Cosmos: Plant Care and Growing Guide

Why should I Grow Cosmos?

There are several reasons why you should consider growing cosmos in your garden or landscape:

Beautiful and colorful flowers: Cosmos flowers are known for their striking colors and attractive appearance. They add a vibrant touch to any garden or landscape.

Easy to grow: Cosmos plants are easy to grow and care for, making them a great choice for gardeners of all skill levels. They are tolerant of a wide range of growing conditions, including poor soil and drought, and can be grown from seed or transplants.

Attract pollinators: The flowers are a favorite of bees, butterflies, and other pollinators, making them a great way to support local wildlife and help promote a healthy ecosystem.

Versatile: Cosmos can be used in a variety of ways, from adding color to a mixed border or flower bed to filling in gaps in a garden to providing a backdrop for other plants.

Long-lasting blooms: The flowers typically bloom from summer to fall, providing a long-lasting source of color and interest in the garden.

Popular Species of Cosmos

Cosmos bipinnatus: Also known as the Mexican aster or garden cosmos, this is the most commonly grown species. Plants typically grow to a height of 2-5 feet (60 to 180 cm) and produce large, daisy-like flowers in a variety of colors, including pink, white, red, orange, and yellow. The flowers can reach a diameter of 5 inches (12 cm) and are borne on long, slender stems. It is native to Mexico and Central America

Cosmos sulphureus: Also known as yellow cosmos or sulfur cosmos, this species is native to North and South America. Plants typically grow to a height of 1-3 feet (30-90 cm) and produce large, bright yellow to orange flowers that can reach a diameter of 2 inches (5 cm). The flowers are borne on long, slender stems and have a slightly spicy fragrance.

Cosmos atrosanguineus: Also called chocolate cosmos due to its chocolate-scented flowers, this species is native to Mexico. Plants typically grow to a height of 1 to 2 feet (30 to 60 cm) and produce dark, maroon to almost black flowers that have a distinctive chocolate fragrance. The flowers can reach a diameter of 1 to 2 inches (2.5 to 5 cm) and are borne on slender stems.

When to Plant Cosmos

• Cosmos flowers are generally planted in the spring after the last frost has passed and the soil has warmed up. In most regions, this is usually around mid to late spring, but it can vary depending on the climate and growing conditions.
• You can plant seeds directly in the ground or start them indoors 4 to 6 weeks before the last expected frost and transplant them outside after the danger of frost has passed.

Where to Plant Cosmos

Cosmos flowers can be grown in a variety of locations and growing conditions. Here are some tips on where to grow them:

• Sunlight: Cosmos prefer full sun and will thrive in areas with at least 6 hours of direct sunlight per day.
• Soil: Cosmos flowers prefer well-drained soil. They like soil that is not too rich, as rich soil will encourage foliage at the expense of bloom. They can grow in a variety of soil types but do best in soil that is slightly neutral to slightly alkaline in pH.
• Climate: Cosmos are annual plants that can be grown in a wide range of climates. They are tolerant of heat, humidity, and drought
• Containers: Cosmos flowers can be grown in containers, making them a good choice for small spaces or balconies. Use a well-draining potting mix and water regularly to ensure healthy growth.
• Garden beds: Cosmos flowers are often used in garden beds and borders and can be planted in groups or mass plantings for a dramatic effect. They can also be used to fill in gaps between other plants or to add color to an empty space.

How to Plant Cosmos

Here are some steps to follow when planting cosmos flowers:

• Choose a sunny location: Cosmos flowers require full sun to thrive, so choose a location that receives at least 6 hours of direct sunlight per day.
• Prepare the soil: Loosen the soil to a depth of 6 to 8 inches (15 to 20 cm).
• Plant the seeds: If planting seeds directly in the ground, scatter them on the soil surface and lightly cover them with soil. Water the area thoroughly after planting.
• Thin: Once the seedlings have grown to a height of 2 to 3 inches (5 to 7.5 cm), thin them out to a spacing of 12 to 18 inches (30 to 45 cm) apart to allow for adequate air circulation and growth. If you are growing tall varieties, stake the plants to keep them upright and prevent them from flopping over.

Cosmos Care

• Watering: Cosmos do not require frequent watering once established, but they do benefit from occasional deep watering during periods of drought or hot weather.
• Fertilizing: Cosmos planted in the garden generally do not require fertilizer, but if grown in containers, they can benefit from fertilization every two weeks or so using an all-purpose garden fertilizer to promote optimal flowering.
• Deadheading: Regular deadheading of spent blooms can help encourage continued blooming and prolong the overall bloom season. Pinch off spent flowers or use sharp pruning shears to remove them.
• Self-seeding: Cosmos flowers are known to readily self-seed. This means that once the flowers have finished blooming and the plant goes to seed, the seeds can drop to the ground and sprout on their own without the need for replanting. This can result in new plants growing in unexpected areas of the garden or landscape. If you want to control the location and number of plants in your garden, deadheading spent blooms before they go to seed can help prevent self-seeding.

Companion Plants for your Cosmos Flowers

Cosmos are versatile and easy to grow, and they can be paired with a wide range of companion plants to create a beautiful garden.

Zinnias: Zinnias are a natural companion plant for cosmos flowers, as they have similar growing requirements and complementary colors.

Marigolds: Marigolds are known for their pest-repelling properties and can help keep harmful insects away from your cosmos flowers.

Nasturtiums: Nasturtiums are a low-growing plant that can help shade the soil around cosmos flowers, helping to retain moisture and prevent weeds.

Sunflowers: Sunflowers are tall and sturdy plants that can provide support for taller varieties of cosmos, as well as add color and interest to the garden.

Salvia: Salvia plants are known for their beautiful, vibrant blooms and can complement the colors of cosmos flowers.

Sweet Alyssum: Sweet Alyssum is a low-growing plant that can be grown as a border or ground cover, adding a delicate and fragrant touch to your cosmos bed.

Cleome: Cleome plants, also known as spider flowers, have tall, spiky blooms that can add height and interest to a cosmos garden.

Sea Holly: Eryngium provides color and textural contrast, adding interest to any garden or landscape.

Tall Verbena: By pairing the delicate purple blooms and thin stems of verbena with cosmos flowers, you can create a contrast of textures that adds visual interest and variety to your garden.

Dahlia: By planting dahlia and cosmos together, you can create a layered effect that adds depth and dimension to your garden.

Cosmos are also perfect for mixing with roses (such as summer-long hybrid musks, English roses), buddlejas, border phlox, penstemon, monardas, campanulas, or veronicas.

They are also excellent companion plants in the orchard or vegetable garden and help crowded peas, lemons, and pomegranates.

How to Propagate

Cosmos can be propagated by seeds or cuttings.

Propagation by seeds:

• Collect the seeds: Cosmos plants produce seed pods that turn brown and dry when the seeds are mature. Harvest the pods when they are dry and crisp and the seeds inside are dark and hard.
• Prepare the soil: Choose a sunny location with well-drained soil, and loosen the soil to a depth of 6 to 8 inches (15 to 20 cm).
• Plant the seeds: Scatter the seeds on the soil surface and lightly cover them with soil. Water the area thoroughly after planting.
• Care for the seedlings: Keep the soil moist but not waterlogged until the seeds have germinated. Once the seedlings have emerged, thin them out to a spacing of 12 to 18 inches (30 to 45 cm) apart to allow for adequate air circulation and growth.

Propagation by cuttings:

• Choose healthy plants: Select healthy, disease-free plants with strong, non-flowering stems.
• Take cuttings: Use a sharp, clean knife or pair of scissors to take cuttings from the stem, making sure each cutting has at least two nodes.
• Prepare the cuttings: Remove the lower leaves from the cutting and dip the cut end into rooting hormone powder.
• Plant the cuttings: Plant the cuttings in a container filled with moist potting soil, and place them in a warm, bright location out of direct sunlight.
• Care for the cuttings: Keep the soil moist but not waterlogged. After 3 to 4 weeks, the cuttings should have rooted and can be transplanted into individual pots or directly into the ground.

Pest and Diseases

Cosmos are resistant to most pests and diseases, but they can be affected by a few common problems. Here are some pests and diseases that can affect them:

Aphids: Aphids are small, soft-bodied insects that feed on the sap of plants. They can be controlled with insecticidal soap or neem oil.

Powdery mildew: Powdery mildew is a fungal disease that can cause a white, powdery coating to appear on the leaves of plants. It can be prevented by ensuring good air circulation around the plants and avoiding overhead watering.

Gray mold: Gray mold, also known as botrytis blight, is a fungal disease that typically appears as grayish-brown spots on the leaves and stems of the plant and can cause the flowers to rot and die. Improve air circulation, avoid overhead watering, remove infected parts, or use fungicides if the gray mold is severe.

Slugs: Slugs can eat holes in the leaves. They can be controlled by using slug bait or placing copper tape around the base of the plants.

Click here to compare all Cosmos varieties

Frequently Asked Questions

Do cosmos flowers come back every year?

Cosmos are typically grown as annuals, meaning they complete their life cycle in one growing season and do not come back the following year. However, in some warmer climates or in sheltered locations, cosmos plants may self-seed and produce new plants the following year.

How long does cosmos flower take to bloom?

Cosmos flowers typically take around 60 to 90 days from the time the seeds are sown to the first blooms appearing.

How long do cosmos flowers last?

The blooming period for cosmos flowers can vary depending on the specific variety, growing conditions, and climate. In general, cosmos flowers will begin to bloom in early to mid-summer and continue blooming until the first frost of fall.

How often should you water cosmos?

As a general guideline, cosmos should be watered deeply once or twice a week, depending on the weather conditions. During periods of drought or hot weather, they may require more frequent watering.