Chrysanthemum in Zone 6A — Mountain West

Chrysanthemums, or mums, bring vibrant color and late-season blooms to our Mountain West gardens when many other flowers are fading. Their cheerful colors brighten up those cool autumn days, and they make fantastic cut flowers for indoor enjoyment. Plus, their relatively compact size makes them perfect for containers or tucking into existing garden beds.

Our high altitude, intense sun, and short growing season present unique challenges, but with careful timing and attention, you can absolutely grow beautiful chrysanthemums here. Starting seeds indoors gives you a head start, and with a 153-day growing season, you'll have plenty of time to enjoy their vibrant colors before the first frost arrives.

Starting chrysanthemum seeds indoors can give you a jump on the season, especially since our springs tend to arrive later in the Mountain West. Begin in late February through mid March, about eight weeks before you plan to transplant. This gives the seedlings a chance to get established before facing our unpredictable spring weather.

Use seed trays or small pots filled with a good seed-starting mix. Keep the soil consistently moist by bottom watering to prevent damping off. Provide warmth and plenty of light – a sunny windowsill or grow lights work well. Remember, even though the sun feels strong here, seedlings still need supplemental light indoors.

Transplant your chrysanthemum seedlings outdoors in early to late May, after the last expected frost. Before planting, harden them off by gradually exposing them to outdoor conditions for about a week. This helps them adjust to the intense sun and cooler nights we often experience.

Choose a sunny spot with well-drained soil, spacing the plants 12-18 inches apart. Keep an eye on the weather forecast. Late spring snow or hail are not uncommon, so be prepared to cover your young plants if necessary. Buying potted mums in the fall, or starts in the spring, are both good options for our region.

Watering is crucial for chrysanthemums in our dry Mountain West climate. While they need consistent moisture, overwatering can be just as detrimental as underwatering. Aim to keep the soil consistently moist, but not soggy.

During the hotter summer months, you'll likely need to water deeply a couple of times per week, providing about an inch of water each time. Use the finger test: stick your finger about two inches deep into the soil. If it feels dry, it's time to water. Water at the base of the plant to avoid wetting the foliage, which can encourage fungal diseases in our low humidity. As the weather cools down in late summer and early fall, reduce watering frequency accordingly.

Signs of underwatering include wilting leaves and dry soil. Overwatering can lead to yellowing leaves and potentially root rot. Adding a layer of mulch around your plants helps retain moisture and regulate soil temperature, which is especially helpful given our fluctuating temperatures.

Pruning chrysanthemums is essential for encouraging bushier growth and more abundant blooms. The key is to pinch off the growing tips regularly early in the season.

Start pinching the tips of the stems every two to three weeks, beginning when the plants are about 6 inches tall. Continue this process through July 4th. This encourages the plant to branch out and produce more flowering stems. After July 4th, stop pinching to allow the flower buds to develop. Remember to remove any dead or damaged leaves throughout the season to keep your plants healthy. Towards the end of the season, after the first frost around early October, you can cut back the plants to a few inches above the ground to prepare them for winter.

Chrysanthemums are fall-blooming plants, so expect your first harvest of flowers from early July through late August, depending on when you planted and the specific variety. The flowers will last for weeks in the garden, providing a long-lasting display of color.

Cut the flowers for bouquets when they are fully open. Use sharp, clean pruners to make the cuts, and try to cut the stems at an angle to encourage water uptake. Remove any leaves that will be below the water line in your vase to prevent bacterial growth. As the first frost approaches in early October, harvest any remaining blooms to enjoy indoors.

Here are some common problems you might encounter growing chrysanthemums in the Mountain West:

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:* Rapid-reproducing sap-sucking insects that thrive in warm weather. Ant colonies may farm them. *How to fix/prevent it:* Strong water spray to knock them off. Encourage ladybugs and lacewings. Insecticidal soap for heavy infestations. Neem oil works systemically. Avoid excess nitrogen fertilizer.

Spider Mites *What it looks like:* Fine stippling on leaves (tiny white dots). Fine webbing on leaf undersides and between stems. Leaves turn bronze or yellow. *What causes it:* Tiny arachnids that thrive in hot, dry conditions. Populations explode in drought stress. *How to fix/prevent it:* Strong water spray (daily for a week) is the best treatment. Increase humidity around plants. Neem oil or insecticidal soap. Release predatory mites. Avoid broad-spectrum pesticides.

Leaf Miners *What it looks like:* *What causes it:* *How to fix/prevent it:*

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. Don't work among wet plants. Rotate crops.

Mountain West Specific Challenges: Our moderate-at-altitude heat, low humidity, and dry climate can both help and hinder chrysanthemum growth. The dry air helps prevent fungal diseases (except when watering habits promote it), but the intense sun requires careful watering to prevent drought stress.

Chrysanthemums benefit from companion plants that either help deter pests or improve growing conditions. Tomatoes and peppers make good companions because they don't compete for the same nutrients and help create a slightly more humid microclimate. Lettuce can be planted nearby as a ground cover, helping to retain soil moisture and suppress weeds.

Avoid planting chrysanthemums near aggressive spreaders as they will compete for water and nutrients, resources that are especially limited in our Mountain West gardens.