Cosmos in Zone 5B — Mountain West

Cosmos are a fantastic addition to any Mountain West garden. Their bright, cheerful blooms bring a burst of color, and they're incredibly easy to grow, even at high altitude. Plus, the cool nights we get even in summer seem to intensify their colors.

Our short growing season and intense sun can be a challenge, but Cosmos are fast growers, maturing in just 50 days. With a little planning, you can easily fit them into our 149-day growing season and enjoy blooms from late June through early October.

Starting Cosmos indoors is possible, but honestly, it’s usually not necessary in our climate. They germinate and grow so readily from seed that direct sowing is the preferred method. However, if you want a *really* early start, you can start seeds indoors from mid-April through early May, about four weeks before you plan to transplant them.

Use seed trays with a good seed-starting mix. Keep the soil consistently moist, but not soggy – bottom watering works well. Provide warmth and plenty of light; a grow light is especially helpful this time of year when our late spring weather can be unpredictable. Remember our late spring can be cool, so a little extra warmth can help with germination.

If you started your Cosmos indoors, transplant them outdoors from mid-May through late June, after the danger of frost has passed. Before transplanting, harden off your seedlings by gradually exposing them to outdoor conditions for about a week. This helps them adjust to the intense sun and cooler nights we get in the Mountain West.

Space your Cosmos plants 12-18 inches apart to allow for good air circulation. Keep an eye on the weather forecast – even in late May and June, we can still get unexpected cold snaps or even hail, so be prepared to cover your young plants if needed.

Direct sowing is the easiest and most successful way to grow Cosmos in the Mountain West. Sow seeds outdoors from early May through late July. Cosmos are pretty forgiving, so you have a good window to get them in the ground.

Prepare your soil by loosening it and removing any large rocks or debris. While our soil tends to be alkaline, Cosmos aren't too fussy about soil pH. 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, which is a definite plus in our dry Mountain West climate. They prefer well-drained soil and don't like to be overwatered. Overwatering, especially in our alkaline soil, can lead to root rot and fewer blooms.

During the peak of summer, when temperatures are consistently in the 80s, water deeply about once a week, providing about an inch of water. Use the finger test to check the soil moisture before watering – stick your finger about two inches into the soil; if it feels dry, it's time to water. In the cooler spring and fall months, you may only need to water every other week, or even less.

Water at the base of the plants to avoid wetting the foliage, which can encourage fungal diseases. While our low humidity usually keeps these issues at bay, it's still a good practice. Signs of underwatering include wilting leaves and dry, crispy edges. Overwatering can cause yellowing leaves and stunted growth. A layer of mulch around the base of the plants helps retain moisture and keeps the soil cooler during our intense summer sun.

Deadheading is key to keeping your Cosmos plants blooming throughout the season. Simply snip off the spent flowers just below the flower head. This encourages the plant to produce more blooms instead of putting energy into seed production.

Pinch back young plants when they are about 6 inches tall to encourage bushier growth. This will result in more stems and, therefore, more flowers. As the first frost approaches in early October, you can let the last blooms go to seed if you want to collect seeds for next year.

You can expect your first Cosmos blooms to appear from late June through early October, about 50 days after sowing. Cut the flowers when the petals are just beginning to open for the longest vase life. Cosmos make excellent cut flowers and will add a splash of color to any bouquet.

Harvesting regularly encourages the plant to produce more flowers, so don't be shy about cutting them. Cut the stems at an angle, just above a leaf node. As the end of the season approaches and the first frost looms, you can harvest any remaining blooms and bring them indoors to enjoy. You can also let some of the flowers dry on the plant and collect the seeds for next year. The seeds are easy to save from dried heads.

Here are some 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.
• What causes it: Overly fertile soil, especially with too much nitrogen. This encourages excessive foliage growth at the expense of strong stems.
• How to fix/prevent it: Amend your soil with compost, but avoid excessive fertilization. Provide support with stakes or netting if necessary.

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: A strong spray of water from the hose can knock them off. Encourage beneficial insects like ladybugs and lacewings. Use insecticidal soap or neem oil for heavy infestations. Avoid over-fertilizing with nitrogen.

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 by spacing plants properly and pruning as needed. Remove affected leaves. Apply neem oil or potassium bicarbonate sprays. A milk spray (1 part milk to 9 parts water) can also be effective.

Mountain West Specific Challenges: Our moderate-at-altitude heat and low humidity generally benefit Cosmos, as they thrive in drier conditions. However, the intense UV radiation can sometimes stress the plants, so providing some afternoon shade during the hottest part of the day can be helpful. Watch out for hail storms, which can shred the delicate foliage and flowers.

Cosmos make great companions for a variety of vegetables in the Mountain West garden. They attract pollinators, which benefit fruiting plants like tomatoes, peppers, and squash. Planting Cosmos near green beans can help to deter bean beetles.

Avoid planting Cosmos near plants that require very rich soil, as Cosmos prefer leaner conditions. Overly fertile soil can cause them to become leggy and flop over.