How to grow cosmos flowers, the complete guide

I cannot remember the first time I grew cosmos; however, I grow them every year. There are several characteristics that I love about cosmos. First, I can grow cosmos in both New Jersey and Florida. Second, they self-seed. Third, they are a lovely addition to a bouquet. Fourth, they are easy flowers to press. Lastly, they are easy to grow. What’s not to like? Let’s explore how to grow cosmos.

Native to Mexico, cosmos is a daisy-like flower that comes in a variety of colors. The flower was named after the Greek word ‘kosmos’ which means order. Each petal on the flower is evenly placed and orderly. In addition, cosmos is one to two birth flowers for the month of October. The other October birth flower is the marigold. In the language of flowers, cosmos symbolize peace and tranquility.

The most common variety of cosmos is the garden cosmos. I like to purchase a seed package that contains a variety of colors. One special variety of cosmos to try is Double Click Snow Puff. While most cosmos are single-petal blooms, the double click variety has fully double or semi-double blooms.

How to grow from seed

You can either direct seed or start cosmos indoors from seed. My preference is to start them indoors from seed. Plan to start indoors from seed 6-8 weeks before the last frost or directed seeded outdoors after the danger of frost has passed. Cosmos will germinate in five to seven days in temperatures above 65°F. You can fertilize the seedling with a half-strength fertilizer after the first set of ‘true leaves’ appear. True leaves are the fern-like leaves you see in the picture below. If you look at the bottom of the stem, just above the soil, you will see the ‘seed leaves’. Seed leaves are the first leaves to appear after germination.

Plant cosmos outdoors when the soil reaches a minimum temperature of 65°F. Once the seedling emerges, it will take anywhere from 45-60 days for the flowers to appear.

Because cosmos seedlings tend to grow tall, I recommend ‘potting up’ the seedling when it reaches a height of about 6-8 inches. Potting up is the process of moving the seedling to a larger container. Before planting outdoors, it’s important to “harden off” the seedlings. Hardening off is the process of gradually acclimating the plant to the outdoors. Harden off by placing seedlings in a cold frame or by simply placing the plants outdoors for a couple of hours each day for one to two weeks. For a full guide on starting plants from seeds, see my blog post titled “The complete guide to starting plants from seeds indoors”.

If you instead chose to direct seed, then lightly rake the bed and plant seeds ¼ inch deep. Space seeds (or thin) to 18 inches apart. Once seedlings emerge, reduce watering to 1x/week.

How to care for cosmos

As noted earlier, cosmos are an easy plant to grow. Here is the growing information.

• Light: Cosmos like full sun with at least 6 hours of sunlight a day.
• Soil: Cosmos can thrive in all soil types sand, clay, and loam.
• Spacing: Follow the guidelines on the back of the seed package for plant spacing. Generally, space 18 inches apart.
• Water: Cosmos prefer dry conditions. After seedlings emerge, reduce watering to 1x per week. Overwatering will result in fewer blooms.
• Fertilizing: Cosmos do not need fertilization. Avoid soil that is overly fertilized or amended with compost.
• Companions: Interplant cosmos within a vegetable garden to attract pollinators and beneficial insects.
• Other characteristics:
  • Deadheading cosmos will result in more blooms.
  • Cosmos are drought and heat tolerant.
  • Disease resistant
  • Because cosmos can grow up to 4 feet tall, staking may be necessary
• Propagation - Cosmos will naturally reseed in your garden. For this to happen, it is important to leave the cosmos as is when they begin to die back in the cooler weather. While this can be unsightly, it’s a good way to ensure that the seed is dispersed. Therefore, if you want this natural propagation, think strategically about where you plant. I know I will find cosmos in the most unexpected places, but that’s part of the fun.

Ideas for using cosmos in the garden

Cosmos are a great cut flower to add to a bouquet.

Because of their height, cosmos can either be centered in the middle of a garden or planted as a backdrop border.

Plant cosmos around your vegetable garden to attract beneficial insects including lacewing, tachinid flies, hoverflies, and various parasitic mini-wasps.

Botanical Information

Cosmos bipinnatus

Type: Annual

Family: Asteraceae

Native Range: Mexico, southern United States

Zone: 2 to 11

Height: 1.00 to 4.00 feet

Spread: 2.00 to 3.00 feet

Bloom Time: June until frost, earlier in zones 8,9,10,11

Bloom Description: Red, pink, white

Sun: Full sun

Water: Medium

Cosmos sulphureus has similar characteristics to the above, however, the flowers are yellow.

Additional Information

You may also be interested in the following:

• Growing and caring for Zinnias, the complete guide
• Saving Zinnia Seeds
• Growing and Caring for Marigolds, the complete guide
• Profusion Zinnias, an awesome flower to grow