Chrysanthemum in Zone 3B — Mountain West

Chrysanthemums are a fantastic addition to any Mountain West garden, bringing a burst of color and cheer right as the season starts to cool down. Their hardy nature and late-season blooms mean you can enjoy their beauty even as other plants are fading, plus they make beautiful cut flowers.

Growing Chrysanthemums in our high-altitude, short-season environment does present a few challenges. But with careful planning and a little extra attention to timing, you can absolutely succeed within our 108-day growing season.

Starting Chrysanthemum seeds indoors can give you a head start in our short growing season, but it's not always necessary. You can often find healthy starts at local nurseries in the spring.

If you do choose to start seeds indoors, aim for late March through mid April, about eight weeks before you plan to transplant. Use seed trays with good drainage and a light, well-draining seed-starting mix. Keep the soil consistently moist (bottom watering is helpful!) and provide plenty of light, either with a grow light or a very sunny windowsill. Remember our springs here in the Mountain West can be late, so be patient.

Transplant your Chrysanthemum seedlings outdoors in late May through mid June, after the last expected frost. Before transplanting, harden off your seedlings by gradually exposing them to outdoor conditions for about a week.

Choose a sunny spot with well-drained soil, and space the plants 12-18 inches apart. Keep an eye on the weather forecast—we can still get some chilly nights and even late-season snow in the mountains, so be prepared to cover your plants if necessary. Alternatively, you can buy potted mums in the fall, or starts in the spring.

Watering Chrysanthemums correctly is key to their success in our dry Mountain West climate. While they need consistent moisture, they don't like to be soggy.

During the hottest part of summer (which is moderate-at-altitude here), water deeply about once a week, providing around 1 inch of water. Use the finger test: stick your finger about 2 inches into the soil near the base of the plant. If it feels dry, it's time to water. Water at the base of the plant to avoid wetting the foliage, as this can encourage fungal diseases in our low-humidity environment.

Watch for signs of underwatering, such as wilting leaves, or overwatering, such as yellowing leaves. A good layer of mulch around the base of the plants will help retain moisture and keep the soil cool.

Pruning Chrysanthemums is essential for encouraging bushy growth and abundant blooms. Begin pinching off the growing tips of your plants every 2-3 weeks starting in spring.

Continue this process through July 4th. This encourages side shoots to develop, resulting in more flowers later in the season. After July 4th, stop pinching the plants and allow the flower buds to form. As the first frost approaches in early September, remove any dead or dying foliage to keep the plants tidy.

You can expect your first Chrysanthemum blooms in early August through mid September, depending on the variety and the weather conditions. Chrysanthemums are fall-blooming plants, so they will bring color when other plants are fading.

Harvest the flowers for bouquets when they are fully open. Cut the stems at an angle, leaving several inches of stem on the plant to encourage further blooming. To prolong the flowering season, deadhead spent blooms regularly. As the first frost approaches in early September, cut the remaining flowers to enjoy indoors, as they won't survive a hard freeze.

Even in our Mountain West gardens, Chrysanthemums can encounter a few common problems. Here's how to identify and address them:

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, often encouraged by warm weather and ant activity. *How to fix/prevent it:* Strong water spray to knock them off. Encourage natural predators like ladybugs. Use insecticidal soap or neem oil for heavy infestations.

Spider Mites *What it looks like:* Fine stippling (tiny white dots) on leaves, fine webbing on leaf undersides. Leaves turn bronze or yellow. *What causes it:* Tiny arachnids that thrive in hot, dry conditions. Populations explode during drought stress. *How to fix/prevent it:* Strong water spray (daily for a week) is often effective. Increase humidity around plants. Use neem oil or insecticidal soap.

Leaf Miners *What it looks like:* Tan winding trails or blotches within the leaves. *What causes it:* The larvae of small flies or moths that burrow between the leaf surfaces. *How to fix/prevent it:* Remove and destroy affected leaves. Use floating row covers to prevent adult insects from laying eggs. Insecticidal soap or neem oil can help control infestations.

Rust *What it looks like:* Rusty orange, yellow, or brown pustules on leaves and stems, mostly on undersides. Leaves yellow and drop prematurely. *What causes it:* Various rust fungi that spread via wind-borne spores, favored by warm, humid conditions with moisture on leaves. *How to fix/prevent it:* Remove affected leaves. Avoid overhead watering. Improve air circulation. Apply sulfur or copper fungicide.

Mountain West Specific Challenges: The moderate-at-altitude heat and low humidity of our region can actually help reduce the incidence of some fungal diseases in Chrysanthemums. However, our dry climate can exacerbate spider mite problems, so be vigilant about providing adequate moisture. Intense UV can also be a stressor; make sure your plants have adequate water.

Companion planting can benefit your Chrysanthemums in the Mountain West garden.

Tomatoes and peppers make good companions because they can provide some shade during the hottest part of the day, protecting Chrysanthemums from the intense sun. Lettuce also benefits from the shade provided by Chrysanthemums, extending its harvest period in our short season. Avoid planting near aggressive spreaders that will compete for resources in our short growing season.