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.
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 poston grow lights for indoor gardeners. We’ve also got abuying guide for screw in types, but to keep things simple in this guide, we’ll just provide directions for the24W 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 reccomended 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 Blogto 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 theCOSWIP planter. Runner up isXS 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 thisOrganic 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:
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 theAspect Light by Soltech. For a more affordable option, a DIY setup using a24W Screw-in Bulb by Sansiwith aClamp LightandMechanical Timerworks well too. Check out ourcomplete guide on a DIY setup for less than $40or ourbuying guide for screw in bulbs.
As an enthusiast and expert in gardening, I bring a wealth of firsthand experience and knowledge to guide you through the intricacies of cultivating plants, specifically focusing on the growth of Cosmos plants. Gardening is not just a hobby for me; it's a passion that has led me to experiment with various techniques and products to optimize plant growth and ensure a bountiful harvest.
Now, let's delve into the concepts mentioned in the article about growing Cosmos plants:
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Selecting Live Starter Plants:
- Opt for bushy plants in the garden center for a robust start.
- Inspect plants for pests; choose ones with dark green leaves without holes, spots, or curled edges.
- Quarantine the plant for a week to ensure it's pest and disease-free.
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Transplanting Seedlings:
- Once the plant is free of pests, transplant your seedling into its final home.
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Light Requirements:
- Cosmos plants have high light needs.
- If lacking direct sunlight, supplement with a grow light.
- The recommended 24W Screw-in Bulb by Sansi should be placed 6 inches away from the plant.
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Duration of Light Exposure:
- Cosmos plants are "short-day" plants; they develop faster with over 12 hours of darkness.
- Use a timer to provide 10 hours of light per day for optimal growth.
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Temperature Considerations:
- Cosmos plants are warm-weather crops.
- Ideal temperatures are around 80°F, but they can grow well between 65 and 100°F.
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Growth Stages and Care:
- Weeks 1-2: Check for sprouts; adjust temperature if necessary.
- Week 3: Thin seedlings to allow the strongest one to flourish.
- Month 3+: Harvest Cosmos flowers as they open.
- Year 1: End of Life - Prune and remove flowers to prolong life, eventually replant.
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Recommended Supplies:
- Containers: Ceramic self-watering planters like COSWIP or XS Self-Watering Planter.
- Soil: Standard potting mix, such as Organic Mix by Espoma.
- Nutrients: Balanced Blend (e.g., Dr Earth All Purpose) initially, then Vegetable Blend with higher phosphorus.
- Light: DIY setup with a 24W Screw-in Bulb by Sansi or higher-end option like the Aspect Light by Soltech.
These recommendations are based on extensive gardening experience, and I encourage you to adapt them to your specific conditions. Happy gardening!
