Cosmos in Zone 3A — Mountain West

Cosmos are a fantastic choice for adding vibrant color to your Mountain West garden, even with our short season. Their daisy-like blooms come in a range of colors and attract pollinators, plus the cool nights here seem to intensify their colors. They're also great for cut flowers, bringing that beauty indoors.

Our high altitude and short growing season present some challenges, but Cosmos are quick to mature. With proper timing and a little attention, you can enjoy a profusion of blooms from late summer into fall. Our intense sun will help them thrive, too.

Starting Cosmos indoors is an option, especially if you're itching to get a jump start on the season. Starting seeds indoors from mid-May through early June, about four weeks before you plan to transplant, gives you a head start. Use seed trays with good drainage and a light, well-draining seed-starting mix.

Keep the trays in a warm spot, around 70-75°F, and provide plenty of light. Bottom watering is a great way to keep the soil consistently moist without overwatering. Remember, though, our late springs mean you need to be ready to protect seedlings from any late frosts.

Transplant your Cosmos seedlings outdoors from mid-June through early July, after the risk of frost has passed. Before transplanting, harden off your seedlings by gradually exposing them to outdoor conditions for about a week. Start with a few hours a day in a sheltered spot and gradually increase the time.

Space your plants 12-18 inches apart in a sunny location with well-drained soil. Be mindful of potential late-season hail; a temporary cover might be wise if storms are predicted soon after transplanting.

Direct sowing is the recommended method for Cosmos in our region, as they germinate easily and don't need a head start. Sow seeds directly into the garden from early June through early July, after the soil has warmed up. Cosmos need that soil temp to germinate well.

Prepare the soil by loosening it and removing any weeds. Sow seeds about ¼ inch deep and space them 12-18 inches apart. Keep the soil consistently moist until the seedlings emerge, usually within a week or two.

Cosmos are drought-tolerant plants that prefer well-drained soil, making them well-suited for our dry Mountain West climate. Overwatering can lead to more foliage than flowers, so be mindful of their needs.

During the peak of our brief summer, give your Cosmos a deep watering about once a week, or when the top two inches of soil feel dry to the touch. Use the finger test to check. Aim for about an inch of water per week. Water at the base of the plants to avoid wetting the foliage, further minimizing the risk of fungal issues in our low-humidity environment.

Signs of underwatering include wilting leaves and stunted growth. Overwatering can cause yellowing leaves and root rot. Adding a layer of mulch around the base of the plants can help retain moisture and regulate soil temperature, which is beneficial with our intense sun.

Deadheading is key to keeping your Cosmos blooming throughout the season. As flowers fade, snip them off just below the flower head, above a set of leaves. This encourages the plant to produce more blooms rather than setting seed.

Pinching young plants when they are about 6 inches tall can also promote bushier growth. Simply snip off the top set of leaves to encourage side branching. As our short season winds down in late summer, you can leave the last blooms on the plants to develop seed heads for saving seeds.

Expect your first Cosmos blooms to appear from late July through early September, about 50 days after sowing. Cut the flowers when the petals have just opened for the longest vase life. They make excellent cut flowers and will continue to open indoors.

To harvest, use sharp scissors or pruning shears to cut the stems at an angle. Removing spent flowers regularly encourages continuous blooming throughout the season. As the first frost approaches in early September, you can harvest any remaining blooms to enjoy indoors.

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

Flopping in Rich Soil

• What it looks like: Tall, leggy plants that bend or fall over, especially after rain or wind.
• What causes it: Overly fertile soil, which encourages excessive foliage growth at the expense of strong stems. This is common if you amend the soil too much.
• How to fix/prevent it: Plant Cosmos in average soil that isn't overly rich. Avoid excessive fertilization. Stake or support the plants if they start to flop.

Aphids

• What it looks like: Clusters of tiny green, black, or white insects on stems and leaf undersides. Sticky honeydew residue and curled or distorted new growth.
• What causes it: Rapidly reproducing sap-sucking insects that thrive in warm weather. They're often brought in by ants.
• How to fix/prevent it: Knock them off with a strong spray of water. Encourage natural predators like ladybugs. Use insecticidal soap or neem oil for heavy infestations.

Powdery Mildew

• What it looks like: White or grayish powdery coating on leaves, stems, and sometimes flower buds. Leaves may curl, yellow, and die.
• What causes it: Fungi that thrive in warm days, cool nights, and moderate humidity. Dry leaf surfaces actually favor it.
• How to fix/prevent it: Improve air circulation by spacing plants adequately and pruning. Remove affected leaves. Apply neem oil or potassium bicarbonate sprays.

Mountain West Specific Challenges: Our intense sun can sometimes scorch leaves if plants are stressed, so ensure adequate watering. The alkaline soil in our region can also affect nutrient availability, so consider a soil test and amend accordingly. Hail can be a major issue, so consider using netting or row covers if severe storms are predicted.

Cosmos make great companions for several vegetables in our Mountain West gardens. Plant them near tomatoes and peppers to attract pollinators, increasing fruit set. They also work well with green beans, as they attract beneficial insects that prey on bean pests.

Squash also benefits from Cosmos nearby, as the pollinators they attract help with squash pollination. Avoid planting Cosmos near fennel, as fennel can inhibit the growth of many plants.