Lazy gardeners, rejoice!
If your goal is a maximum return for minimum effort, cosmos are the flowers for you!
These native Mexican, Central American, and South American plants, with delicate, feathery foliage and a broad color palette, are the embodiment of resilience. They thrive in poor soil, demand minimal care, and still produce alluring blossoms.
Ignore them all you want. They’ll love you no matter what and reward you with abundant flowers.
Their name originates from the Greek word kosmos, meaning “harmony” or “order” — a tip of the hat to their balanced appearance and regular growth habits. So, let’s bring a little harmony and order to your garden!
Cosmos are usually grown as annual flowers, completing their life cycle in one growing season. However, in their native tropical regions, some varieties behave as short-lived perennials. They cannot survive frost but are remarkably easy to reseed, ensuring they return year after year with little effort on your part.
Choosing a Variety for Southern California
Cosmos are hardy plants. Still, some varieties do exceptionally well in Southern California’s Mediterranean climate. These include (but are not exclusive to):
- Cosmos bipinnatus: The most popular variety is simply called “cosmos.” It comes in various colors, including white, pink, purple, and orange. These plants can grow up to 6 feet tall and have single or double blooms.
- Cosmos sulphureus: Sulfur cosmos has bright yellow to orange flowers and can reach 3–4 feet. It’s more heat tolerant than other varieties.
- Cosmos atrosanguineus: This chocolate cosmos has a deep red-brown color and a distinctive chocolate fragrance. It reaches 2–3 feet high and adds richness to your garden.
- Cosmos Sonata: A compact variety ideal for smaller spaces and containers, growing only 1–2 feet tall. It features a mix of colors, including pink, white, and crimson, and produces continuous blooms throughout the season.
Cosmos bipinnatus, the most popular variety is, simply called “cosmos.”
Ideal Growing Conditions
Light
- Cosmos need at least 6–8 hours of direct sunlight daily.
- They can tolerate partial shade, but reduced light may lead to fewer blooms.
Temperature
- Cosmos love warm to hot temperatures. California, here we come!
- They perform well in USDA zones 2–11 when planted after the last frost. Metro Los Angeles includes zones 9–11.
- Daytime temperatures above 70°F are ideal for producing lush blooms.
Soil
- Cosmos prefer poor to medium-quality soils with good drainage and a neutral to slightly acidic pH, ideally ranging from 6.0 to 7.0.
- Avoid overly rich, fertile soil, which encourages excessive foliage growth instead of flowers.
- Simple soil preferences make cosmos an easy choice for various garden setups.
Cosmos atrosanguineus has a deep red-brown color and a distinctive chocolate fragrance.
Steps for Planting Cosmos
Follow these easy steps to plant cosmos and kickstart their growth.
- Pick a sunny location with well-draining soil. If your soil retains too much water, blend in sand or perlite to improve drainage.
- Loosen the soil with a garden fork or tiller to ensure good aeration. You don’t need to add compost unless your soil is extremely poor.
- Cosmos grow best from seeds planted directly outdoors after the danger of frost has passed. Sprinkle seeds 12–18 inches apart on the soil surface and lightly cover with 1/4 inch of soil.
- Water gently but deeply to ensure the soil stays moist—but not soggy—until germination occurs, typically within 7–10 days.
- Once seedlings grow 3–4 inches tall, thin them out, leaving the most vigorous plants about 12 inches apart. This gives them the space to flourish.
Caring for Cosmos
Water
- Cosmos are drought-tolerant and can survive with minimal watering once established.
- However, they’ll produce more flowers if watered regularly, especially during hot and dry spells.
Fertilizer
- As mentioned earlier, cosmos prefer poor soil, so they don’t need much fertilizer.
- If you must fertilize, use a balanced all-purpose fertilizer sparingly after the plants have established themselves. Overfertilizing can lead to excessive foliage growth rather than blooms.
Sulfur cosmos has bright yellow to orange flowers and can reach 3–4 feet.
Deadheading
- Regular deadheading (removing spent flowers) encourages continuous seasonal bloom.
- It also prevents seeds from forming and reseeding in unwanted areas.
Pests & Diseases
- Cosmos are generally pest and disease-free.
- Still, watch for aphids, spider mites, and whiteflies. Treat infestations with insecticidal soap or neem oil.
Cosmos as Companion Plants
- Since cosmos are so easy to grow, they make fantastic companion plants for many other flowers and vegetables.
- Their bright colors also attract pollinators. Try planting them near fruits and vegetables that require pollination.
- They also deter pests like cabbage worms, tomato hornworms, and corn earworms.
Cosmos Sonata is a compact variety ideal for smaller spaces and containers.
FAQs: Growing Cosmos
Q: When should I plant cosmos?
Plant cosmos outdoors after the danger of frost has passed and the soil has warmed to at least 60°F.
Q: Can cosmos be grown in pots?
Cosmos grow wonderfully in pots. A container at least 12 inches wide allows room for root growth.
Q: How tall do cosmos grow?
Varieties typically range from 1 to 6 feet tall, depending on the type and growing conditions.
Q: When do cosmos start blooming?
Cosmos start blooming about 8 weeks after planting and will continue until the first frost.
Q: Do cosmos reseed themselves?
Yes, cosmos are excellent self-seeders. If left to their own devices, they’ll often come back year after year.
Q: Why aren’t my cosmos blooming?
Excessive fertilizer, low sunlight, or overcrowding could all be to blame. Adjust these factors for better bloom production.
Q: Should I deadhead cosmos?
Deadheading (removing spent blooms) encourages more flowers. Leave a few blooms toward the end of the season to allow for reseeding.
Q: Can I grow cosmos indoors?
While cosmos prefer the outdoors, you can start them indoors 4–6 weeks before your last frost date. They must be transplanted for best results.