How to Grow Cosmos Indoors

Apr 09, 2021Danah Fabian

Cosmos flowers are often used to symbolize order and harmony, due largely to the flower’s harmonious petals

Timing

Sprouts in 1-2 weeks. Harvest from Month 3+ on.

Full sun

Equivalent of 7+ hours of direct sun [DLI of 21+ mol/m²/day].

Care

Beginner friendly. You’ll sprout, thin, and deadhead.

Are Cosmos Edible?

Yes, certain species of cosmos are edible. They are Cosmos sulphureus (C. bipinnatus, C. caudatus or Mexican Aster). Especially if you grow organic cosmos seeds and use non-chemical plant food, then yes, you can eat the petals of cosmos flowers and add them to your salads.

Growing Cosmos: Best Cosmos varieties to grow inside.

Cosmos flowers can be successfully grown in containers. They can grow as much as 6 feet tall, so we recommend that you look for dwarf or compact cultivars for indoor growing. Here are our most favorite ones:

Garden

Has very attractive flowers throughout the summer. Also attracts bees, butterflies, and birds to your garden.

Amazon

Sulphur

They are a lovely addition to your home and edible, too! Toss a few of the zesty petals into your favorite soup or salad, or just eat straight off the plant as a snack!

Urban Leaf

Chocolate

Makes wonderful and exotic cut flowers and does a great job at attracting butterflies to your garden

Amazon

How to Grow Cosmos: Best Setup for Cosmos Plants

You’ll need:

Planter or Pot for Your Cosmos:

Ceramic Self Watering Planter (preferred) or pot that is at least 12″ / 5 gal.

Soil:

Standard Potting Mix

Plant Food:

At the start: Balanced Blend. This should be equal parts nitrogen, phosphorus, and potassium (with NPK numbers like 10-10-10).

Ongoing: Vegetable Blend. This should be high in phosphorus and low in nitrogen (with NPK numbers like 4-10-6).

Do Cosmos Need Full Sun? Grow Lights for Your Cosmos:

A strong grow light that can give the equivalent of 7+ hours of direct sun [DLI of 21+ mol/m²/day].

Jump to: Our product recommendations

Preparing your Planter & Watering Schedule for Cosmos

Cosmos plants do well in moist soil conditions. If the soil dries out completely the roots will die back and it will be tough for the plant to recover. On the other hand, if the roots are exposed to standing water for too long, they can rot.

A Ceramic Self Watering Planter filled with a standard potting mix self-regulates to keep the soil at consistent moisture for your plant to thrive (and no watering guesswork for you).

To set one up:

Fill up the planter with dry soil from the bag, gently tamping down the top.
Dump the soil into a large mixing bowl and add water until the soil is moist, but not sopping wet (about ½ Cup)
Mix in 1 tablespoon of the Balanced Blend Plant Food.

If you are using a regular pot instead, it should be a little bit bigger (at least 12″ / 5 gal and will need drainage holes to prevent it from being over watered. Let the top of the soil dry out between watering.

Starting your Cosmos: Seed vs Cutting vs Nursery Plant

New Cosmos plants can be started from seed, propagated from an established plant, or purchased live at many garden centers. We prefer to sprout from seed or propagate from a stem cutting, as it results in plants that are adapted to your growing conditions and limit the chances that you accidentally bring home pests.)

Growing Cosmos: How to Plant Cosmos seeds

Cosmos grow quickly from seed. Plant 2 sites in a 12″ / 5 gal container. In larger containers, space sites 4″ apart. For each site plant 2 seeds 1/4 inches deep. Keep the soil warm ( 70-90°F, ideally 75°F). Sprouts typically appear in 7 days but can be as quick as 5 days or as long as 10 days depending on your conditions.

Propagating Cosmos: How to Clone from a Stem Cutting

If you’ve already got a Cosmos plant you love (or a friend does!) you can easily “clone” it with just sharp scissors and a clean glass of water. First, cut a couple 6” shoots of new growth (avoid anything woody). Next, remove the lower leaves, so the bottom half is just stem. Place in a glass of 3” of water, making sure the cut leaf spots are underwater. Place the glass on a bright windowsill and change the water every few days. In a couple of weeks, roots should emerge and you can transplant them into your container. While using additional rooting hormones won’t hurt, it’s not necessary with Cosmos plants.

Cut 6” section of new growth
Remove leaves halfway and place them in the water on a sunny window sill
Wait 14 days for a few ½ inch roots to form and carefully transplant into it final container

How to Transplant Cosmos

Live starter plants give you a big jump start on your first harvest. When you’re in a garden center – pick the bushiest plant available (tall and lanky ones will be weak growers) and give it a good inspection for pests. Leaves should be dark green without holes, spots, or curled edges. A best practice is to actually “quarantine” your plant for about a week after bringing it home to make sure it’s free and clear of ride-on pests.

Ensuring it’s pest and disease-free it’s time to transplant your seedling into its final home.

Remove some soil from its final planter – leaving enough space for the bottom of the seedling to be just higher than the soil surface.
Hold on to the base of the stem with one hand, and turn the pot over while gently pulling the seedling. Giving the pot a few squeezes can help dislodge it.
Place in its final container and fill around it with soil so that it’s tight, but not compacted.

Growing Cosmos: Where to grow your Cosmos plants

Cosmos plants have the highest light needs of any plant – so unless you have a totally unobstructed southern-facing window and plan on only growing in the summer – you’ll need a grow light. We still recommend taking advantage of your bright window (sunlight is free and great for plants!) and supplement it with a grow light. For an introduction to grow lights, head over to our post on grow lights for indoor gardeners. We’ve also got a buying guide for screw in types, but to keep things simple in this guide, we’ll just provide directions for the 24W Screw in Bulb by Sansi, which we think is a good middle-of-the-road option.

How bright should your grow light be?

Cosmos plants need the equivalent of 7+ hours of direct sunlight [DLI of 21+ mol/m²/day] to grow their best. In order to provide an equivalent amount with a grow light, it needs to be pretty bright! The 24W Sansi bulb should be placed 6 inches away from the top of the plant. This will give your PPFD (the standard measure of brightness) of 500 μmol/m²/s.

How many hours per day do your Cosmos plants need under a grow light?

Cosmos plants are known as “short-day,” meaning they’ll develop faster if they sense over 12 hours of darkness. We want them to progress into flowering as soon as possible, so we recommend setting up a timer to leave it on for only 10 hours per day.

Growing Cosmos: Cosmos Plants Grow Faster in Warmer Temps

Cosmos plants are called “warm-weather crops” and will speed up their metabolism when temperatures are warmer. On the other hand, if things get too hot they’ll wilt and become prone to disease. Ideal temperatures are around 80°F but anything between 65 and 100°F grows well.

Week 1-2: Check for Sprouts

You could see seedlings in as little as 5 days (though 7 days is more typical). If it’s been 10 days and you still don’t have any sprouts, it’s likely that your setup is too cold.

Week 3: Thin Your Seedlings

Thin your planter to only have 1 seedling per site – leaving the largest plant. If you are using the recommended planter (at least 12″ / 5 gals) this will mean you’ve got 2 plants after thinning. By getting rid of the smaller seedlings, you’re allowing the biggest and strongest one to flourish by reducing its competition for water, food, and space.

If your seedlings are under 1 inch, stretching out, or folding over, it’s likely that they don’t have quite enough light.

Month 3+: How to Harvest Cosmos

Pick flowers right as they open to encourage more flowers to grow. If you let flowers go to seed on the plant it will start to end their life cycle so you should “deadhead” (remove the old flowers) to prolong the flowering season. If you fall behind, you can cut the plant by one-third which produces a second flush of leaves and flowers.

Year 1: End of Life

You can prolong your cosmos life by pruning it and removing flowers before they have a chance to go to seed, but eventually, it will die. At this time is best to clear the plant and start over.

Shop This Blog to Start Growing Cosmos

The right supplies can take the guesswork out of caring for your plants – and turn care from a daily to weekly routine. Through our grow tests, we’ve found these products to produce the best indoor Cosmos (and also have simple maintenance). Plants are adaptable and can grow in many different conditions, so they are by no means necessary if you already have other supplies.

Best Containers for Cosmos: Ceramic Self Watering Planters

Plants thrive on consistent moisture but can suffer if they’re waterlogged. A semi-porous ceramic self regulates ideal conditions. Our favorite is the COSWIP planter. Runner up is XS Self Watering Planter by Wet Pot.

Best Soil for Cosmos: Standard Potting Mix

Cosmos likes a rich and moist root zone – so you are best off with a standard potting mix – we like this Organic Mix by Espoma.

Best Nutrients for Cosmos: Balanced Blend followed by Vegetable Blend

Cosmos likes to start with nutrients that are equal parts nitrogen, phosphorus, and potassium (with NPK numbers like 10-10-10). For this Balanced Blend we recommend: Dr Earth All Purpose

Once they are growing, it’s better to use plant food that is high in phosphorus and low in nitrogen (with NPK numbers like 4-10-6). For this Vegetable Blend, we recommend: Joyful Dirt Tomato & Herb

Best Light for Cosmos: DIY or Soltech

There is a very small chance that you have the bright windows needed to grow these without a grow light. If you are looking for a higher-end option – we love the Aspect Light by Soltech. For a more affordable option, a DIY setup using a 24W Screw-in Bulb by Sansi with a Clamp Light and Mechanical Timer works well too. Check out our complete guide on a DIY setup for less than $40 or our buying guide for screw in bulbs.