warm windowsill. It will ensure that the germination happens within a week.


How Far Apart To Plant Cosmos?

Cosmos can get bushy. Hence, they need adequate space to spread out. So, ideally, you must plant the shorter varieties of cosmos sulphureus at least 10 to 12 inches apart.


However, if growing the taller varieties like Cosmos bipinatus, a gap of at least one to two feet is recommended. In addition, you must consider staking the cosmos if they get too tall and gangly.

Cosmos Post Planting Care

Cosmos Post Planting Care


You have learned how to germinate cosmos seeds, but you must also take measures towards their after-care so that they bloom nicely. So, here’s what you need to do:


A. Soil

Cosmos enjoy neutral soil with a pH between 6.0 to 8.0. However, they will also grow in poor soil where most flowering plants languish but perform best in well-drained, medium-moisture soil. You will see them perform ok even in dry soil.


You must never grow them in rich soil. It can result in the plants becoming too tall and flopping over. To avoid drooping, grow them close to other plants that can offer them support or stake the plants.


B. How much light do cosmos seeds need to germinate?

Cosmos love sunlight and thrive in full sun. Of course, they can grow in partial shade but might not produce a lot of blooms. So, when you pick a sunny spot, you can offer conditions similar to their native habitat in Central America and Mexico. It will allow your cosmos to thrive.


C. Watering

They are not water enthusiasts. So, after they establish, they rarely require water unless there is a situation of prolonged drought. The cosmos are the last plants to demand irrigation in an area with less water.


D. Fertilizer

Often fertilizer harms the cosmos. Cosmos can endure poor soil. Excessive fertilizing can result in robust plants with excessive foliage and fewer blooms. So, unless your plant shows apparent signs of struggle, you do not need to fertilize them per se.


E. Staking

Cosmos often demand staking if you reside in areas with harsh winds or when your plants have a heavy top with many blooms. Here’s what you need to do to stake your cosmos:


  1. Firstly, pick the correct stake. You can choose from metal, bamboo, or wooden stakes. Regardless of the selection, ensure that it offers the requisite support to the plant and is tall enough to cover even the tallest blooms.
  2. Drive the stake into the ground next to the plant. Be cautious while you do so to avoid damage to the roots.
  3. Tie the plant’s stem to the stake with a flexible, soft material like a plant tie, cloth, or twine.
  4. Repeat the process as required, ensuring that you stake every stem to keep it standing tall and prevent it from drooping downwards.
  5. Regularly monitor the ties and adjust them as required to ensure that the plant stays securely annexed to the stake sans any damage.


Staking is usually necessary for the cosmos to stay upright and not fall over while simultaneously allowing them to bloom.

Do Cosmos Come Back Every Year?

Sadly, most cosmos varieties are annuals. It implies that they do not return year after year. 


So, if you desire more Cosmos, you must always collect the seeds and sow new plants every spring. However, Chocolate Cosmos or the Cosmos atrosanguineus is a tender perennial and will come back every year, provided you shield them against the winter cold.
Related: Winter Flowering Trees | Perennials That Bloom All Summer | Trees With White Flowers


How To Grow Cosmos From Seeds Indoors?

Grow Cosmos From Seeds Indoors


Growing Cosmos indoors is not very different how you plant cosmos seeds outdoors. Follow the steps given below.


A. Starting the Seeds

Use cell trays and plastic pots to start the cosmos seeds indoors. Any small container works well, provided it has drainage holes at the bottom.


In the container, add moist-seed starting compost that has perlite. It can allow the soil to hold moisture well.


Leave the top ¼ inch of the container vacant. Now add one or two cosmos seeds on the soil surface and spread a thin soil layer. Please do not conceal the seeds completely because darkness intervenes with germination.


Now, keep the container in a sunny and bright spot that receives good southern exposure. You can drape a plastic wrap onto the container to allow it to lock the moisture in the soil. Inspect the soil moisture daily, or when it feels dry, consider misting heavily.


The cosmos seeds will germinate a week or two after sowing, provided the temperature is between 70-75 degrees Fahrenheit. Move the pots to a propagation mat if the temperature is cool. It will warm the soil and help with germination.


B. Further Growing the Seeds for Lovely Blooms

Learning how to plant cosmos seeds is very simple, but you’ll fail in getting beautiful blooms if you don’t transfer them to the correct location timely.


Transplant the cosmos seedlings in individual five-inch pots after they are three to four inches tall. Now, employ fresh potting soil and add some slow-release fertilizer to it. Shift the container to a bright window and rotate them daily to ensure that every part receives light.


It will help avoid leaning and stretching. Be cautious, and water sparingly. Splashing water around the seedling base will suffice when the soil feels completely dry at the surface. Do not splash water onto the foliage.


As per the University of Minnesota Extension, you must commence the hardening-off two weeks before planting seedlings in the garden. Now, shift them to a lightly shaded, sheltered location outdoors for a few hours in the afternoon. It will help the plant meet its light requirements.


But at night, when the temperature outside is cold, you must bring the container indoors. Further, try increasing the time you leave your pot outdoors every day by 30 minutes so.


In addition, expand the exposure to direct sunlight. After 14 days, you can move your plant outdoors, provided there is no frost forecast. Once all the frost danger has ceased, you can transplant the cosmos into a sunny garden bed.


You will notice them bloom soon in vibrant colors!


Why Are My Cosmos Seeds Not Germinating?

We have discussed how to plant cosmos flower seeds, and if you followed all the listed measures to the T, and still your seeds do not germinate, it might be because of the following reasons:


  1. They do not have access to the required water for germination to occur.
  2. You have overwatered the seeds, resulting in patchy, poor, or non-existent germination. Overwatering might also lead to compaction and waterlogging.
  3. Your seeds are not getting adequate oxygen for metabolism until photosynthesis.
  4. Temperature is too low or too high.
  5. You did not store the seeds correctly.
  6. You picked stale seeds.