How To Grow And Care For Cosmos

Even if you aren't fortunate enough to live next to a meadow of wildflowers, you can still capture their untamed beauty in a container or garden this summer by planting cosmos. Cosmos come in various colors, from burgundy to pink to white. Mix them up for a more natural look, like a wildflower meadow. Bonus: this colorful flower, native to tropical America, attracts birds and butterflies with its cheery blooms. Because of their wildflower characteristics, cosmo flowers are considered invasive in some environments. Cosmos are prime not only for containers but for creating a mass of color in borders or backgrounds or as a filler among shrubs.

Plant Attributes

Plant Attributes
Common Name:	Garden Cosmos, Mexican Aster
Botanical Name:	Cosmos bipinnatus
Family:	Asteraceae
Plant Type:	Annual, Tuber
Mature Size:	1-6 ft. tall, 1-3 ft. wide
Sun Exposure:	Full
Soil Type:	Well-drained, Sandy
Soil pH:	Neutral to Alkaline (7.0 to 7.5)
Bloom Time:	Summer
Flower Color:	Red, Pink, Orange, Yellow, White
Hardiness Zones:	Zones 2-11 (USDA)
Native Area:	North America, South America, Central America

Cosmos Care

Establishing cosmos in your garden or containers is relatively easy because these flowers are drought-tolerant, not particular about soil conditions, and don't require much attention. Plant cosmos in an area with plenty of airflow to help prevent diseases from emerging, but since this plant is considered invasive in the southeast region of the United States, be mindful of the location.

Light

A true summer flower, the cosmos thrive in full sunlight—at least eight hours daily. Some cosmos varieties will grow in partial sunlight but tend to bloom less than those that receive proper sunlight. The native regions for cosmo flowers have hot summer conditions, so replicating this environment is best.

Soil

Cosmos are not particular about soil but overall prefer well-drained soils that lean more towards dry, sandy conditions. Cosmos will perform poorly in rich soils because the extra nutrients cause the plants to overdevelop and droop. If this occurs, adding stakes can help save some of the cosmos.

Water

Cosmos are relatively drought-tolerant, so these plants only need limited watering after establishment. Overwatering can lead to overfertilization and less showy blooms—only water when the soil is dry and ensure proper irrigation.

Temperature and Humidity

Like their native regions, the cosmos thrive in hot, humid environments during the summer. Don't plant seeds until after the last frost to help promote healthy blooms.

Fertilizer

As a maintenance-free plant, the cosmos do not require fertilization. Too much fertilizer can cause less showy blooms and the foliage to overdevelop.

Types of Cosmos

• 'Purity': Ideal for garden borders, this white-petaled flower highlights a golden center in its large blooms. This ideal cut flower grows well in moist, well-drained soil and full sun.
• 'Sea Shells': Tube-shaped petals in white, pink, purple, or red are great for attracting pollinators to your garden. 
• 'Velouette': This bold flower highlights deep pink or crimson-colored petals with striped features. 
• 'Pink Popsocks': A bright-pink flower that attracts wildlife and grows in full sun.

Pruning

Encourage new blooms by deadheading spent flowers. During the middle of the growing season, trim the branches about a third of the way to the ground to see a late-season display of blooms. The flowers can be cut to the ground or pulled up from the roots in the fall or towards the end of the cosmos growing season. Leaving the cosmos with a few flowers on the plant can help encourage self-seeding.

Propagating Cosmos

Cosmos can self-seed and take about seven to 21 days to germinate. While germinating takes place, keep the soil moist. It is best to propagate the cosmos after the last frost and give the plant enough space to grow by placing seeds or cuttings about 12 to 18 inches apart. Here is how to propagate using stem cuttings:

1. After the last frost, take clean pruning shears or a sharp knife to cut the plants. 
2. Choose plants with at least three to five leaf nodes on the branch and cut under the last node. Remove the leaves, but leave the node intact to encourage new growth. 
3. Use a container filled with moist potting soil and create holes about one to two inches deep. Use your fingers or a pencil tip to create these shallow holes. 
4. Plant the stem cuttings into the shallow holes but keep the last leaf node above the soil line. Gently press the soil around the stem, so it stays upright. 
5. Keep the stem cuttings moist and wait for new leaf growth—usually about three weeks. 
6. After new growth emerges, remove the root ball from the container and transplant the cosmos to its new location.

How to Grow Cosmos From Seed

Cosmos grow relatively quickly from seeds, but waiting until the last frost past is essential. Expect seeds to germinate within seven to 21 days of planting, followed by continuous flowering for about two months. Here's how to grow cosmos from seed:

1. Decide if you want to sow the seeds indoors or directly in the garden outside. If you begin the seeds indoors, plant four to six weeks before the last frost, or wait until the threat of frost has passed if you want to plant them directly in the garden. 
2. Sow the seeds in a shallow hole, usually 1/4 inch deep, and spaced one to two feet apart. You can sow seeds closer together if you want the plants to support each other's growth. Covering the soil with moss will help protect the plant's roots during the summer heat.
3. Keep the soil moist while germinating. The cosmos should bloom continuously until the fall and will likely self-sow throughout the garden if you allow the flower heads and seeds to remain. 

Overwintering

Cosmo flowers do not require much winter maintenance as they are annuals. Beyond deadheading spent flowers and trimming back the branches, the cosmos will die in the fall. However, cosmos flowers can self-seed for new growth the following year as the seeds go dormant throughout the winter. One way to encourage new growth the next year is to allow spent flowers to remain intact so the seeds drop.

Common Pests & Plant Diseases

While relatively easy to grow, cosmo flowers still suffer from specific pest infestations. In particular, aphids and cucumber beetles eat away at the petals and foliage, which is most harmful to small, young plants. A protective covering can help prevent pests from eating the cosmos.

Other diseases that impact the cosmos from developing are stem canker, powdery mildew, and Botrytis blight. Overwatering increases the likelihood of these diseases forming on the foliage or in the soil, so maintaining proper care will help prevent these diseases. Remove diseased stem sections or use insecticidal soap if these diseases are present.

How to Get Cosmos to Bloom

Maintaining proper care is the best way to get the cosmos to bloom. Proper maintenance includes plenty of sunshine and moist, well-draining soil. Deadheading spent flowers will also encourage new growth. The lower you cut back the stems, the longer it will take for new blooms to emerge.

Common Problems With Cosmos

Cosmos thrive in full sun, well-drained soil, and with proper maintenance. If pests or diseases are present, the cosmos might show signs of distress in the following ways:

Leaves Turning Yellow

When cosmo flowers do not receive enough daily, direct sunlight, diseases such as powdery mildew can attach to the foliage. The first sign of this disease will be a powdery white coating on the leaves, which will then cause the foliage to turn yellow, wilt, and eventually fall off. Planting cosmos in an area with direct sunlight and proper circulation will help prevent powdery mildew from forming. Also, maintaining adequate water without oversaturating the plants will help. If powdery mildew is present, try spraying a fungicide on the leaves according to the direction.

Flowers Not Blooming

Overfertizlized soil can cause the cosmos to produce too much foliage instead of consistently blooming. Soil with a high nitrogen concentration can also negatively impact the flowers' potential for full blooms.