Cosmos in Zone 7A — Mountain West

Cosmos are a fantastic choice for adding vibrant color to your Mountain West garden. Their daisy-like blooms are beautiful in bouquets, attract pollinators like crazy, and thrive in our intense sunlight. Plus, the cool nights we get, even in summer, seem to intensify their colors.

Our short growing season and alkaline soil present challenges, but Cosmos are surprisingly adaptable. With a little planning, you can easily enjoy blooms throughout our 173-day season.

Starting Cosmos indoors isn't essential, as they transplant easily, but it can give you a jump start. If you want to get a head start, sow seeds indoors from late March through mid April, about four weeks before you plan to transplant them outside. Use seed trays with a good seed-starting mix, keep them warm (around 70°F), and provide plenty of light. Bottom watering helps prevent damping-off, which can be a problem in our cool spring conditions.

Transplant your Cosmos seedlings outdoors from early May through mid June, once the danger of frost has passed. Before transplanting, harden off your seedlings by gradually exposing them to outdoor conditions for about a week. Space them 12-18 inches apart in well-draining soil. Keep an eye on the weather forecast; we can still get unexpected late frosts at that time of year.

Direct sowing is the easiest and most reliable way to grow Cosmos in the Mountain West. Sow seeds from mid April through early August, after the soil has warmed up a bit. Prepare the soil by loosening it and removing any rocks or debris. Sprinkle the seeds lightly over the soil and cover them with a thin layer of soil.

Keep the soil moist until the seeds germinate, which usually takes about a week. Once the seedlings emerge, thin them to 12-18 inches apart. Cosmos don't need a head start, and direct sowing ensures they adapt well to our alkaline soil.

Cosmos are drought-tolerant plants that thrive on neglect, which is perfect for our dry Mountain West climate. Overwatering is a far bigger problem than underwatering with these guys. During the peak of summer, when temperatures reach around 85°F, water deeply about once a week if we haven't had any rain.

Before watering, use the finger test: stick your finger about two inches into the soil. If it feels dry, it's time to water. Aim for about an inch of water per week, delivered at the base of the plant to avoid wetting the foliage unnecessarily. Signs of underwatering include wilting leaves, while yellowing leaves often indicate overwatering. A good layer of mulch, like shredded bark, helps retain moisture and regulate soil temperature.

Deadheading is key to keeping your Cosmos blooming all season long. Regularly remove spent flowers by cutting the stem just above a leaf node. This encourages the plant to produce more blooms instead of setting seed. Pinching young plants when they are about 6 inches tall also encourages bushier growth.

As the first frost approaches in early October, you can let the last blooms fade and allow the plants to set seed if you want to collect them for next year. Otherwise, you can pull up the plants and compost them.

You can expect your first Cosmos blooms about 50 days after sowing, so typically from early June through early October in our region. Cut the flowers when the petals have just opened but are not fully unfurled. This will give you the longest vase life. Cut the stems long, just above a leaf node, to encourage more blooms.

Harvesting regularly encourages continued production. As the end of the season approaches and the first frost is on the horizon, harvest any remaining blooms to enjoy indoors. You can also let some of the flower heads dry on the plant to collect seeds for next year.

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

Flopping

• What it looks like: Stems bending over or collapsing, especially after rain or watering.
• What causes it: Rich soil with too much nitrogen. Our alkaline soil is not usually the cause, but amending it too heavily can be.
• How to fix/prevent it: Avoid over-fertilizing. Plant in well-draining soil. Stake plants if necessary.

Aphids

• What it looks like: Clusters of tiny green, black, or white insects on stems and leaf undersides. Sticky honeydew residue. Curled or distorted new growth.
• What causes it: Rapid-reproducing sap-sucking insects that thrive 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.

Mountain West Specific Challenges: Cosmos are generally well-suited to our climate, but the intense sun can sometimes scorch the leaves, especially on young plants. Make sure to provide adequate water, especially during hot, dry spells. Hail can also damage the plants, so consider using row covers or other protection if hail is in the forecast.

Cosmos are excellent companions for many vegetables in the Mountain West garden. They attract pollinators, which benefit tomatoes, peppers, green beans, and squash. The pollinators love them! Planting Cosmos near these vegetables can increase yields. Avoid planting Cosmos near plants that are susceptible to powdery mildew, as they can potentially spread the disease.