Cosmos flowers are among the easiest and most rewarding annuals to grow from seed. With their feathery foliage, vibrant colours, and long-lasting blooms, cosmos can brighten any garden from late spring until the first frost. These beauties are also pollinator magnets, making them a fantastic choice for eco-conscious gardeners.
This guide will walk you through growing cosmos flowers from seeds, whether you prefer to start indoors or sow them directly outside. Let’s dive in!
Why Choose Cosmos for Your Garden?
Cosmos flowers are drought-tolerant, thrive in poor soils, and bloom profusely with minimal care. Available in an array of colours—from whites and pinks to deep purples and vibrant oranges—cosmos provide an effortless way to create a stunning garden display. These flowers also make excellent cut flowers, often lasting over a week in a vase.
Choosing Your Cosmos Variety
There are two main types of cosmos to grow:
- Cosmos bipinnatus: Classic garden cosmos with feathery foliage and colours ranging from white to deep crimson.
- Cosmos sulphureus: More compact plants with fiery hues like orange and yellow. Try ‘Bright Lights’ or ‘Cosmic Orange’ for a bold pop of colour.
When to Start Cosmos Seeds
- Starting Indoors (recommended) : 4 to 6 weeks before the last expected frost date (e.g., mid- to late-April for many Canadian gardeners).
- Direct Sowing: After all danger of frost has passed and the soil has warmed, usually late May or early June in most areas.
Starting Cosmos Seeds Indoors
Materials Needed:
- Seed trays or small pots
- Seed-starting mix (light, well-draining soil)
- Clear plastic dome or plastic wrap (optional for humidity)
- Cosmos seeds
- Spray bottle for watering
Step-by-Step Instructions:
- Prepare Your Pots: Fill seed trays or pots with seed-starting mix. Moisten the soil lightly before planting.
- Plant the Seeds: Cosmos seeds are relatively large, making them easy to handle. Place 1 seeds about ¼ inch (6 mm) deep in each pot.
- Cover Lightly: Gently cover the seeds with a thin layer of soil. Do not compact the soil; cosmos prefer a light touch.
- Water: Mist the soil lightly with a spray bottle.
- Provide Warmth and Light: Place the trays in a warm spot (21–26°C or 70–80°F). Once seedlings emerge, move them to a bright location or use grow lights for at least 12–14 hours a day.
- Transplant: Once seedlings have two sets of true leaves and the outdoor soil temperature is consistently above 15°C (60°F), transplant them into your garden. Space plants 12–18 inches apart for adequate airflow.
Pro Tip: Don’t rush to transplant your cosmos. They dislike root disturbance, so handle seedlings with care.
Direct Sowing Cosmos Flowers Outdoors
Materials Needed:
- Prepared garden bed or large pots
- Cosmos seeds
- Garden trowel or small rake
Step-by-Step Instructions:
- Choose the Right Spot: Pick a sunny location with well-draining soil. Cosmos tolerate poor soil but bloom best when soil isn’t overly rich (too much nitrogen can lead to lush foliage but fewer flowers).
- Prepare the Soil: Loosen the top 2–3 inches of soil. Remove weeds and mix in some compost if needed.
- Plant the Seeds: Scatter seeds lightly over the area or plant them individually ¼ inch (6 mm) deep, spaced 12–18 inches apart.
- Cover Gently: Use your hands or a rake to cover the seeds with a thin layer of soil.
- Water Lightly: Use a gentle spray to water the area without washing away the seeds.
- Wait for Germination: Cosmos seeds germinate in about 7–20 days, depending on soil warmth.
Tips for Growing Cosmos Successfully
Soil Preferences
- Type: Well-draining soil is key. If your garden soil is heavy clay, amend it with sand or compost.
- pH Range: Cosmos thrive in slightly acidic to neutral soils (6.0–7.0).
- Fertility: Keep it moderate! Overly fertile soil can result in fewer blooms.
Watering
- Water sparingly once the plants are established. Cosmos are drought-tolerant but need consistent moisture during their early growth stage.
Pinching Back for Bushier Growth
- When plants are about 12–15 cm (5–6 inches) tall, pinch back the growing tips (cut off the top inch) to encourage branching. This will result in fuller, bushier plants with more blooms. Larger tips (i.e. 2" or more) that you pinch off can be placed in a cup of water to promote new roots and then transplanted for even more cosmos!
Companion Planting and Pests
Good Companions:
- Plant cosmos with marigolds, zinnias, or sunflowers for a vibrant display.
- Cosmos attract bees, butterflies, and other beneficial pollinators, making them ideal companions for vegetables like tomatoes and cucumbers.
Pest Issues:
- Cosmos are largely pest-free! Aphids may occasionally appear but can be managed with a blast of water or insecticidal soap.
Harvesting and Deadheading
For Bouquets:
- Cut cosmos in the morning when blooms are partially open. Place the stems in water immediately.
For Continuous Blooms:
- Deadhead spent flowers (cut just above a leaf node) to encourage more blooms throughout the growing season.
End-of-Season Care
- Saving Seeds: Allow some blooms to mature fully and turn brown. Collect the dried seed heads, remove the seeds, and store them in a cool, dry place.
- Composting: Pull up spent plants after frost and compost them to return nutrients to the soil.
Growing Cosmos Flowers From Seeds FAQs
1. Can I grow cosmos in pots? Absolutely! Choose a large container (at least 12 inches deep) with good drainage. Water regularly and avoid overly rich potting soil.
2. Why are my cosmos growing tall and spindly? This usually happens when they don’t get enough sunlight. Make sure your plants receive at least 6 hours of direct sunlight daily.
3. Will cosmos reseed themselves? Yes, cosmos readily reseed. If you’d like a naturalized look, let some seed heads mature and scatter in the garden.
Conclusion
Growing cosmos flowers from seeds is an easy and rewarding project for gardeners of all skill levels. These cheerful blooms are perfect for brightening your garden, attracting pollinators, and filling your vases with vibrant colour. By following the steps above—whether starting indoors or sowing directly outdoors—you’ll enjoy a summer filled with effortless beauty. So, grab those seeds and start planting today!
View our selection of Cosmos Flower seeds here.
Happy gardening!