Cosmos in Zone 6B — Mountain West

Cosmos are a fantastic addition to any Mountain West garden. Their bright, cheerful blooms bring vibrant color to our landscapes, and they're incredibly easy to grow, even at high altitude. Plus, the cool nights we get here intensify their colors and make them perfect for cut flower arrangements.

Our short growing season and intense sun can be a challenge, but Cosmos are fast growers, maturing in about 50 days. With a little planning within our 163-day growing season, you can enjoy a continuous display of blooms from mid-summer right up until our first frosts arrive.

Starting Cosmos indoors is definitely an option, but not usually necessary here in the Mountain West. Since they direct sow so readily, it's often easier to just pop the seeds in the ground. However, if you're aiming for the earliest possible blooms, or want to get a jump start on the season, starting seeds indoors makes sense.

Start your seeds indoors from early to late April, about four weeks before you plan to transplant them outdoors. Use seed trays filled with a good seed-starting mix, keep them warm (around 70°F), and provide plenty of light, either from a sunny window or a grow light. Try bottom watering to keep the seedlings moist without damping off. Remember our springs here can be slow to warm, so patience is key.

Once the danger of frost has passed, and the soil has warmed up a bit (usually early May through mid June), you can transplant your Cosmos seedlings outdoors. Before transplanting, be sure to harden them off for about a week by gradually exposing them to outdoor conditions. This helps them adjust to the intense sun and cooler nights we experience here.

When transplanting, space the seedlings 12-18 inches apart to allow for good air circulation. Even though our last frost is usually around late April, keep an eye on the weather forecast, as those unexpected cold snaps can still happen.

Direct sowing is the easiest and most reliable method for growing Cosmos in the Mountain West. You can start sowing seeds directly into the garden from late April through early August. Cosmos seeds germinate best when the soil temperature is around 65-75°F.

Prepare the soil by loosening it and removing any weeds. Sow the seeds about 1/4 inch deep and space them 12-18 inches apart. Keep the soil consistently moist until the seedlings emerge, which usually takes about a week. Since Cosmos don't need a head start, direct sowing saves time and effort.

Cosmos are exceptionally drought-tolerant, and in our dry Mountain West climate, it's crucial to avoid overwatering. During the spring when establishing seedlings, water regularly to keep the soil consistently moist, but not soggy. Once established, Cosmos require very little supplemental water.

In our moderate-at-altitude summer heat, with low humidity and infrequent rainfall, water deeply but infrequently. Aim for about 1 inch of water per week, only if rainfall is lacking. Use the "finger test" – if the top 2 inches of soil are dry, it's time to water. Water at the base of the plants to avoid wetting the foliage, which helps prevent fungal diseases in our low-humidity environment.

Overwatering can lead to leggy growth, fewer flowers, and root rot. Underwatering will cause the plants to wilt and the blooms to fade. Mulching around the base of the plants with organic materials like shredded bark or straw helps retain moisture and suppress weeds, which is helpful in our dry climate.

Deadheading is essential for keeping your Cosmos blooming throughout the season. Regularly remove spent flowers to encourage the plant to produce more blooms. Simply snip off the flower head just below the next set of leaves.

Pinching young plants early in the season can also promote bushier growth. When the plants are about 6 inches tall, pinch off the top set of leaves to encourage branching. As the first frost approaches in early October, you can stop deadheading to allow the plants to set seed if you want to collect them for next year.

You can expect your first Cosmos blooms about 50 days after sowing, typically from mid June through early October in our Mountain West climate. Cut the flowers when the petals have just opened, as they will continue to open indoors. Cosmos make excellent cut flowers and will last for several days in a vase.

When harvesting, cut the stems at an angle to encourage new growth. Regular harvesting encourages continuous production of blooms throughout the season. As the first frost approaches, harvest any remaining blooms to enjoy indoors. Cosmos seeds are easy to save – simply allow the flower heads to dry completely on the plant, then collect the seeds from the dried heads.

Here are three common problems you might encounter with Cosmos in the Mountain West:

Flopping in Rich Soil

• What it looks like: Tall, leggy plants that fall over easily, especially after rain or wind.
• What causes it: Overly fertile soil, often combined with too much water. Our alkaline soil is usually not the problem.
• How to fix/prevent it: Plant Cosmos in well-drained soil that is not overly rich. Avoid over-fertilizing. You can also stake the plants for support.

Aphids

• What it looks like: Clusters of tiny soft-bodied insects (green, black, or white) on stems and leaf undersides. Sticky honeydew residue. Curled or distorted new growth.
• What causes it: Rapid-reproducing sap-sucking insects. Populations explode in warm weather. Ant colonies may farm them for honeydew.
• How to fix/prevent it: Strong water spray knocks them off. Encourage ladybugs and lacewings (natural predators). Insecticidal soap for heavy infestations. Neem oil works systemically. Avoid excess nitrogen fertilizer (it attracts them).

Powdery Mildew

• What it looks like: White or grayish powdery coating on leaves, stems, and sometimes fruit. Leaves curl, yellow, and die. Reduces plant vigor.
• What causes it: Various fungi that thrive in warm days, cool nights, and moderate humidity. Dry leaf surfaces actually favor it (unlike most fungi).
• How to fix/prevent it: Improve air circulation through spacing and pruning. Remove affected leaves. Neem oil or potassium bicarbonate sprays help. Milk spray (1 part milk to 9 parts water) is surprisingly effective. Plant resistant varieties.

Mountain West Specific Challenges: The intense sun at our high altitude can stress Cosmos, making them more susceptible to problems. Low humidity generally helps prevent fungal diseases but can also dry out plants quickly, so consistent monitoring is key. Our dry climate means watering practices are important.

Cosmos are great companions for many vegetables in the Mountain West garden. They attract beneficial insects like bees and butterflies, which help pollinate your crops.

Good companions include:

• Tomatoes: Cosmos attract beneficial insects that prey on tomato pests.
• Peppers: Similar to tomatoes, peppers benefit from the pest control provided by Cosmos.
• Green Beans: Cosmos attract pollinators that improve bean production.
• Squash: Cosmos help to attract pollinators to squash plants, increasing fruit set.

I haven't found any truly bad companions for cosmos.